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Population 3 stars are like “the original” stars. Those formed with material that comes straight from the Big Bang. It would be very (like, a lot!) cool to see them with gravitational-wave detectors. But can we tell them apart? Or do they look like all the other stars? Here is an attempt with a fancy machine-learning classifier.

Filippo Santoliquido, Ulyana Dupletsa, Jacopo Tissino, Marica Branchesi, Francesco Iacovelli, Giuliano Iorio, Michela Mapelli, Davide Gerosa , Jan Harms, Mario Pasquato.
Astronomy & Astrophysics 690 (2024) A362.
arXiv:2404.10048 [astro-ph.HE].

Not sure what happened here, how the hell did I end up writing a paper with actual radio data that needed to be reduced … Call me an ambulance.

The guy here is 3C186 which is not a postcode but a quasar. A funny one because it’s not centered on the galaxy (it’s a bit off) and it’s also going at another velocity (ciao ciao). One of the leading explanations is that 3C186 is a recoiling black hole, the remnant of black-hole merger is being kicked away (yeah these things can happen). 3C186 also has a radio jet, and that should point in the direction of the black-hole spin. The funny thing is that spin and the kick appear perpendicular to each other, and this is fun because theory says they should actually be parallel. We looked into this a bit carefully and discovered it’s all a lie! The spin and the kick both point along the line of sight and appear perpendicular only because of a super strong projection effect. If this is true, the radio jet should also point straight to us! We then tried to test this with whatever ratio data we could grab (where is that ambulance) and found that… mmh, well, it’s a maybe.

Matteo Boschini, Davide Gerosa , Om Sharan Salafia, Massimo Dotti.
Astronomy & Astrophysics 686 (2024) A245.
arXiv:2402.08740 [astro-ph.GA].

The Milky Way, our own Galaxy, is not alone. We’re part of a galaxy cluster, but closer in we have some satellites. The bigger ones are the Large and Small Magellanic Clouds (which unfortunately I’ve never seen because they are in the southern hemisphere) but also other smaller ones: faint groups of stars in the outskirts of the Milky Way. Much like all galaxies, these faint satellites will have white dwarfs, those white dwarf will form binaries, which will be observable by LISA. There’s a new population of gravitational-wave sources there waiting to be discovered!

Valeriya Korol, Silvia Toonen, Antoine Klein, Vasily Belokurov, Fiorenzo Vincenzo, Riccardo Buscicchio, Davide Gerosa , Christopher J. Moore, Elinore Roebber, Elena M. Rossi, Alberto Vecchio.
Astronomy & Astrophysics 638 (2020) A153.
arXiv:2002.10462 [astro-ph.GA].

ps. The second half of the story is here.

Looks like some of the LIGO black holes have low spins (better, low effective spins). In this paper we show these values can be accommodated with standard “field binaries”, i.e. formation channels where binary black holes form from binary stars.

Krzysztof Belczynski, Jakub Klencki, Carl E. Fields, Aleksandra Olejak, Emanuele Berti, Georges Meynet, Christopher L. Fryer, Daniel E. Holz, Richard O’Shaughnessy, Duncan A. Brown, Tomasz Bulik, Sching C. Leung, Ken’ichi Nomoto, Piero Madau, Raphael Hirschi, Etienne Kaiser, Samuel Jones, Samaresh Mondal, Martyna Chruslinska, Paweł Drozda, Davide Gerosa , Zoheyr Doctor, Mirek Giersz, Sylvia Ekström, Cyril Georgy, Abbas Askar, Vishal Baibhav, Daniel Wysocki, T. Natan, Will M. Farr, Grzegorz Wiktorowicz, M. Coleman Miller, Ben Farr, Jean-Pierre Lasota.
Astronomy & Astrophysics, in press.
arXiv:1706.07053 [astro-ph.HE].

Hierarchical mergers are the new black. LIGO is seeing black holes that are just too big to be there. The reason is that stars, which collapse and produce black holes, do some funny things when they get too massive. Notably, they start to spontaneously produce positrons and electrons instead of keeping their own photons. Long story short: those missing photons make the temperature go up, ignite an explosion that disrupts the core and prevents black-hole formation. This “mass gap” is a solid prediction from our astrophysics friends. In some previous papers, we and other groups pointed out that one can bypass stars and form black holes from previous black holes (and goodbye my dear maximum mass limit!). But now our astrophysics friends are telling us they can also evade the limit with some more elaborate astro-magic (winds, rotation, dredge-up, reaction rates, accretion). Today’s paper is about telling the two apart, with a key prediction: a black hole with large mass but low spin would raise a glass to the astro-wizards.

Davide Gerosa , Nicola Giacobbo, Alberto Vecchio.
Astrophysical Journal, 915 (2021) 56.
arXiv:2104.11247 [astro-ph.HE].

The LISA data analysis problem is going to be massive: tons of simultaneous sources all together at the same time. In Birmingham we are developing a new scheme to tackle the problem, and here are the first outcomes. We populate satellite galaxies of the Milky Way with double white dwarfs and show that LISA… can actually do it! LISA will detect these guys, tell us which galaxies they come from, etc. It might even discover new small galaxies orbiting the Milky Way! Surprise, surprise, LISA is going to be amazing…

Elinore Roebber, Riccardo Buscicchio, Alberto Vecchio, Christopher J. Moore, Antoine Klein, Valeriya Korol, Silvia Toonen, Davide Gerosa , Janna Goldstein, Sebastian M. Gaebel, Tyrone E. Woods.
Astrophysical Journal Letters, 894 (2020) L15.
arXiv:2002.10465 [astro-ph.GA].

ps. Here is the first half of the story.
ps2. The code still needs a name. Suggestions?

LIGO and Virgo are up and running like crazy. They started their third observing run (O3) and in just a few months doubled the catalogs of observing events. And there’s so much more coming! In this paper we try to work out “how much” using our astrophysical models. Figure 4 is kind of shocking: we’re talking about thousands of black holes in a few years, and millions of them in 20 years. Need to figure out what to do with them…

Vishal Baibhav, Emanuele Berti, Davide Gerosa , Michela Mapelli, Nicola Giacobbo, Yann Bouffanais, Ugo N. Di Carlo.
Physical Review D 100 (2019) 064060.
arXiv:1906.04197 [gr-qc].

The 2017 paper “Are merging black holes born from stellar collapse or previous mergers? ” that I wrote with Emanuele Berti was selected 2025 Frontiers of Science Award. These prizes are awarded by the International Congress of Basic Science (ICBS), sponsored by the City of Beijing and the Yanqi Lake Beijing Institute of Mathematical Sciences and Application (BIMSA). Every year, they select influential recent papers in Physics, Maths, and Computer Science.

The complete list of Physics papers selected for awards is available here. Ours is one of only three papers that were selected in the category Astrophysics and Cosmology – Theory. We’ll collect the award in July at the Great Hall of the People of China in Beijing.

I’m so happy to see how a seemingly simple idea we had (“What if LIGO’s black holes merge multiple times?”) went so far! Our paper was published in Physical Review D in 2017, selected as an Editor’s Suggestion back then… and now got an award!

Huge congrats to Arianna Renzini and Nick Loutrel who won two of this year’s “Giovani Talenti” (Young Talents) prizes from the University of Milano-Bicocca. These are internal grants for postdocs: there were four grants awarded in Physics in total and two of them are from our group! Let’s gooooooooooo

Huge huge congrats to Zacharias Roupas who was awarded a Marie Curie Fellowship with us! Zachos is currently based at the British University in Egypt and will be joining my group in Milan in the Fall of 2024. The Marie Curie Fellowship program is a prestigious postdoctoral scheme operating at the EU level and, together with Arianna, we’ll now have two Marie Curie grantees in the group. Zachos’ winning proposal is titled “Black hole spin and mass function in gaseous proto-clusters” (nickname: protoBH).

Looks like my name is on a list of the 2% top scientists worldwide. Take these rankings with a grain (or a block) of salt… but this is kind of cool! The list was compiled by Stanford University and bounced by our press office.

Happy to report we got a grant from the Italian PRIN program! This is in collaboration with Andrea Maselli from GSSI in L’Aquila. The title is “Gravitational-wave astronomy as a mature field: characterizing selection biases and environmental effects”. Stay tuned for more research (and more positions to join our group!).

I was just awarded a large allocation on the Italian national supercomputer at CINECA. My PhD student Viola De Renzis (our parameter-estimation expert!) is the co-I on our proposal. Our award is part of the so-called ISCRA Class B program (which is their medium-size allocation scheme) and amounts to 1.2M CPUh on the Galileo cluster (that is: we’re going to have to crunch a ton of numbers now!). Viola and I will study the extraction of spin-spin couplings from black-hole binaries using gravitational-wave data and stochastic sampling techniques. Stay tuned!

I’m very happy to share that I was awarded a 330k EUR grant from the Cariplo Foundation. Cariplo is a trust that supports scientific research in the Lombardy region of Italy. Our proposal is titled “Deep into the relativistic two-body problem“, though arguably a better title could have been “more black-hole fun coming our way”!

It is a true honor to receive the career Prize for Young Researchers of the Italian Society for General Relativity and Gravitational Physics (SIGRAV). I was awarded the prize in the class of relativistic astrophysics. It’s amazing to be recognized in my home country; it’s great to be back! Let me thank all my mentors, advisors, collaborators, and now students who are walking with me in the adventure of science.

Here is me with the president of the society Fulvio Ricci. And here are press releases from the University of Milan-Bicocca and the INFN.

I am truly honored to receive this year’s research prize of the Italian Culture Ministry and the Lincei National Academy. The prize is given to Italian researchers in various fields, and this year was awarded in the physical sciences.
For more information see here and here (in Italian only).

I was awarded a Starting Grant from the European Research Council for my program titled “Gravitational-wave data mining”. My team and I will look into gravitational-wave data, machine-learning tools, black-hole binary dynamics, stellar-evolution simulations, etc. The total awarded amount is 1.5M EUR. Here is the press release from the Birmingham news office.

Thank you Europe, you’re great.

I am the recipient of the 2020 IUPAP General Relativity and Gravitation Young Scientist Prize. The prize is awarded by the International Society on General Relativity and Gravitation (ISGRG) through its affiliation with the International Union of Pure and Applied Physics (IUPAP) to “recognize outstanding achievements of scientists at early stages of their career”.

The citation reads: “ For his outstanding contributions to gravitational-wave astrophysics, including new tests of general relativity. “

A huge thank you to all my supervisors and advisors who supported me in these past years. For more see the Birmingham press release, the Springer press release, and the IUPAP newsletter.

My Leverhulme Trust grant proposal titled “Black-hole spins and accretion discs with gravitational waves from space” has been selected for funding (PI Davide Gerosa, University of Birmingham). The total awarded amount is GBP 191,417 over 3 years.

I was selected by the European Space Agency to join the Voyage 2050 Topical Teams. Voyage 2050 is ESA’s long-term programmatic plan to select scientific missions to be launched between 2035 and 2050. I am part of the review panel tasked to evaluate mission proposals focussed on “ The Extreme Universe, including gravitational waves, black holes, and compact objects “.

This summer I’ll be working with two undergraduate research students. Luca Reali is finishing his master at my alma mater (University of Milan, Italy) and is visiting Birmingham with a scholarship from the HPC Europa 3 cluster. Daria Gangardt just finished her 3rd year in Birmingham. Their projects concentrate on spin effects in black hole binaries and the properties of merger remnants. Welcome Daria and Luca, hope you’ll have a very rewarding summer!

The COST action GWverse is an impressive network of European researchers and institutions tackling gravitational waves, black holes, etc (i.e. the things I like… sweet!). Together with conferences and outreach, they support collaborative visits between the network members, so here we come. Hey wait a minute, Caltech is kind of far from Europe isn’t it? Here’s the news: Caltech is now an international partner of GWverse, and we’re very happy to host European researchers who want to collaborate with us in sunny southern California.

We’re having our first visitors. Serguei Ossokine from the AEI, is here to work with me on a black-hole binary spin project. Yann Bouffanais from University of Padova (Italy) is coming to collaborate on formation channels. Welcome Serguei and Yann, and thanks to COST for supporting our science!

I was recently awarded the 2018 Giulio Rampa Thesis Prize for Outstanding Research in General Relativity. The prize is sponsored by the University of Pavia (Italy) and the Italian Society for Relativity and Gravitational Physics (SIGRAV), and was officially awarded at the 23rd SIGRAV Conference. The prize announcement reads:

Dr. Gerosa’s Ph.D. Thesis on “Source modelling at the dawn of gravitational-wave astronomy” shows an impressive ability to master a rather broad range of topics in relativistic astrophysics and gravitational wave physics. The research initiated by Dr. Gerosa in these areas has triggered follow-up work, providing new important insights and new physical scenarios. The large impact that the work of Dr. Gerosa has already had can only continue to grow.

I was awarded the 2016 Stefano Braccini PhD Thesis Prize by the Gravitational Wave International Committee (GWIC). The prize announcement reads:

Dr. Gerosa received his Ph.D. from the University of Cambridge and was nominated by his adviser, Prof. Ulrich Sperhake. Dr. Gerosa’s thesis includes a wide variety of topics relevant to gravitational waves, as well as other topics in astrophysics: astrophysical explorations of accretion disks, analytically challenging work in mathematical relativity and post-Newtonian theory, and numerical relativity coding of supernova core-collapse in relativity and modified gravity.

The prize was officially awarded at the 12th Edoardo Amaldi Conference on Gravitational Waves. Here is a picture tweeted by Salvo :

Today I’ve defended my PhD at the University of Cambridge! My thesis is titled “Source modelling at the dawn of gravitational-wave astronomy” (with two l’s, British spelling here!). Huge thanks Uli and everyone else for the journey!

I was awarded a NASA Einstein Fellowship to conduct three years of postdoctoral research at Caltech. My proposal is titled “Strong gravity to the realm of observational astronomy”. Here is a passage from NASA’s press release:

“We are very pleased to welcome this talented group of young scientists as the incoming Einstein Fellows,” said Belinda Wilkes, Director of the Chandra X-ray Center at the Smithsonian Astrophysical Observatory that manages the Einstein Fellows program for NASA. “Their research will advance the quest to better understand the physics of the cosmos in a variety of directions.”

Sunny California, here I come!

Daria Gangardt has just defended her PhD thesis at the University of Birmingham. The thesis is called “Black-hole dynamics and their environments” and jumps from black-hole spins all the way to AGN discs. Daria, it has been a true pleasure working with you, all the way since your very first summer project and through your supervisor changing countries. I’m both honored and proud that you completed your PhD with me, all the best with everything. Time for drinks now! Go Dr. Daria!

And the second paper on the arxiv today is Daria’s masterpiece! pAGN (which Daria says you should read “pagan”) is a brand new, super cool code that implements the hydrodynamics of AGN disks, at least in their most popular one-dimensional fashion. Those solutions have been around for a long time but their details were, well, let’s say unclear. Daria went through everything from beginning to end, coming up with the “one-stop solution for your AGN disc needs” (that was actually the working title of the paper…). So pip install pAGN and have fun.

Daria Gangardt, Alessandro Alberto Trani, Clément Bonnerot, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society, 530 (2024) 3986–3997.
arXiv:2403.00060 [astro-ph.HE].

Much like Spotify, here is our group “Wrapped”, 2023 edition!

Some of the group highlights include… We welcomed Pippa, Nick, Arianna, Sshorab, and Matteo. We said bye to Matt who moved to MIT and Nate who moved to Canada, while Daria remains our UK stronghold. Michele got a faculty job, Viola got a postdoc, Davide got a PRIN grant, and Giulia got a SigmaXi grant. We graduated something like 12 BSc students and 4 MSc students (and all 4 of them now have PhD positions). A few long-term visitors (Francesco, Giulia, Harrison) made the group even better for a while. We wrote lots of papers, gave lots of talks, and ate lots of cakes. LIGO is taking data, LISA is being adopted, Virgo has seen better days, and GR is still true. Arianna was in the newspaper, Sshorab broke Davide’s ribs, Alice danced Greek dances, and Costantino got his first American coffee ever. Our gwpopnext conference was a blast and we discussed too much, thunderstorms included.

… now get ready for all the 2024 surprises!

Please let me introduce Dr Matthew Mould… After N papers (where N is a lot) and a 4h+15min viva discussion, Matt has completed his PhD in gravitational-wave astronomy at the University of Birmingham. WooooO! The examiners were Annelies Mortier from Birmingham and Uli Sperhake from Cambridge, who went through a thesis with more than 600 references…. Matt will be continuing his already successful career with a postdoc at MIT, LIGO lab. From my side, Matt is (actually, was!) my first PhD student and spending 3+ years working with him has been amazing. Thanks, Matt for teaching me Bayesian stats and never letting go when I was saying crap.

We’re having four (!) summer students joining the group this year!

Welcome all! We look forward to seeing your summer discoveries!

Big stars burn everything they have, die fast, and produce big black holes. So when you see two black holes together, it’s likely that the big black hole comes from the big star. Or maybe not? Before dying, the big star can drop some mass onto the other guy, making it bigger! So now, the initially big star still produces the first black hole, but, at the end of the day, that might not be the more massive black hole anymore! This scenario is called “mass-ratio reversal” and our astrophysics friends have put together many models out there showing this is indeed possible for a good fraction of the black holes that produce gravitational-wave events. So here we ask the data: given the events LIGO and Virgo have seen so far, what’s the evidence for mass-ratio reversal in binary stars? Read Matt’s paper to find out.

Matthew Mould, Davide Gerosa , Floor S. Broekgaarden, Nathan Steinle.
Monthly Notices of the Royal Astronomical Society 517 (2022) 2738–2745.
arXiv:2205.12329 [astro-ph.HE].

Observing gravitational waves from the ground (i.e. LIGO, Virgo, etc) give us a unique view on “the last three minutes” of the life of compact objects before they merge with each other. Going to space (I’m talking to you, LISA!) will instead give us “the last three years”. Completed together with the rest of the Birmingham crowd, this paper provides a realistic view of this truly amazing landscape. LISA observations at low frequencies in the 2030s will be paired with high-frequency data from LIGO’s successors (the so-called 3rd generation detectors). Together (and that’s crucial, together!) LISA and 3g detectors will tell us the full story of the life of merging black holes. LIGO alone is like catching up with a movie because you were late at the theatre, LISA alone is like a huge cliffhanger before the series finale… multiband observations are a bingewatching experience!

Antoine Klein, Geraint Pratten, Riccardo Buscicchio, Patricia Schmidt, Christopher J. Moore, Eliot Finch, Alice Bonino, Lucy M. Thomas, Natalie Williams, Davide Gerosa , Sean McGee, Matt Nicholl and Alberto Vecchio.
arXiv:2204.03423 [gr-qc].

It took a while (so many technical challenges…) but we made it! Matt‘s monster paper is finally out!

Let me introduce a fully-fledged pipeline to study populations of gravitational-wave events with deep learning. If it sounds cool, well, it is cool (just look at the flowchart in Figure 1!). We can now perform a hierarchical Bayesian analysis on GW data but, unlike current state-of-the-art applications that rely on simple functional form, we can use populations inferred from numerical simulations. This might sound like a detail but it’s not: it’s necessary to compare GW data directly against stellar physics. While we don’t do that yet here (our simulations are admittedly too simple), there’s a ton of astrophysics already in this paper. Whether you care about neural networks or hierarchical black-hole mergers (or, why not, both!), sit tight, fasten your seatbelt, and read Matt’s paper.

Matthew Mould, Davide Gerosa , Stephen R. Taylor.
Physical Review D 106 (2022) 103013.
arXiv:2203.03651 [astro-ph.HE].

Great Scott, a new paper! When analyzing gravitational-wave data, looking at one black hole at a time is not enough anymore, the fun part is looking at them all together. The issue Matt and I are tackling here is that one needs to be consistent with putting together different events when fitting the entire population. This is obvious for things that do not change (say the masses of the black holes, those are what they are), but becomes a very tricky business for varying quantities (say the spin directions, which is what we look at here). In that case, it’s dangerous to put together events taken at different stages of their evolution. And the solution to this problem is…. time travel! We show that but propagating binaries backward in time, one can put all sources on the same footing. After that, estimating the impact of the detector requires traveling forward in time, so going “back to the future”. After all, we all know that post-Newtonian black-hole binary integrations look like this:

Matthew Mould, Davide Gerosa.
Physical Review D 105 (2022) 024076.
arXiv:2110.05507 [astro-ph.HE].

Nathan Steinle is officially starting his postdoc in the group today! Nate graduated with Mike Kesden at the University of Texas at Dallas and is now working with me and the rest of the Birmingham crowd. Welcome Nate! Hope you enjoy this side of the pond.

No black hole is an island entire of itself.

We’ve got many gravitational wave events now. One can look at each of them individually (aka “parameter estimation”), all of them together (aka “population”), or each of them individually while they’re together. That’s what we do in this paper: we look at the properties of individual gravitational-wave events in light of the rest of the observed population. The nice thing is that all of these different ways of looking at the data are part of the same statistical tool, which is a hierarchical Bayesian scheme. Careful, heavy stats inside, don’t do this at home.

Christopher J. Moore, Davide Gerosa.
Physical Review D 104 (2021) 083008.
arXiv:2108.02462 [gr-qc].

Huge congrats to Maciej (Max) Dabrowny, who just graduated from the University of Birmingham after a very successful research project with us (Max’s project ended up in a paper!). Well done and all the best for the future.

I was recently interviewed for the Italian Week of Astronomy (“Settimana dell’Astronomia”), a science festival organized by Fondazione Lombardia per l’Ambiente and supported by various Italian associations, universities, and research centers. Here is the nice clip they put together (I mean, I’m terrible at speaking to a camera, but they assembled it very well!).

Today we go deep into the perilous world of binary population synthesis! Using Nicola’s code MOBSE, our master student Maciej has implemented some new prescriptions for how supernovae explode and produce compact objects. In practice, we use the compactness (that’s mass over radius) of the stellar core before the explosion to decide if that specific star will form a neutron star or a black hole. This now needs to be compared carefully with gravitational-wave data, but we suggest that there are two key signatures one should look for: the lowest black hole masses and the relative merger rates between black holes and neutron stars.

Maciej Dabrowny, Nicola Giacobbo, Davide Gerosa.
Rendiconti Lincei. Scienze Fisiche e Naturali 32 (2021) 665–673.
arXiv:2106.12541 [astro-ph.HE].

LISA is going to be great and will detect stuff from white dwarfs to those supermassive black-hole that live at the center of galaxies. If we’re lucky (yeah, who knows how many of these we will see), LISA might also detect some smaller black holes, similar to those that LIGO now sees all the time, but at a much earlier stage of their lives. But if we’re indeed lucky, the science we would take home is outstanding. Using simulated data from the LISA Data Challenge we unleash the new amazing parameter-estimation code Balrog (don’t ask what it means, it’s just a name, not one of those surreal astronomy acronyms) at this problem. Dive into the paper for some real data-analysis fun!

Riccardo Buscicchio, Antoine Klein, Elinore Roebber, Christopher J. Moore, Davide Gerosa , Eliot Finch, Alberto Vecchio.
Physical Review D 104 (2021) 044065.
arXiv:2106.05259 [astro-ph.HE].

We have a new friend in the group! Meredith Vogel is joining us for her undergraduate summer research project. Meredith is e-visiting us from Missouri State University (but will soon start her grad school at the University of Florida*) and will be working with Matt on numerical-relativity surrogate models. Meredith’s project is part of the IREU (International Summer Research) program, which is a great opportunity for US students to visit groups abroad, including us! Welcome Meredith, looking forward to seeing your great science.

• That’s the place were I saw a real alligator. On campus!

Who are the parents of LIGO’s black holes? Stars, most likely. Things like those we see in the sky at night will eventually surrender to gravity and collapse. Some of them will form black holes. Some of them will form binary black holes. Some of them will merge. Some of them will be observed by LIGO. That’s the vanilla story at least, but it might not apply to all of the black holes that LIGO sees. For some of those, stars might be the grandparents or the great grandparents. And the parents are … just other black holes! This is today’s paper lead by Vishal Baibhav. Instead of just measuring the properties of the black holes that LIGO observes, we show we can also say something about the features of the black hole parents. Read on to explore the black-hole family tree.

Vishal Baibhav, Emanuele Berti, Davide Gerosa , Matthew Mould, Kaze W. K. Wong.
Physical Review D 104 (2021) 084002.
arXiv:2105.12140 [gr-qc].

The quest of finding their astrophysical origin of merging black-hole binaries is now a key open problem in modern astrophysics. Stars are the natural progenitor of black holes: at the end of their lives, the core collapses and leaves behind a compact object. But once those “first-generation” black holes are around, they can potentially meet again and form “second generation” LIGO events. I first got interested in this problem in 2017 and, together with many many others researchers in the community, we explored the consequences of this “hierarchical merger” scenario in terms of both gravitational-wave physics and astrophysical environments. In this Nature Astronomy review article, Maya and I tried to condense all this body of work into a few pages. The result is (we hope) a broad and informed overview of this emerging research strand, with a whopping number of more than 270 citations! Hope you like it.

Davide Gerosa , Maya Fishbach.
Nature Astronomy 5 (2021) 749-760.
arXiv:2105.03439 [astro-ph.HE].
Review article.
Press release : Birmingham.
Other press coverage: SciTechDaily, techexplorist, sci-news, Media INAF, globalscience, futura-sciences, europapress, la Razon, astroblogs, phys.org, ScienceDaily, Mirage News Australia, World News Monitor, nanowerk, newsbeezer, SpaceDaily.

Hierarchical mergers are the new black. LIGO is seeing black holes that are just too big to be there. The reason is that stars, which collapse and produce black holes, do some funny things when they get too massive. Notably, they start to spontaneously produce positrons and electrons instead of keeping their own photons. Long story short: those missing photons make the temperature go up, ignite an explosion that disrupts the core and prevents black-hole formation. This “mass gap” is a solid prediction from our astrophysics friends. In some previous papers, we and other groups pointed out that one can bypass stars and form black holes from previous black holes (and goodbye my dear maximum mass limit!). But now our astrophysics friends are telling us they can also evade the limit with some more elaborate astro-magic (winds, rotation, dredge-up, reaction rates, accretion). Today’s paper is about telling the two apart, with a key prediction: a black hole with large mass but low spin would raise a glass to the astro-wizards.

Davide Gerosa , Nicola Giacobbo, Alberto Vecchio.
Astrophysical Journal, 915 (2021) 56.
arXiv:2104.11247 [astro-ph.HE].

General Relativity works well. But we still want to test it, and I guess that’s because it actually works too well (you know, all those quantum things that don’t really fit, etc). And we want to test it with gravitational-wave data, and not just because it’s the new cool thing to do (though it is!) but also because they gravitational waves give us insight into the strong-field regime of gravity where new things, if they are there at all, should show up. Now, all of this sounds great but, in practice, one has to deal with the actual model used to analyze the data. Errors in these signal models (aka waveforms), which are somewhat inevitable, can trick us into thinking we have seen a deviation from General Relativity. So, before you go out on the street and shout that Einstein was wrong, keep calm and mind your waveform.

Christopher J. Moore, Eliot Finch, Riccardo Buscicchio, Davide Gerosa.
iScience 24 (2021) 102577.
arXiv:2103.16486 [gr-qc].
Other press coverage: indiescience, sciencedaily, phys.org, astronomy.com, physicsworld.

ps. The codename for this paper was SANITY: S ystemA tics usiN g populatI ons to T est general relativitY.

This is a quick update some of our group activities… In the past few months we’ve been busy learning about the formation of stellar-mass black-hole binaries in the disks of active galactic nuclei. We organized a journal club and studied one paper each week on this “new” formation channel for LIGO sources. We discussed a ton of topics, going from disk accretion to migration traps, LIGO rates, AGN variability, GW counterparts, hierarchical mergers, all the way to EMRIs.

Here is a log of all the sessions: davidegerosa.com/bhbin-agndisks

Let me thanks all those who took part and presented papers including Daria, Matt (1), Chris, Eliot, Matt (2), Alberto, Evan, Riccardo, and Sean.

Here is the latest in our (by now long) series of papers on black-hole binaries spin precession. This work was is championed by two outstanding PhD students, Daria (in my group) and Nate (UT Dallas). The key idea behind this paper is that, for black-hole spins, one cannot really talk about precession without talking about nutation (although we only say “precession” all the time…). The spin of, say, the Earth also does both precession (azimuthal motion) and nutation (polar motion). But, unlike in the Earth problem, for black-hole spins the two motions happen on roughly the same timescale meaning that you cannot really take them apart. Or can you? We stress the role of five parameters that characterize the combined phenomenology of precession and nutation. The hope is now to use them as building blocks for future waveforms… stay tuned!

Daria Gangardt, Nathan Steinle, Michael Kesden, Davide Gerosa , Evangelos Stoikos.
Physical Review D 103 (2021) 124026.
arXiv:2103.03894 [gr-qc].

ps. Stupid autocorrect! It’s nutation, not mutation.

Orbital eccentricity in gravitational-wave observations has been long neglected. And with good reasons! Gravitation-wave emission tends to circularize sources. By the time black holes are detectable by LIGO/Virgo/LISA/whatever, they should have had ample time to become circular. Unless something exciting goes on in their formation, things like clusters, triples, Kozai-Lidov oscillations, etc. And if that happens, we want to see it! This paper contains the first model for gravitational waveforms and black-hole remnants (final mass, spin) trained directly on eccentric numerical relativity simulations. Because eccentric is the new circular.

Tousif Islam, Vijay Varma, Jackie Lodman, Scott E. Field, Gaurav Khanna, Mark A. Scheel, Harald P. Pfeiffer, Davide Gerosa , and Lawrence E. Kidder.
Physical Review D 103 (2021) 064022.
arXiv:2101.11798 [gr-qc].

We are running a virtual workshop with my group (Bham) and Emanuele Berti’s group at Johns Hopkins University (Hop). It’s an attempt to feel a bit less lonely during the COVID pandemic. Hope this is the opportunity to start new projects! And we’re a funny crowd…

For more: davidegerosa.com/hopbham-workshop/

Up-down instability S01-E03.
“Previously on the up-down instability. After finding out that the instability exists (S01-E01) and calculating its analytic endpoint (S01-E02), one terrifying prospect remains. What if it’s just PN? Can all of this disappear in the strong-field regime? This challenge now needs to be faced”.

Today’s paper is the latest in our investigations of the up-down instability in binary black holes. If the primary black hole is aligned and the secondary is anti-aligned to the orbital angular momentum, the entire system is unstable to spin precession. We found this funny thing using a post-Newtonian ( read : approximate)__ treatment but we couldn’t be 100% sure that this would still be true when the black holes merge and our approximation fails. So, we got our outstanding SXS friends on board and ask them if they could see the same effect with their numerical relativity (read : the real deal!) code. And the answer is… yes! The instability is really there! And by the way, these are among the longest numerical relativity simulations ever done.

Vijay Varma, Matthew Mould, Davide Gerosa , Mark A. Scheel, Lawrence E. Kidder, Harald P. Pfeiffer.
Physical Review D 103 (2021) 064003.
arXiv:2012.07147 [gr-qc].
Supporting material available here.

Spin precession is cool, and we want to measure it. In General Relativity, the orbital plane of a binary is not fixed but moves around. This effect is related to the spin of the orbiting black holes and contains a ton of astrophysical information. The question we try to address in this paper is the following: how does one quantify “how much” precession a system has? This is typically done by condensing information into a parameter called \(\chi_{\rm p}\), which is here generalize to include two- spin effects. There are two black holes in a binary and we received numerous complaints from the secondaries: they want to join the gravitational-wave fun!

Davide Gerosa , Matthew Mould, Daria Gangardt, Patricia Schmidt, Geraint Pratten, Lucy M. Thomas.
Physical Review D 103 (2021) 064067.
arXiv:2011.11948 [gr-qc].
Open-source code: homepage, repository.

It’s a great pleasure to welcome Nicola Giacobbo, who starts his postdoc with us today. Nicola completed his PhD and first postdoc year in Padova, and is an expert in population-synthesis simulations, compact binary progenitors, stellar physics, and all those funny things. Welcome Nicola!

If you want to know what’s out there, you need to figure out what’s missing. And gravitational-wave astronomy is no exception. We are trying to infer how things like black holes and neutron stars behave in the Universe given a limited number of observations, which are somehow selected by our detectors. This is a very general problem which is common to a variety of fields of science. We provide a hopefully pedagogical introduction to population inference, deriving all the necessary statistics from the ground up. In other terms, here is what you always wanted to know about this population business everyone is talking about but never dared to ask.

This document is going to be part of a truly massive “Handbook of Gravitational Wave Astronomy” soon to be published by Springer (not really a handbook I would say, you probably need a truck to carry it around).

Salvatore Vitale, Davide Gerosa , Will M. Farr, Stephen R. Taylor.
Chapter of “Handbook of Gravitational Wave Astronomy”; Springer Singapore (2021).
arXiv:2007.05579 [astro-ph.IM].

I am truly honored to receive this year’s research prize of the Italian Culture Ministry and the Lincei National Academy. The prize is given to Italian researchers in various fields, and this year was awarded in the physical sciences.
For more information see here and here (in Italian only).

I was awarded a Starting Grant from the European Research Council for my program titled “Gravitational-wave data mining”. My team and I will look into gravitational-wave data, machine-learning tools, black-hole binary dynamics, stellar-evolution simulations, etc. The total awarded amount is 1.5M EUR. Here is the press release from the Birmingham news office.

Thank you Europe, you’re great.

I am very happy to welcome Daria Gangardt back in my group. We worked together last summer for a short but successful summer project. Now Daria is starting her PhD. I’m honored we can be part together of the next great discoveries of our field

Congratulations to my Master’s students that graduate this year: **Abdullah Aziz** and Julian Chan from the University of Birmingham, and Beatrice Basset from the University of Lyon. Well done all, and good luck with your future adventures.

And here is the latest episode in the series of our massive scalar-tensor gravity papers… After stellar collapse, we now look at how neutron stars look like in this strange theory of gravity (recap: “massive scalar-tensor” means that gravity is mediated by the usual metric plus a scalar field which as a mass). Result: not only the theory is strange, stars are strange too! If you want to get a neutron star of 40 solar masses, look no further, massive scalar-tensor is the theory for you. More seriously, we explore all the different families of static solutions, highlighting a remarkable phenomenology. This is the kind of predictions we need to test gravity with astrophysical sources!

Roxana Rosca-Mead, Christopher J. Moore, Ulrich Sperhake, Michalis Agathos, Davide Gerosa.
Symmetry 12 (2020) 1384.
arXiv:2007.14429 [gr-qc]

And here is my latest lockdown effort: some experiments in the wonderful and perilous world of machine learning. The idea of this paper is to teach a computer to figure out by itself if a gravitational-wave signal will be detectable or not. The problem is very similar to that of image recognition: much like classifying if an image is more likely to contain a dog or a cat, here we classify black-hole mergers based on the imprints they have in the LIGO and Virgo detectors. This is important to quantify the so-called “selection effects”: in order to figure out what the Universe does based on what we observe, we need to know very well “how” we observe and thus what we are going to miss. Our code is built using Google’s TensorFlow and it is public on Github, feel free to play with it!

Davide Gerosa , Geraint Pratten, Alberto Vecchio.
Physical Review D 102 (2020) 103020.
arXiv:2007.06585 [astro-ph.HE]
Open-source code: homepage, repository.

Supermassive black hole inspiral and spin evolution are deeply connected. In the early stages when black holes are brought together by star scattering and accretion, spin orientations can change because of interactions with the environment. Later on, when gravitational waves are driving the mergers, spins change because of relativistic couplings. In this paper we try to follow this complicated evolution in a full cosmological framework, using products of the Illustris simulation suite, a new sub-resolution model, and post-Newtonian integrations.

Mohammad Sayeb, Laura Blecha, Luke Zoltan Kelley, Davide Gerosa , Michael Kesden, July Thomas.
Monthly Notices of the Royal Astronomical Society 501 (2020) 2531–2546.
arXiv:2006.06647 [astro-ph.GA].

If General Relativity is too boring, couple it to something else. In this paper we study what happens to stellar collapse and supernova explosions if gravity is transmitted not only with the usual metric of Einstein’s theory (aka the graviton) but also an additional quantity. If this extra scalar field has a mass, it dramatically impacts the emitted gravitational waves… Which means that maybe, one day, one can use gravitational-wave data to figure out if scalar fields are coupled to gravity. Here we try to explore all the related phenomenology of stellar collapse with a large set of simulations covering the parameter space. And the overall picture is remarkably neat and simple!

Roxana Rosca-Mead, Ulrich Sperhake, Christopher J. Moore, Michalis Agathos, Davide Gerosa , Christian D. Ott.
Physical Review D 102 (2020) 044010.
arXiv:2005.09728 [gr-qc].

The latest news from our LIGO/Virgo friends (including some colleagues here in Birmingham) was an astrophysical surprise. The black-hole binary GW190412 is just different from every other one we have had so far. One of the two black holes is about three times larger than the other one, it’s spinning relatively fast, and that spin might even be misaligned with respect to the binary axis. That’s a lot of new things, which makes this event very challenging (but we like challenges!) to be explained with a coherent astrophysical setup. That’s what I meant by an astrophysical surprise. Today’s paper is our attempt to, first of all, quantify that GW190412 is indeed very unusual. Maybe it comes from a second-generation merger (that is, an event where one of the two black holes is the result of a previous merger). This might explain its features, but then the astrophysical host must be very unusual. So, yet another challenge.

Davide Gerosa , Salvatore Vitale, Emanuele Berti.
Physical Review Letters 125 (2020) 101103.
arXiv:2005.04243 [astro-ph.HE].
Press release : Birmingham, MIT.
Other press coverage: International Business Times, SciTechDaily, VRT, notimerica, allnewsbuzz, canaltech.

A black-hole binary starts its life as two single black holes, and finish it as a single black hole. In between there’s all the complicated dynamics predicted by General Relativity: many orbits, dissipation of energy via gravitational waves, spins that complicate the whole business, and finally the merger which leaves behind a remnant. In this paper we put together different techniques to map this entire story beginning to end, connecting the two asymptotic conditions of a black-hole binary. This work started as a summer project of my student Luca: well done!

Luca Reali, Matthew Mould, Davide Gerosa , Vijay Varma.
Classical and Quantum Gravity 37 (2020) 225005.
arXiv:2005.01747 [gr-qc].

New paper today! We’ve been working on this for a very long time but three weeks of lockdown forced us to finish it. It’s about distorted (aka warped) accretion discs surrounding black holes. If the black hole is spinning and part of a binary system, the disc behaves in a funny way. First, it’s not planar but warped to accomodate these external disturbances. Second, disc and black hole interacts and tend to reach some mutual agreement where the disc is flat and the black-hole spin is aligned. We find it’s not that easy and things are actually much more complicated: read the paper to know more about non-linear fluid viscosities, critical obliquity, mass depletion, etc.

Davide Gerosa , Giovanni Rosotti, Riccardo Barbieri.
Monthly Notices of the Royal Astronomical Society 496 (2020) 3060-3075.
arXiv:2004.02894 [astro-ph.GA].

ps. Here is a Twitter thread by P. Armitage.

We’ve been knowing about the mass gap for a while, but I bet “spin gap” sounds new to you, uh? The gap in the spectrum of binary black hole masses is due to pair-instability supernovae (i.e. what happens if a giant ball of carbon and oxygen burns all at the same time). As for the spin gap, it might be that stars collapse into black holes which have a tiny tiny spin. But that’s only for black holes that come from stars: those come out of the merger of other black holes, on the other hand, are very rapidly rotating. So, there’s a gap between these two populations. Our paper today shows that, together, mass gap and spin gap are powerful tools to figure out where black holes come from. Cluster or field? Gaps will tell.

Vishal Baibhav, Davide Gerosa , Emanuele Berti, Kaze W. K. Wong, Thomas Helfer, Matthew Mould.
Physical Review D 102 (2020) 043002.
arXiv:2004.00650 [gr-qc].

I am the recipient of the 2020 IUPAP General Relativity and Gravitation Young Scientist Prize. The prize is awarded by the International Society on General Relativity and Gravitation (ISGRG) through its affiliation with the International Union of Pure and Applied Physics (IUPAP) to “recognize outstanding achievements of scientists at early stages of their career”.

The citation reads: “ For his outstanding contributions to gravitational-wave astrophysics, including new tests of general relativity. “

A huge thank you to all my supervisors and advisors who supported me in these past years. For more see the Birmingham press release, the Springer press release, and the IUPAP newsletter.

Sometimes you have to look into things twice. We found the up-down instability back in 2015 and still did not really understand what was going on. Three out of four black hole binaries with spins aligned to the orbital angular momentum are stable (in the sense that the spins stay aligned), but one is not. The impostors are the “up-down” black holes –binaries where the spin of the big black holes is aligned and the spin of the small black hole is antialigned. These guys are unstable to spin precession: small perturbation will trigger large precession cycles. Matt’s paper today figures out what’s the fate of these runaways. We find that these binaries become detectable in LIGO and LISA with very specific spin configurations: the two spins are aligned with each other and equally misaligned with the orbital angular momentum. There’s a lot of interesting maths in this draft (my first paper with a proof by contradiction!) as well as some astrophysics (for you, AGN disks lover).

Matthew Mould, Davide Gerosa.
Physical Review D 101 (2020) 124037.
arXiv:2003.02281 [gr-qc].
Supporting material available here.

The Milky Way, our own Galaxy, is not alone. We’re part of a galaxy cluster, but closer in we have some satellites. The bigger ones are the Large and Small Magellanic Clouds (which unfortunately I’ve never seen because they are in the southern hemisphere) but also other smaller ones: faint groups of stars in the outskirts of the Milky Way. Much like all galaxies, these faint satellites will have white dwarfs, those white dwarf will form binaries, which will be observable by LISA. There’s a new population of gravitational-wave sources there waiting to be discovered!

Valeriya Korol, Silvia Toonen, Antoine Klein, Vasily Belokurov, Fiorenzo Vincenzo, Riccardo Buscicchio, Davide Gerosa , Christopher J. Moore, Elinore Roebber, Elena M. Rossi, Alberto Vecchio.
Astronomy & Astrophysics 638 (2020) A153.
arXiv:2002.10462 [astro-ph.GA].

ps. The second half of the story is here.

The LISA data analysis problem is going to be massive: tons of simultaneous sources all together at the same time. In Birmingham we are developing a new scheme to tackle the problem, and here are the first outcomes. We populate satellite galaxies of the Milky Way with double white dwarfs and show that LISA… can actually do it! LISA will detect these guys, tell us which galaxies they come from, etc. It might even discover new small galaxies orbiting the Milky Way! Surprise, surprise, LISA is going to be amazing…

Elinore Roebber, Riccardo Buscicchio, Alberto Vecchio, Christopher J. Moore, Antoine Klein, Valeriya Korol, Silvia Toonen, Davide Gerosa , Janna Goldstein, Sebastian M. Gaebel, Tyrone E. Woods.
Astrophysical Journal Letters, 894 (2020) L15.
arXiv:2002.10465 [astro-ph.GA].

ps. Here is the first half of the story.
ps2. The code still needs a name. Suggestions?

LISA is going to be cool. And not just for your astro-related dreams. Theoretical physicists can have fun too! This community-wide manifesto illustrates just how cool things are going to be with LISA. LISA will constitute a major milestone to test gravity, cosmology, the nature of black holes, etc. A big thanks to all those involved.

Enrico Barausse, et al. (322 authors incl. Davide Gerosa).
General Relativity and Gravitation 52 (2020) 8, 81.
arXiv:2001.09793 [gr-qc].

The Institute for Gravitational Wave Astronomy at the University of Birmingham, UK, invites applications for postdoctoral positions.

The Institute provides a vibrant and diverse environment with expertise covering theoretical and experimental gravitational-wave research, with applications to present and future-generation detectors, theoretical astrophysics, transient astronomy, gravitational-wave source modeling, and general relativity theory. Applications from top researchers in all areas related to gravitational-wave and transient astronomy are encouraged.

Institute faculty members include Andreas Freise, Davide Gerosa, Denis Martynov, Haixing Miao, Christopher Moore, Conor Mow-Lowry, Matt Nicholl, Patricia Schmidt, Silvia Toonen, and Alberto Vecchio.

One postdoctoral appointment is funded by the UK Leverhulme Trust (PI Dr. Davide Gerosa) and is focused on developing astrophysical and statistical predictions for the LISA space mission. The successful candidate will have ample opportunities to explore other areas of gravitational-wave astronomy as well.

Appointments will be for a three-year term starting in the Fall of 2020 and come with generous research and travel budget.

Applications should include a CV with a list of publications, and a two-page statement covering research interests and plans. Complete applications should be received by 27 January 2020 for full consideration. Applications should be sent to Ms. Joanne Cox at: [email protected].
Applicants should also arrange for 3 reference letters to be sent by 27 January 2020 to the same email address.

For further information and informal inquiries please contact Dr. Davide Gerosa ([email protected]) and Prof. Alberto Vecchio ([email protected]).

My Leverhulme Trust grant proposal titled “Black-hole spins and accretion discs with gravitational waves from space” has been selected for funding (PI Davide Gerosa, University of Birmingham). The total awarded amount is GBP 191,417 over 3 years.

I was recently interviewed for Scientific American about my recent paper on multiple-generation black holes in stellar clusters. Here is the article: “Black Hole Factories May Hide at Cores of Giant Galaxies”. Very happy to be quoted saying “I don’t think we’ve been hitting this problem hard enough”. I think it’s a nice summary of scientific research –so much to discover!

I was selected by the European Space Agency to join the Voyage 2050 Topical Teams. Voyage 2050 is ESA’s long-term programmatic plan to select scientific missions to be launched between 2035 and 2050. I am part of the review panel tasked to evaluate mission proposals focussed on “ The Extreme Universe, including gravitational waves, black holes, and compact objects “.

We’re accepting applications from prospective PhD students. The deadline is Dec 31, 2019 for positions starting in the Fall of 2020.

Here below is my project description:

_Astrophysics and phenomenology of gravitational-wave sources with LIGO and LISA
This project concentrates on developing theoretical and astrophysical prediction s of gravitational-wave sources. The first observations of gravitational waves by LIGO have ushered us into the golden age of gravitational-wave discoveries. Thousands of new events are expected to be observed in the next few years as detectors reach their design sensitivities. Such large catalogs of gravitational-wave observations will open new, unprecedented opportunities in terms of both fundamental physics and astrophysics. Crucially, they will need to be faced with increasingly accurate predictions. First, among large catalogs, there will be “golden” events. We expect systems that, because of their properties, are particularly interesting to carry out some specific measurements (perhaps because of their favorable orientations, or because they are very massive, or very rapidly rotating, etc). Second, large catalogs need to be exploited with powerful statistical techniques. In the long run, future facilities like LISA will deliver new kinds of sources providing access to a whole new set of phenomena in both astrophysics and fundamental physics. New theoretical tools and techniques need to be developed (and immediately applied!) to maximize the scientific payoff of current and future gravitational-wave observatories. _

This week I am organizing the GrEAT PhD winter school. GrEAT (which stands for Gravitational-wave Excellence through Alliance Training) is a synergy network between the UK and China. Our program features informal talks in the mornings and hands-on sessions in the afternoons, covering both theoretical and experimental gravitational-wave physics.

After the school in Birmingham, students will move on to various UK nodes to complete longer projects. In particular, Mingyue Zhou will stay here working with me.

Two close collaborators will be visiting my group this winter.

Vijay Varma, postdoc at Caltech and expert of numerical relativity surrogate models, will be here on October 7-11. Get ready for his talk “Binary black hole simulations: from supercomputers to your laptop” (aka: Everything you ever wanted to know about waveform surrogates).

Giovanni Rosotti, Veni fellow in Leiden, will be here on November 4-15. He will also give a talk: “The observational era of planet formation“. What do planets have to do with black holes? Turns out some stages of their evolution are set by the same equations. We have a lot to learn from each other! Giovanni’s visit is supported by the GWverse COST Action (thanks EU!).

Today’s paper is about superkicks. These are extreme configurations of black hole binaries which receive a large recoil. Black hole recoils work much like those of, say, a cannon. As the cannonball flies, the cannon recoils backwards. Here the binary is shooting gravitational waves: as they are emitted, the system recoils in the opposite direction. In this paper we show that superkicks might be up to 25% larger if the binary is mildly eccentric. This means it’s a bit easier to kick black holes out of stellar clusters and galaxies.

Ulrich Sperhake, Roxana Rosca-Mead, Davide Gerosa , Emanuele Berti.
Physical Review D 101 (2020) 024044.
arXiv:1910.01598 [gr-qc].

I am very excited to welcome Matthew Mould in my research group. Matt is starting his Ph.D. with me in Birmingham. We already have too many ideas…

Gravitational-wave astronomy is, seems obvious to say, about doing astronomy with gravitational waves. One has gravitational-wave observations (thanks LIGO and Virgo!) on hand and astrophysical models on the other hand. The more closely these two sides interact, the more we can hope to use gravitational-wave data to learn about the astrophysics of the sources. Today’s paper with JHU student Kaze Wong tries to further stimulate this dialog. And, well, one needs to throw some artificial intelligence in the game. There are three players now (astrophysics, gravitational waves, and machine learning) and things get even more interesting.

Kaze W.K. Wong, Davide Gerosa.
Physical Review D 100 (2019) 083015.
arXiv:1909.06373 [astro-ph.HE].

ps. The nickname of this project was sigmaspops

I am very excited to welcome Matthew Mould in my research group. Matt is starting his Ph.D. with me and will be part of the Birmingham Institute for Gravitational Wave Astronomy. And now ready for discovery!

Blog at WordPress.com.

What’s in between neutron stars and black holes? It looks like neutron stars have a maximum mass of about 2 solar masses while black holes have a minimum mass of about 5. So what’s in between? That’s the popular issue of the ‘low mass gap’. Actually, now we know something must be in there. LIGO and Virgo have seen GW170817, a merger of two neutron stars, which merged in to a black hole with the right mass to populate the gap. Can this population be seen directly with (future) gravitational-wave detectors? That’s today’s paper.

Anuradha Gupta, Davide Gerosa , K. G. Arun, Emanuele Berti, Will Farr, B. S. Sathyaprakash.
Physical Review D 101 (2020) 103036.
arXiv:1909.05804 [gr-qc].

This summer I’ll be working with two undergraduate research students. Luca Reali is finishing his master at my alma mater (University of Milan, Italy) and is visiting Birmingham with a scholarship from the HPC Europa 3 cluster. Daria Gangardt just finished her 3rd year in Birmingham. Their projects concentrate on spin effects in black hole binaries and the properties of merger remnants. Welcome Daria and Luca, hope you’ll have a very rewarding summer!

Funny things happen in supernova explosions. Funny and complicated. If the star is too massive, the explosion is unstable. The black hole it formed it not as massive as it could have been. In gravitational-wave astronomy, this means that we should not observe black holes heavier than about 50 solar masses. This does not apply, of course, to black holes that are not formed from stars, but from other black holes (yes! more black holes!). If black holes resulting from older gravitational wave events somehow stick around, they could be recycled in other generations of mergers. We point out that this can work only if their astrophysical environment is dense enough. Can we measure the escape speed of black holes “nurseries” using gravitational-wave events that should not be there because of supernova instabilities?

Davide Gerosa , Emanuele Berti.
Physical Review D Rapid Communications 100 (2019) 041301R.
arXiv:1906.05295 [astro-ph.HE].
Press release : Birmingham.
Other press coverage: Scientific American, astrobites, interestingengineering, metro.co.uk, Media INAF, Great Lakes Ledger, sciencealert, sciencetimes, mic.com.

LIGO and Virgo are up and running like crazy. They started their third observing run (O3) and in just a few months doubled the catalogs of observing events. And there’s so much more coming! In this paper we try to work out “how much” using our astrophysical models. Figure 4 is kind of shocking: we’re talking about thousands of black holes in a few years, and millions of them in 20 years. Need to figure out what to do with them…

Vishal Baibhav, Emanuele Berti, Davide Gerosa , Michela Mapelli, Nicola Giacobbo, Yann Bouffanais, Ugo N. Di Carlo.
Physical Review D 100 (2019) 064060.
arXiv:1906.04197 [gr-qc].

Spoiler alert: this paper is a bit sad.

Stellar-mass black-hole binaries are now detected by LIGO on a weekly basis. It would be really cool if LISA (a future space mission targeting low-frequencies gravitational waves) could see them as well. We could do a lot of cool stuff, in both the astro and the theory side of things. In today’s paper, we try to figure out how easy or hard it will be to extract these signals from the LISA noise. Well, it’s hard. In terms of the minimum signal-to-noise ratio required, we find that this is as high as 15. The number of expected detection becomes discouragingly low unless the detector behaves a bit better at high frequencies or black holes with 100 solar masses start floating around.

Christopher J. Moore, Davide Gerosa , Antoine Klein.
Monthly Notices of the Royal Astronomical Society Letters 488 (2019) L94–L98.
arXiv:1905.11998 [astro-ph.HE].

Where do black holes come from? Sounds like a scify book title, but it’s real. These days, that’s actually the million dollar question in gravitational-wave astronomy. LIGO sees (lots of!) black holes in binaries, and those data encode information on how their stellar progenitors behave, what they like or did not like to do. This is paper is the latest attempt to understand if black holes formed alone (i.e. a single binary star forms a single binary black hole) or together (i.e. many stars exchange pairs in dense stellar environments).

Yann Bouffanais, Michela Mapelli, Davide Gerosa , Ugo N. Di Carlo, Nicola Giacobbo, Emanuele Berti, Vishal Baibhav.
Astrophysical Journal, 886 (2019) 25.
arXiv:1905.11054 [astro-ph.HE].

Surrogate models are the best of both worlds. Numerical-relativity simulations are accurate but take forever. Waveform models have larger errors but can be computed cheaply, which means they can be used in the real world and compared with data. Surrogates are as fast as the approximate waform models, but as accurate as the numerical-relativity simulations they are trained on. Don’t believe me? I don’t blame you, this does sound impossible. Check out our new paper, where we pushed this effort to binaries with spins and more unequal masses.

Vijay Varma, Scott E. Field, Mark A. Scheel, Jonathan Blackman, Davide Gerosa , Leo C. Stein, Lawrence E. Kidder, Harald P. Pfeiffer.
Physical Review Research 1 (2019) 033015.
arXiv:1905.09300 [gr-qc].

We moved (back) to the UK. I’m now a lecturer at the University of Birmingham (which is equivalent to assistant professor in the US). Really excited to start this new adventure!

My group is part of the Birmingham Institute for Gravitational Wave Astronomy. If you’re interested in joining us, have a look at this page. And see you in the Midlands!

The prospect of multiband gravitational-wave astronomy is so so so exciting (I mean, really!). So exciting that we want to make sure once again it’s true; and this is today’s paper. Multiband means seeing the same black hole binary with both LIGO at high frequencies and LISA at low frequencies. LISA observations can serve as precursors for the LIGO mergers, and you can a whole lot of new science (astrophysics, tests of GR, smart data analysis, cosmology, etc). Here we have a new semi-analytic way to estimate the rate (i.e. how many) of multiband events, and we also explore some of the stellar physics one could constraint with them. Enjoy!

Davide Gerosa , Sizheng Ma, Kaze W.K. Wong, Emanuele Berti, Richard O’Shaughnessy, Yanbei Chen, Krzysztof Belczynski
Physical Review D 99 (2019) 103004.
arXiv:1902.00021 [astro-ph.HE].
Supporting material available here.

Latest in the series of our spin-precession papers, here we found a thing that was worthy of a new name: wide nutation(we had wide precession before, but this is better). These are black-hole binary configurations where the angle between any of the two spins and the orbital angular momentum changes a lot. Can’t change more actually: spins goes from full alignment to full anti-alignment. And they do it many times.

ps. We found this wide precession during Alicia’s SURF undergraduate summer project at Caltech. Jackpot!

Davide Gerosa , Alicia Lima, Emanuele Berti, Ulrich Sperhake, Michael Kesden, Richard O’Shaughnessy.
Classical and Quantum Gravity 36 (2019) 10, 105003.
arXiv:1811.05979 [gr-qc].
Supporting material available here.

LISA is going to be amazing: supermassive black-holes, galactic white dwarfs, EMRIs… Besides all of that, LISA can help us doing LIGO’s science better. Some LIGO sources (notably, things like GW150914) will show up in LISA years in advance. LISA is going to tell us when (in time) and where (in frequency) LIGO will see these sources. In this paper, we explore the idea of adapting the LIGO noise curve if one knows that a source is coming in (because LISA told us). We apply this idea to ringdown tests of GR, and show how powerful they become.

Rhondale Tso, Davide Gerosa, Yanbei Chen.
Physical Review D 99 (2019) 124043.
arXiv:1807.00075 [gr-qc].
Other press coverage: astrobites.

… and we’re back to selection effects. That means modeling what you cannot see. The black holes that gravitational-wave detectors observe are not representative of those that are out there in the Universe. Some are easier to see, some are harder. Quantifying how much easier and harder is crucial to properly understand the underlying astrophysics. In this paper (which came out of a BSc student project!), we go back to the basics and work out gravitational-wave selection effects one step after the other, using and refining the most common approximation. Two things to remember: including noise fluctuations is easy, and a signal-to-noise ratio threshold of 11 is probably ok.

Davide Gerosa , Malvina Bellotti.
Classical and Quantum Gravity 41 (2024) 125002.
arXiv:2404.16930 [astro-ph.HE].

Spinning black holes are weird (well, all black holes are weird but those that spin are the worse!). They have a funny thing called ergoregion where orbiting particles can have negative energy. Penrose was the first to realize that this can be exploited to extract energy from the black hole itself. The thing is, even if you figure out how to do it, you’re inevitably going to spin the black hole down. At the end of the day, you’re left with a fossil black hole that does not have any spin. The mass of that leftover black hole (“ What’s for lunch dear? Fancy some sushi or prefer a black hole?”) is called irreducible mass. Hawking (another giant!) figured out this has to do with thermodynamics.

Long story short, in this paper we compute the irreducible mass of the black holes detected in gravitational waves by LIGO. It was funny to re-discover that gravitational wave detection was indeed the motivation behind Hawking original proof of the area theorem (he had Weber‘s claimed detection in mind at the time). The story behind our paper starts as a toy calculation with my undergraduate student Cecilia and ended up in a neat, hopefully informative exploitation of LIGO data. We reparametrized LIGO’s black-hole properties using the rotational and rotational contributions to their total energy, we ranked current gravitational-wave events according to their “irreversibility”, and we compute a sort of population version of the area law. Enjoy!

Davide Gerosa , Cecilia Maria Fabbri, Ulrich Sperhake.
Classical and Quantum Gravity 39 (2020) 175008.
arXiv:2202.08848 [gr-qc].

A black-hole binary starts its life as two single black holes, and finish it as a single black hole. In between there’s all the complicated dynamics predicted by General Relativity: many orbits, dissipation of energy via gravitational waves, spins that complicate the whole business, and finally the merger which leaves behind a remnant. In this paper we put together different techniques to map this entire story beginning to end, connecting the two asymptotic conditions of a black-hole binary. This work started as a summer project of my student Luca: well done!

Luca Reali, Matthew Mould, Davide Gerosa , Vijay Varma.
Classical and Quantum Gravity 37 (2020) 225005.
arXiv:2005.01747 [gr-qc].

As you can imagine, I’m kind of obsessed with black hole binaries. So easy (let’s face it, a black hole is easy! Just mass and spin), but at the same time so terribly complicated… Happy to present our attempt to see the binary dynamics in real time. Technical blah blah: we attach a visualization tool to a numerical relativity surrogate model. Are you ready to be a binary black hole explorer?

ps. Kids can have fun with black holes too! From mikesmathpage.

Vijay Varma, Leo C. Stein, Davide Gerosa.
Classical and Quantum Gravity 36 (2019) 9, 095007.
arXiv:1811.06552 [astro-ph.HE].
Supporting material available here.

Latest in the series of our spin-precession papers, here we found a thing that was worthy of a new name: wide nutation(we had wide precession before, but this is better). These are black-hole binary configurations where the angle between any of the two spins and the orbital angular momentum changes a lot. Can’t change more actually: spins goes from full alignment to full anti-alignment. And they do it many times.

ps. We found this wide precession during Alicia’s SURF undergraduate summer project at Caltech. Jackpot!

Davide Gerosa , Alicia Lima, Emanuele Berti, Ulrich Sperhake, Michael Kesden, Richard O’Shaughnessy.
Classical and Quantum Gravity 36 (2019) 10, 105003.
arXiv:1811.05979 [gr-qc].
Supporting material available here.

This is a massive review born out of the European COST Action CA16104 Gravitational waves, black holes and fundamental physics (GWverse). We summarize the status of the field of gravitational-wave astronomy and lie down a roadmap for the immediate future.

Leor Barack, et al. (199 authors incl. Davide Gerosa).
Classical and Quantum Gravity 36 (2019) 14, 143001.
arXiv:1806.05195 [gr-qc].
Editor’s coverage in physicsworld.com.

Equal-mass binaries correspond to a discontinuous limit in the spin precession equations. A new constant of motion pops up, which can be exploited to study the dynamics. This is a really neat calculation done with Jakub, a Cambridge undergraduate student. Also, my first paper at Caltech!

Davide Gerosa , Ulrich Sperhake, Jakub Vošmera.
Classical and Quantum Gravity 34 (2017) 6 ,064004.
arXiv:1612.05263 [gr-qc].

Here we present 1+1 numerical-relativity simulation of stellar collapse in scalar-tensor theories, where gravity is mediated by the usual metric coupled to an additional scalar field. Bottom line: you can test General Relativity with supernovae explosions!

Davide Gerosa , Ulrich Sperhake, Christian D. Ott.
Classical and Quantum Gravity 33 (2016) 13 , 135002.
arXiv:1602.06952 [gr-qc].
Supporting material available here.

What happens if you throw a scalar field into General Relativity? And if you throw more than one? Here is a paper on the phenomenology of neutron stars in theories with more than one scalar field coupled to gravity.

Michael Horbatsch, Hector O. Silva, Davide Gerosa , Paolo Pani, Emanuele Berti, Leonardo Gualtieri, Ulrich Sperhake.
Classical and Quantum Gravity 32 (2015) 20, 204001.
arXiv:1505.07462 [gr-qc].
Featured in CQG+. Selected as IOPselect.

A massive review on testing gravity, which came out of a nice meeting we had at University of Mississippi in January 2014. See the numbers.

Emanuele Berti, et al. (53 authors incl. Davide Gerosa).
Classical and Quantum Gravity 32 (2015) 24, 243001.
arXiv:1501.07274 [gr-qc].

The 2017 paper “Are merging black holes born from stellar collapse or previous mergers? ” that I wrote with Emanuele Berti was selected 2025 Frontiers of Science Award. These prizes are awarded by the International Congress of Basic Science (ICBS), sponsored by the City of Beijing and the Yanqi Lake Beijing Institute of Mathematical Sciences and Application (BIMSA). Every year, they select influential recent papers in Physics, Maths, and Computer Science.

The complete list of Physics papers selected for awards is available here. Ours is one of only three papers that were selected in the category Astrophysics and Cosmology – Theory. We’ll collect the award in July at the Great Hall of the People of China in Beijing.

I’m so happy to see how a seemingly simple idea we had (“What if LIGO’s black holes merge multiple times?”) went so far! Our paper was published in Physical Review D in 2017, selected as an Editor’s Suggestion back then… and now got an award!

The prospect of multiband gravitational-wave astronomy is so so so exciting (I mean, really!). So exciting that we want to make sure once again it’s true; and this is today’s paper. Multiband means seeing the same black hole binary with both LIGO at high frequencies and LISA at low frequencies. LISA observations can serve as precursors for the LIGO mergers, and you can a whole lot of new science (astrophysics, tests of GR, smart data analysis, cosmology, etc). Here we have a new semi-analytic way to estimate the rate (i.e. how many) of multiband events, and we also explore some of the stellar physics one could constraint with them. Enjoy!

Davide Gerosa , Sizheng Ma, Kaze W.K. Wong, Emanuele Berti, Richard O’Shaughnessy, Yanbei Chen, Krzysztof Belczynski
Physical Review D 99 (2019) 103004.
arXiv:1902.00021 [astro-ph.HE].
Supporting material available here.

The COST action GWverse is an impressive network of European researchers and institutions tackling gravitational waves, black holes, etc (i.e. the things I like… sweet!). Together with conferences and outreach, they support collaborative visits between the network members, so here we come. Hey wait a minute, Caltech is kind of far from Europe isn’t it? Here’s the news: Caltech is now an international partner of GWverse, and we’re very happy to host European researchers who want to collaborate with us in sunny southern California.

We’re having our first visitors. Serguei Ossokine from the AEI, is here to work with me on a black-hole binary spin project. Yann Bouffanais from University of Padova (Italy) is coming to collaborate on formation channels. Welcome Serguei and Yann, and thanks to COST for supporting our science!

As you can imagine, I’m kind of obsessed with black hole binaries. So easy (let’s face it, a black hole is easy! Just mass and spin), but at the same time so terribly complicated… Happy to present our attempt to see the binary dynamics in real time. Technical blah blah: we attach a visualization tool to a numerical relativity surrogate model. Are you ready to be a binary black hole explorer?

ps. Kids can have fun with black holes too! From mikesmathpage.

Vijay Varma, Leo C. Stein, Davide Gerosa.
Classical and Quantum Gravity 36 (2019) 9, 095007.
arXiv:1811.06552 [astro-ph.HE].
Supporting material available here.

Latest in the series of our spin-precession papers, here we found a thing that was worthy of a new name: wide nutation(we had wide precession before, but this is better). These are black-hole binary configurations where the angle between any of the two spins and the orbital angular momentum changes a lot. Can’t change more actually: spins goes from full alignment to full anti-alignment. And they do it many times.

ps. We found this wide precession during Alicia’s SURF undergraduate summer project at Caltech. Jackpot!

Davide Gerosa , Alicia Lima, Emanuele Berti, Ulrich Sperhake, Michael Kesden, Richard O’Shaughnessy.
Classical and Quantum Gravity 36 (2019) 10, 105003.
arXiv:1811.05979 [gr-qc].
Supporting material available here.

Black hole mergers are like a scattering problem. Two black holes come in, and one black hole comes out. The difference is a bunch of gravitational waves. Those are nice, of course, but the remnant black hole is important too! Here we provide accurate predictions of the mass, spin and kick of this remnant given the properties of the two merging black holes. If you need those numbers (want to build a waveform family? or test GR perhaps?) just use our python module surfinBH!

Bonus note. What if you collide ducks instead of black holes?

Vijay Varma, Davide Gerosa , François Hébert, Leo C. Stein, Hao Zhang.
Physical Review Letters 122 (2019) 011101.
arXiv:1809.09125 [gr-qc]
Press release: Caltech, Ole Miss.
Other press coverage: Space Daily, phys.org, longroom, tasnim, europapress (Spanish), Media INAF (video in Italian).

We all know Doppler shifts, right? That’s like the biibouuubiiiiboouuuuuu of an ambulance. That happens to gravitational waves as well. Suppose you have a merging binary which is emitting gravitational waves (bibooou). If that binary is going somewhere (say it’s falling into the gravitational potential of a third body), much like the ambulance, the emitted signal will be Doppler shifted. This paper shows a very nice calculation to incorporate Doppler shifts into gravitational waves.

ps. This started out as Katie’s undergraduate summer project at Caltech. Congrats Katie!

Katie Chamberlain, Christopher J. Moore, Davide Gerosa , Nicolas Yunes.
Physical Review D 99 (2019) 024025.
arXiv:1809.04799 [gr-qc].

Today’s paper celebrates the wedding of startrack and precession (the nickname for this project was pretrack 😉 ). We use population synthesis evolution from startrack to predict the parameters of spinning black-hole binaries observed by LIGO. The spin distribution is then propagated from formation to detection using post-Newtonian evolutions from my precession code. The bottom line is that spin measurements can be used to truly reconstruct the binary formation channels, and some specific mechanisms (like mass transfers, tides, natal kicks, supernova’s instabilities etc.). Our database is publicly available (play with it!), as well as a little code to compute gravitational-wave detectabilities.

Update : this is my 25th published paper! That’s silver, right?

Davide Gerosa , Emanuele Berti, Richard O’Shaughnessy, Krzysztof Belczynski, Michael Kesden, Daniel Wysocki, Wojciech Gladysz.
Physical Review D 98 (2018) 084036.
arXiv:1808.02491 [astro-ph.HE].
Supporting material available here.

LISA is going to be amazing: supermassive black-holes, galactic white dwarfs, EMRIs… Besides all of that, LISA can help us doing LIGO’s science better. Some LIGO sources (notably, things like GW150914) will show up in LISA years in advance. LISA is going to tell us when (in time) and where (in frequency) LIGO will see these sources. In this paper, we explore the idea of adapting the LIGO noise curve if one knows that a source is coming in (because LISA told us). We apply this idea to ringdown tests of GR, and show how powerful they become.

Rhondale Tso, Davide Gerosa, Yanbei Chen.
Physical Review D 99 (2019) 124043.
arXiv:1807.00075 [gr-qc].
Other press coverage: astrobites.

Gravitational-wave astronomy is moving. Quickly. In a few years we are going to have large catalogs of many detections, and a whole lot of information to extract from them. Instead of focussing on parameters (masses, spins, redshifts) of single sources, we will want to extract hyperparameters describing physical features of the population (metallicity, natal kicks, common envelope, stellar winds, etc). Here we show how to do this in practice: read our new paper for an amazing journey through hyperlateral cubes, Gaussian process emulators, selection biases, hierarchical modeling and more.

Our tools are publicly available! Here is Steve’s Webpage and our public code.

Stephen R. Taylor, Davide Gerosa.
Physical Review D 98 (2018) 083017.
arXiv:1806.08365 [astro-ph.HE].
Editor’s coverage in APS’s Kaleidoscope.

This is a massive review born out of the European COST Action CA16104 Gravitational waves, black holes and fundamental physics (GWverse). We summarize the status of the field of gravitational-wave astronomy and lie down a roadmap for the immediate future.

Leor Barack, et al. (199 authors incl. Davide Gerosa).
Classical and Quantum Gravity 36 (2019) 14, 143001.
arXiv:1806.05195 [gr-qc].
Editor’s coverage in physicsworld.com.

LIGO can measure spins. Well, effective spins actually. These are special combinations of the two spins (magnitude and direction) and the binary mass ratio. There’s a ton of astrophysics that can be done just with this quantity, but one should always be careful. Today’s paper points out a few important shortcomings when dealing with effective spin measurements. Want to know more about asymmetries and selection biases?

ps. You can hardly find a better day to post a paper on the arxiv than May 4th

Ken K. Y. Ng, Salvatore Vitale, Aaron Zimmerman, Katerina Chatziioannou, Davide Gerosa , Carl-Johan Haster.
Physical Review D 98 (2018) 083007.
arXiv:1805.03046 [gr-qc].

Surrogate models are fancy interpolation schemes developed to provide accurate (well, really accurate) waveforms directly from numerical relativity simulations. The first surrogate able to model fully precessing systems came up recently (and it’s really an amazing work!). Here we exploit these advances to explore how linear momentum is emitted in generic black-hole mergers, and well as its back-reaction. Black holes get kicked!

Davide Gerosa , François Hébert, Leo C. Stein.
Physical Review D 97 (2018) 104049.
arXiv:1802.04276 [gr-qc].
Open-source code: homepage, repository.

These are proceedings of the IAU Symposium 338 “Gravitational Wave Astrophysics”, held in Baton Rouge LA on October 16-19, 2017. My contribution is based on arXiv:1707.04637, where we look at the first binary black hole data using different Bayesian priors. During that conference, we had the announcement of the first neutron start event, GW170817, and I was presenting black-hole science: so obsolete…

Davide Gerosa , Salvatore Vitale, Carl-Johan Haster, Katerina Chatziioannou, Aaron Zimmerman.
Proceedings of the International Astronomical Union, 13 (S338), 22-28.
arXiv:1712.06635 [astro-ph.HE].

These are my proceedings for the 12th Edoardo Amaldi Conference on Gravitational Waves (July 9-14, 2017, Pasadena CA). I summarize how to use black-hole spin dynamics to learn about the lives of stars using gravitational-wave data. There are surprises…

Before the talk, I was awarded the 2016 Stefano Braccini Thesis prize (here is Salvo’s tweet about it).

Davide Gerosa.
Journal of Physics: Conference Series 957 (2018) 1, 012014.
arXiv:1711.10038 [astro-ph.HE].

Natal kicks imparted to neutron stars and black holes at birth can be constrained using LIGO data. Kicks cause misalignments between the spins and the orbital angular momentum. Here we compare large banks of population synthesis simulations to LIGO data using hierarchical Bayesian statistics and show that (already with 4 events!) natal kicks are constrained from both above and below. Simulated binaries are produced merging Startrack evolutions to my precession code. More on this very soon…

Update : here it is!

Daniel Wysocki, Davide Gerosa , Richard O’Shaughnessy, Krzysztof Belczynski, Wojciech Gladysz, Emanuele Berti, Michael Kesden, Daniel Holz.
Physical Review D 97 (2018) 043014.
arXiv:1709.01943 [astro-ph.HE].

Supernova can be used to test gravity! …and if there’s a massive scalar field around, things get terribly interesting. Here we generalize arXiv:1602.06952 to study stellar collapse in massive scalar-tensor theories of gravity (that is, the graviton is coupled to a massive scalar field) with numerical simulations. The scalar-field mass introduces a dispersion relation, and different GW frequencies travel at different speeds. It might even make sense to target historic supernovae: maybe the tail of the signal is still coming to us!

Ulrich Sperhake, Christopher J. Moore, Roxana Rosca, Michalis Agathos, Davide Gerosa, Christian D. Ott.
Physical Review Letters 119 (2017) 201103.
arXiv:1708.03651 [gr-qc].

Bayesian statistics is really cool. It lets you specify clearly and openly all the assumptions that enter an analysis. What’s the effect of these prior assumptions on current inference with gravitational-wave data from black-hole binaries? Here we tackle this question head-on, and perform parameter estimation runs on LIGO data with many (astrophysically motivated!) prior assumptions. Some parameters (like chirp mass) do not suffer from prior choices but others (like the effective spin) do! Specifying the astrophysics as priors is a powerful tool to extract more science from GW data

Update : at the time of publication, this was the first independent reanalysis of any GW data! (There are many more now…). Also, use our data for your research!

Salvatore Vitale, Davide Gerosa , Carl-Johan Haster, Katerina Chatziioannou, Aaron Zimmerman.
Physical Review Letters 119 (2017) 251103.
arXiv:1707.04637 [gr-qc].
Posterior sample data release.

Looks like some of the LIGO black holes have low spins (better, low effective spins). In this paper we show these values can be accommodated with standard “field binaries”, i.e. formation channels where binary black holes form from binary stars.

Krzysztof Belczynski, Jakub Klencki, Carl E. Fields, Aleksandra Olejak, Emanuele Berti, Georges Meynet, Christopher L. Fryer, Daniel E. Holz, Richard O’Shaughnessy, Duncan A. Brown, Tomasz Bulik, Sching C. Leung, Ken’ichi Nomoto, Piero Madau, Raphael Hirschi, Etienne Kaiser, Samuel Jones, Samaresh Mondal, Martyna Chruslinska, Paweł Drozda, Davide Gerosa , Zoheyr Doctor, Mirek Giersz, Sylvia Ekström, Cyril Georgy, Abbas Askar, Vishal Baibhav, Daniel Wysocki, T. Natan, Will M. Farr, Grzegorz Wiktorowicz, M. Coleman Miller, Ben Farr, Jean-Pierre Lasota.
Astronomy & Astrophysics, in press.
arXiv:1706.07053 [astro-ph.HE].

Part of our series of spin precession papers, here we study nutational resonances. Those are configurations where the precession of L about J, and that of the two spins are in resonance with each other. These configurations are very generic (virtually every binary will go through resonances), but their effect on the dynamics seems to be small, unless… unless you end up in transitional precession! Transitional precession (great paper!) turns out to be a very special nutational resonance.

Xinyu Zhao, Michael Kesden, Davide Gerosa.
Physical Review D 96 (2017) 024007.
arXiv:1705.02369 [gr-qc].

Black-hole data can be used to probe the lives of stars. That’s the promise of gravitational-wave astronomy, right? Here we give it a go. We present a (admittedly) very simple model showing that the misalignment of GW151226 can be easily explained with large natal kicks. I like simple things…

Richard O’Shaughnessy, Davide Gerosa , Daniel Wysocki.
Physical Review Letters 119 (2017) 011101.
arXiv:1704.03879 [astro-ph.HE].
Press release : Rochester Institute of Technology, Caltech’s tweet.
Editor’s coverage in physics.aps.org.
Other press coverage: IOP’s physicsworld.com, Science Daily, Phys.org, astronomy.com, sciencenews, iflscience.

My little latex project to compile bibliographies in a smart way was published by JOSS. I really liked JOSS: it’s an innovative way to get recognition for your carefully crafted software, encouraging open science and good code practice. It’s really about publishing your code, not a paper that describes the code: they peer-review the repository, openly with pull requests.

Davide Gerosa , Michele Vallisneri.
The Journal of Open Source Software 2 (2017) 13.
Open-source code: homepage, repository.

What if the black holes LIGO sees are the results of a merger? I mean, we see mergers, but maybe those are second-generation ones, and the two merging black holes come from first-generation mergers. Or (more likely…) stellar mass black holes form from stars and only merge once…

Davide Gerosa , Emanuele Berti.
Physical Review D 95 (2017) 124046.
arXiv:1703.06223 [gr-qc].
Selected as PRD Editors’ Suggestion.
Other press coverage: Ars Technica.

Equal-mass binaries correspond to a discontinuous limit in the spin precession equations. A new constant of motion pops up, which can be exploited to study the dynamics. This is a really neat calculation done with Jakub, a Cambridge undergraduate student. Also, my first paper at Caltech!

Davide Gerosa , Ulrich Sperhake, Jakub Vošmera.
Classical and Quantum Gravity 34 (2017) 6 ,064004.
arXiv:1612.05263 [gr-qc].

Equal-mass binaries correspond to a discontinuous limit in the spin precession equations. A new constant of motion pops up, which can be exploited to study the dynamics. This is a really neat calculation done with Jakub, a Cambridge undergraduate student. Also, my first paper at Caltech!

Davide Gerosa , Ulrich Sperhake, Jakub Vošmera.
Classical and Quantum Gravity 34 (2017) 6 ,064004.
arXiv:1612.05263 [gr-qc].

Today I’ve defended my PhD at the University of Cambridge! My thesis is titled “Source modelling at the dawn of gravitational-wave astronomy” (with two l’s, British spelling here!). Huge thanks Uli and everyone else for the journey!

Black hole kicks are cool: powerful (up to thousands of km/s!) recoils that black holes receive following a merger. Here we show these events might leave an imprint on the emitted gravitational waves, which is potentially detectable by future interferometers.

Davide Gerosa , Christopher J. Moore.
Physical Review Letters 117 (2016) 011101.
arXiv:1606.04226 [gr-qc].
Selected as PRL Editors’ Suggestion.
Press release : Cambridge University, Cambridge Center for Theoretical Cosmology
Other press coverage: astrobites, particlebites, Daily Mail, phys.org, Particle Bites, egno.gr, Daily Galaxy, Register, Media INAF, IneffableIsland, AstronomyNow, Accademia delle Stelle, noticiasdelaciencia, Cambridge TV.

Here we present my numerical code precession, which implements our multi-timescale way to look at spinning black-hole binaries. The paper has a detailed description of the various functions as well as lots of examples.

Update : typos in Eq. (36-37) have been fixed in v3 on the arXiv.

Davide Gerosa , Michael Kesden.
Physical Review D 93 (2016) 124066.
arXiv:1605.01067 [astro-ph.HE].
Open-source code: homepage, repository, documentation.

Here we present 1+1 numerical-relativity simulation of stellar collapse in scalar-tensor theories, where gravity is mediated by the usual metric coupled to an additional scalar field. Bottom line: you can test General Relativity with supernovae explosions!

Davide Gerosa , Ulrich Sperhake, Christian D. Ott.
Classical and Quantum Gravity 33 (2016) 13 , 135002.
arXiv:1602.06952 [gr-qc].
Supporting material available here.

This is a follow up of arXiv:1403.7147, just done better. Instead of overlaps, we do real injections in LIGO parameter-estimation codes to show that spin-orbit resonances are indeed detectable.

Daniele Trifirò, Richard O’Shaughnessy, Davide Gerosa , Emanuele Berti, Michael Kesden, Tyson Littenberg, Ulrich Sperhake.
Physical Review D 93 (2016) 044071.
arXiv:1507.05587 [gr-qc].

I wrote a post for The Birth of an Idea, which is a really beautiful blog collecting insights on how scientists start their science. Thanks Vitor for the opportunity to contribute! Here is my post:

An idea, a good one at least, is like a gift. It’s something which is not yours (indeed, you didn’t have it before!) but comes to you, it’s given to you.

I bike to work, it’s kind of ten minutes from my place to the Cambridge Maths department, but those ten minutes can be more productive than ten hours or ten days in front of my computer’s screen. It’s morning, your mind should be clear (you should pay attention to cars while biking!), but it’s actually already getting full of what you have to do today. You get to the office, sit down, turn your computer on, and start looking at your problem. You write the equations down, try putting them in a computer, it doesn’t work, just nans coming out. You ask a collaborator who hopefully knows something, write the equations down again, it doesn’t work. You check in a paper if someone else did something similar, take a break, get annoyed (and here I typically open football websites…). Oh, and you write the same equations down again, it simply doesn’t work.

At some stage, it’s time to go home, and that moment is precious to me. You know your problem so well, those equations, that crashing piece of code, but you were looking too close. When I close my laptop and get on my way home, fresh air on my face, I can look at the problem from afar. It’s like looking at those beautiful ancient mosaics. If you look very close, you only see one colored piece, but you can’t see any meaning in it. Each piece is crucial to the final piece of art, but the value of each piece is its relation to the bigger picture. You can only appreciate a mosaic if you take one step back and look to the whole picture from afar. Wow. Biking home is my step back. You’ve been looking at all pieces for days, weeks, you know the color of each piece so well that you can finally grasp the relation which puts them together.

An idea, a good one at least, is like a gift you can say thanks for.

Here we study the stability of black-hole binaries with spins (anti)aligned with the orbital angular momentum. Aligned configurations are points of equilibrium, but are they stable? If the heavier black-hole is aligned and the lighter one is anti-aligned, this turns out to be unstable! And the onset of this instability can be in the LIGO or LISA band!

Davide Gerosa , Michael Kesden, Richard O’Shaughnessy, Antoine Klein, Emanuele Berti, Ulrich Sperhake, Daniele Trifirò.
Physical Review Letters 115 (2015) 141102.
arXiv:1506.09116 [gr-qc].
Selected as PRL Editors’ Suggestion.
Supporting material available here.

Detailed analysis of 2PN black-hole binary spin precession using multi-timescale methods. Follow-up of the Letter arXiv:1411.0674, this paper contains the full calculation and the description of the underlying phenomenology.

Davide Gerosa , Michael Kesden, Ulrich Sperhake, Emanuele Berti, Richard O’Shaughnessy.
Physical Review D 92 (2015) 064016.
arXiv:1506.03492 [gr-qc].
Supporting material available here.

What happens if you throw a scalar field into General Relativity? And if you throw more than one? Here is a paper on the phenomenology of neutron stars in theories with more than one scalar field coupled to gravity.

Michael Horbatsch, Hector O. Silva, Davide Gerosa , Paolo Pani, Emanuele Berti, Leonardo Gualtieri, Ulrich Sperhake.
Classical and Quantum Gravity 32 (2015) 20, 204001.
arXiv:1505.07462 [gr-qc].
Featured in CQG+. Selected as IOPselect.

Supermassive black holes in binaries and their accretion discs… Spins align on some timescale, but migration also takes place. Do gas discs have enough time to align the spins? Well, the secret is the mass ratio: light secondaries might prevent primaries from aligning. A great collaboration between gravitational-wave and planet researchers!

Davide Gerosa , Benedetta Veronesi, Giuseppe Lodato, Giovanni Rosotti.
Monthly Notices of the Royal Astronomical Society 451 (2015) 3941-3954.
arXiv:1503.06807 [astro-ph.GA].

A massive review on testing gravity, which came out of a nice meeting we had at University of Mississippi in January 2014. See the numbers.

Emanuele Berti, et al. (53 authors incl. Davide Gerosa).
Classical and Quantum Gravity 32 (2015) 24, 243001.
arXiv:1501.07274 [gr-qc].

2PN black-hole binary spin precession works exactly like Kepler’s two-body problem. Not kidding: just define effective potentials and divide the phase space into morphologies. The only things you need are a few timescales to play with.

Michael Kesden, Davide Gerosa , Richard O’Shaughnessy, Emanuele Berti, Ulrich Sperhake.
Physical Review Letters 114 (2015) 081103.
arXiv:1411.0674 [gr-qc].
Press release : Cambridge University, Cambridge Center for Theoretical Cosmology, Ole Miss, UT Dallas.
Other press coverage: Science Daily, phys.org, phys.org (2), Media INAF, Astroblogs, RIA, Daily News, Science World Report, Tech Times, Tech Times (2), SpaceRef, Space Daily, ECN, R&D, Daily Galaxy, scitechdaily, nanowerk.
Supporting material available here.

These are my proceedings of the 3rd Session of the Sant Cugat Forum on Astrophysics, held in beautiful Sant Cugat, near Barcelona on April 22-25, 2014. My contribution is this paper: have a look if you want to know more about rival families in spinning black-hole binaries.

Davide Gerosa.
Astrophysics and Space Science Proceedings 40 (2015) 137-145.

Black-hole kicks are powerful. I mean, really powerful. They can even eject supermassive black holes from the heavier galaxies in our Universe. And then these galaxies are left “empty”…

Davide Gerosa , Alberto Sesana.
Monthly Notices of the Royal Astronomical Society 446 (2015) 38-55.
arXiv:1405.2072 [astro-ph.GA].

Spinning black-hole binaries might belong to two spin-orbit resonances, or families. Can you tell them apart using gravitational-wave observations? Spoiler: yes!

Bonus note: check out the title in v1 on the arxiv…

Davide Gerosa , Richard O’Shaughnessy, Michael Kesden, Emanuele Berti, Ulrich Sperhake.
Physical Review D 89 (2014) 124025.
arXiv:1403.7147 [gr-qc].

This week we’re all at the Gran Sasso Science Institute (GSSI) in beautiful L’Aquila for the second edition of our joint workshop with the local GW group. Thanks for having us!

davidegerosa.com/thuram

(If you’re asking, the title of the workshop is a totally legit acronym that just happens to make up the name of FC Inter’s striker… So weird, it happened last year already, I really don’t know how.)

The Italian Society of General Relativity and Gravitation (SIGRAV) announces the 26th SIGRAV Conference, hosted by the University of Milano-Bicocca, to be held in Milan, Italy, from September 8-12, 2025.

https://sites.google.com/unimib.it/sigrav2025

The conference will cover various aspects of classical and quantum gravity, including tests of General Relativity, cosmology, gravity experiments, and gravitational waves from experimental, theoretical, and data-analysis perspectives.

Participation is open to SIGRAV members and non-members alike, both nationally and internationally. The program will feature a series of broad review talks on various aspects of gravitational physics, as well as contributed talks. The SIGRAV Amaldi medals, the SIGRAV prizes for young researchers, and the Giulio Rampa PhD thesis prize will be awarded during the conference. There will also be a public event dedicated to the 10-year anniversary of the first direct detection of gravitational waves, GW150914.

Abstracts for contributed talks should submitted by May 31, 2025. We aim to announce the full conference program by the end of June. Registrations will be accepted until July 15, 2025.

Milan is a beautiful, international city in the north of Italy and is served by three major airports with worldwide connections. The city is home to art, history, and great food; you can also explore nearby lakes or venture into the stunning Alps.

Together with Ilya Mandel, last week I organized a workshop titled “Gravitational-wave snowballs, populations, and models” in Sexten (Italy). Both the science and the scenery were just stellar! We had almost zero talks, and the entire conference was made of brainstorming sessions on three topics “Parametrization”, “Correlation,” and “Falsification.” There are already several emails circulating with several paper ideas coming out of it. Huge thanks to all those who led and participated in the discussions. Here is the conference website…

https://sites.google.com/unimib.it/gwsnowballs

… and here is us! We should definitely do it again. And remember: if you run population synthesis once, you shall be cursed forever.

The workshop “ Challenges and future perspectives in gravitational-wave astronomy: O4 and beyond ” will take place at the Lorentz Center (Leiden, Netherlands) from October 14th to October 18th, 2024.

Our goal is to foster an interdisciplinary discussion (with astrophysicists, data analysts, and machine learners) about how current and future observations of gravitational and electromagnetic waves can be used to shed light on the physics of compact-object formation and evolution.

We encourage interested participants to apply by July 21st, 2024 at:
https://www.lorentzcenter.nl/challenges-and-future-perspectives-in-gravitational-wave-astronomy-o4-and-beyond.html

Lorentz Workshops@Oort are scientific meetings for small groups of up to 55 participants, including both senior and junior scientists. We will dedicate a considerable amount of time to discussion sessions, thus stimulating an interactive atmosphere and encouraging collaboration between participants. The venue Lorentz Center@Oort is located at the Faculty of Science campus of Leiden University, the Netherlands. The Lorentz Center provides each participant with office space as well as various practical services such as arranging accommodations at the nearby hotel Van der Valk Hotel Leiden/Tulip Inn Leiden at a special rate, visa assistance, and bike rental. For more information see: www.lorentzcenter.nl

SOC : Fabio Antonini (chair), Maya Fishbach, Davide Gerosa, Laura Nuttall, Rosalba Perna, Simon Portegies Zwart.

We are organizing “Gravitational-wave snowballs, populations, and models” — a workshop to be held in Sexten, in the Dolomites region of Italy, January 20-24, 2025:
https://sites.google.com/unimib.it/gwsnowballs

Our goal is to bring together researchers at the forefront of both forward astrophysical modeling of compact object binary formation and gravitational-wave data analysis in preparation for the upcoming O4 data release of LIGO/Virgo, for discussions focused on population-level modeling and inference.

The meeting will be held at Bad Moos Hotel right next to the ski slopes and the conference program will have appropriate breaks for snow activities; more details are available at
https://sites.google.com/unimib.it/gwsnowballs/logistics

We hope you will consider applying to participate. Space is limited to 40 people. Please apply online at
https://sites.google.com/unimib.it/gwsnowballs/registration
by July 15, 2024. We plan to notify accepted participants by the end of July.

Ilya Mandel
Davide Gerosa
Salvatore Vitale

This week we’re hosting researchers from the Gran Sasso Science Institute (GSSI) for a joint mini-conference / workshop / group meeting. More here:

davidegerosa.com/lautaro

This is part of a PRIN grant we have together (thanks Italy) with support from other grants as well (thanks Europe). The meeting has the best title ever (that was actually my idea…), the best logo ever (that was Giulia’s idea), and the best organization ever (huge thanks Costantino and Sara!).

Last week my group and I hosted the international workshop “Gravitational-wave populations: what’s next?.” It’s been a blast!

An unconventional conference, with almost zero talks and the vast majority of the time dedicated to discussions. I report the program here below, just to give you a feeling of what we discussed. The conference started with the question “ How many of you entered the field after GW150914? ” and virtually everyone raised their hand! It was so refreshing to see our field is alive.

We then went through population synthesis simulations, fancy statistical methods (I promise I’ll understand nonparametric methods one day!), intricacies of injections, catalogs, and overlap with our EM observer friends. We took a break on Wednesday for a social activity on Lake Como, with some folks diving into the lake and others hiking up to a small castle. All before dinner with a fascinating lake (and thunderstorm!) view.

Thanks all for joining and participating so actively. Huge thanks to Emanuele Berti and Salvo Vitale for co-organizing this with me, as well as the local GW group for assistance. Finally, congrats to Amanda Farah and Alex Criswell who won our SIGRAV early career prize.

And if you couldn’t make it for whatever reason no worries, we’ll do it again!

Conference program in a nutshell. These are our discussion sessions

The advanced class “Big data within science and industry” will take place on September 22nd at the University of Milano-Bicocca (Milan, Italy).

https://sites.google.com/unimib.it/bigdatamasterclass

Data are everywhere. Exploring scientific data is now at the heart of both scientific advances as well as industrial applications. This one-day master class provides a “learn by example” introduction to the fascinating world of big data, namely pieces of information that are so rich and structured that require targeted analysis techniques loosely referred to as machine learning or artificial intelligence.

The class is suitable for advanced MSc students, PhD students, and postdocs who wish to expand their proficiency in handling scientific data. The program features the participation of three world-leading experts from both academia and the private sector, as well as a hands-on experience for all participants.

For students enrolled in the Physics and Astronomy PhD program here at Milano-Bicocca, this 8-hour program will be recognized with 1 CFU. In any case, we are happy to provide attendance certificates.

Interested students should register by ** September 8th, 2023**. Participation is free of charge. We hope to accommodate everyone, but depending on the number of people registering, participants might need to be selected.

Davide Gerosa, Michele Fumagalli (Milano-Bicocca)

It is a pleasure to announce the workshop “Gravitational-wave populations: what’s next?” which we are currently organizing for next summer:

https://sites.google.com/unimib.it/gwpopnext

As the catalog of detected gravitational-wave events grows from O(10) to O(100) sources (but think millions in a few decades!), such increasingly detailed information is allowing us to dig deeper into the (astro)physics of compact objects. At the same time, new and more data require appropriately powerful statistical tools to be fully exploited. This highly interactive workshop (fewer talks, more working together!) will be the opportunity to share recent progress, identify what new steps are now needed, and hopefully set the stage for substantial progress in the field.

The workshop will take place on July 10-14, 2023 at the University of Milano-Bicocca, which is located near the city center of Milan, Italy. Milan is a beautiful, international city in the north of Italy and is served by three major airports with worldwide connections. The city is home to art, history, and great food; nearby excursions will take you to the Italian lakes and the stunning Alps.

While we are unable to provide travel support, the workshop will have no registration fee. The workshop will be in person without remote options.

Interested participants should register on the conference website by March 1st, 2023. Depending on the number of people registering, participants might need to be selected. We will be in touch soon after the registration deadline, so please do not make travel plans until you hear back from us. When registering please indicate which of the discussion session(s) you would like to contribute to. Early career scientists will have the opportunity to give flash talks highlighting their science.

Davide Gerosa (Milano-Bicocca), Emanuele Berti (Johns Hopkins), Salvatore Vitale (MIT)

We’re at Johns Hopkins University (Baltimore) today, for a brainstorming workshop we organized together with the gravity groups at JHU and Penn State. A ton of interesting people, cool science, fun numerics, big black holes, future detectors, and many new exciting projects we all want to start. The idea is to get “a little help from my (gravity) friends”. Have a look at what we’re up to: davidegerosa.com/with-a-little-help-from-my-friends-workshop/

This is a quick update some of our group activities… In the past few months we’ve been busy learning about the formation of stellar-mass black-hole binaries in the disks of active galactic nuclei. We organized a journal club and studied one paper each week on this “new” formation channel for LIGO sources. We discussed a ton of topics, going from disk accretion to migration traps, LIGO rates, AGN variability, GW counterparts, hierarchical mergers, all the way to EMRIs.

Here is a log of all the sessions: davidegerosa.com/bhbin-agndisks

Let me thanks all those who took part and presented papers including Daria, Matt (1), Chris, Eliot, Matt (2), Alberto, Evan, Riccardo, and Sean.

We are running a virtual workshop with my group (Bham) and Emanuele Berti’s group at Johns Hopkins University (Hop). It’s an attempt to feel a bit less lonely during the COVID pandemic. Hope this is the opportunity to start new projects! And we’re a funny crowd…

For more: davidegerosa.com/hopbham-workshop/

This week I am organizing the GrEAT PhD winter school. GrEAT (which stands for Gravitational-wave Excellence through Alliance Training) is a synergy network between the UK and China. Our program features informal talks in the mornings and hands-on sessions in the afternoons, covering both theoretical and experimental gravitational-wave physics.

After the school in Birmingham, students will move on to various UK nodes to complete longer projects. In particular, Mingyue Zhou will stay here working with me.

Happy to report about the great success of our workshop ”Numerical Relativity beyond General Relativity”. This was organized by me, Helvi Witek, and Leo Stein at the Benasque physics center (Spain), in the beautiful region of the Pyrenees, on June 3-9, 2018. Was great to see world-leading experts from so many different fields (numerical relativity, gravitational-wave data analysis, self-force, theoretical physics, cosmology, etc) interacting and reporting their progress on innovative uses of computational techniques in gravitation. Here are the conference program and (some of) the talk’s slides.

I only wish the rain would have stopped for more than a few hours over the entire week. This is us with Einstein; we’re all beyond!

The 34th edition of the Pacific Coast Gravity Meeting, sponsored by the APS, was held at Caltech on March 16-17, 2018. This year’ edition was organized by me, Leo Stein and a few others, and was dedicated to Jim Isenberg who first started the Pacific Gravity meetings 34 years ago. We had a beautiful blend of people (including some very talented undergrads!) and topics (from numerical relativity, to quantum gravity, high-energy physics and gravitational-wave astronomy). I hope everybody had fun. I surely did!

Here is the conference program, and this below is the logo that I designed (It’s supposed to be Newton’s apple with some gravitational waves in Caltech’s orange color; I know, I’m a scientist, not an artist…). And congrats to Maria Okounkova who won the best student talk award of the APS.

Our workshop “The disc migration issue: from protoplanets to supermassive black holes” took place in May (2017) at the Cambridge Institute of Astronomy. Chaired by Cathie Clarke and co-organized by me, Giovanni Rosotti and a few other people, we tried to bring together people working on both planetary and black-hole physics, to understand what we have in common… Much like planets migrate in protoplanetary discs, supermassive black holes are also brought together by gas interactions. Same physics, different scales, right?

Here is the conference program (with some of the talk’s slides) and below is our beautiful logo (there are discs, waves, inspirals, and King’s College!). Thanks to the KAVLI and Templeton foundations for making this possible.

LISA is going to be cool. And not just for your astro-related dreams. Theoretical physicists can have fun too! This community-wide manifesto illustrates just how cool things are going to be with LISA. LISA will constitute a major milestone to test gravity, cosmology, the nature of black holes, etc. A big thanks to all those involved.

Enrico Barausse, et al. (322 authors incl. Davide Gerosa).
General Relativity and Gravitation 52 (2020) 8, 81.
arXiv:2001.09793 [gr-qc].

Four BSc+MSc students just graduated with projects in our group!

… congrats all!

I’m so so proud to see my PhD student Viola De Renzis defending her PhD thesis today. Viola’s thesis is titled “Gravitational-wave astronomy at the crossroads: from current to future detectors, from single events to populations” and was examined by Maya Fishbach (Toronto), Laura Sberna (Nottingham) as external referees, as well as Walter Del Pozzo (Pisa), Stephen Green (Nottingham) and Alberto Sesana (Milano-Bicocca) as defense committee members. What should I say, from the first “off you go and learn Bilby” meeting we had, to all those discussions at the board, learning how to ski, those codes that did (not) work, and that distinctive laughter across the corridor. Our group will not be the same without Viola. You turned into a great scientist: now “spacca tutti” in Marseille!

That’s me, Steve, Walter, Viola, and Alberto…

Four BSc students and one MSc student defended their research project with us this month.

Thanks all for spending some time in our research group!

Four students just graduated with projects in our group…

First, huge congrats to Cecilia Fabbri who got her MSc in Astrophysics. Cecilia (you might remember her) worked on an exciting applied statistics problem (which has already ended up in a poem, but soon in a paper). Her problem got like 10 more people hooked beside us, so we really have to finish it now! From my side, it’s always amazing to see scientists like her growing so much. Cecilia be moving on with a PhD in Nottingham (UK) with Steve Green (and when you come back to visit you’ll tell me everything I don’t understand about simulation-based inference!). Good luck!

We also supervised three BSc students who defended their short projects:

Congrats all, Spritz time now.

Usually my students graduate in Physics, but not this time… Together with Matteo Boschini, I had the pleasure of supervising a student majoring in Computer Science. Alessandro Crespi got his BSc degree with a project on Simulation Design, which is really a computing thing but has lots of physics applications. That was so much fun! It is truly true that putting different expertise/approaches/ideas makes things better.

Daria Gangardt has just defended her PhD thesis at the University of Birmingham. The thesis is called “Black-hole dynamics and their environments” and jumps from black-hole spins all the way to AGN discs. Daria, it has been a true pleasure working with you, all the way since your very first summer project and through your supervisor changing countries. I’m both honored and proud that you completed your PhD with me, all the best with everything. Time for drinks now! Go Dr. Daria!

Three more students graduated in March with research projects completed in our group!

Our student Lisa Merlo defended her BSc 3rd year project today! Lisa worked with Pippa Cole and me on computing rates for mergers of primordial black holes, also considering a new detector prototype that the experimental group here is developing (nickname BAUSCIA, from the Milan dialect). Short answer: the rate is low but now is more accurately low. Lisa’s presentation was amazing and working with her has been a real pleasure. Stay tuned for her future astro career!

We had another graduation session in November, and a whopping 4 people graduated with research projects in our group. Here are the new BSc physicists who just defended:

Congrats all (and twice congrats to Marco and Serena, who graduated with full marks and honors). It was great working with you. Matteo and Martin are now enrolled in an MSc degree in Artificial Intelligence (good luck!), while Marco and Serena are starting our MSc degree in Astrophysics.

We’ve had four amazing research students graduating with us in October!

After the Master’s defenses, students turned the graduation party into a football supporter thing, with chants and all the rest!

Three of our BSc students graduated today.

And, last but not least, let me add Simone Piscitelli, who last week defended his MSc degree at Milano Statale (“the other” University of Milan) supervised by Costantino Pacilio and myself. Simone worked on a cool test of GR. Stay tuned…

Congrats all!

Two students just completed their Bachelor’s degree with research projects in our group.

I had the honor of heading their graduation committee and could call them “physicists” for the very first time (and the Italian ceremonial sentence is quite imposing: “ coi poteri conferitami… “). Congrats Simone and Leonardo!

Please let me introduce Dr Matthew Mould… After N papers (where N is a lot) and a 4h+15min viva discussion, Matt has completed his PhD in gravitational-wave astronomy at the University of Birmingham. WooooO! The examiners were Annelies Mortier from Birmingham and Uli Sperhake from Cambridge, who went through a thesis with more than 600 references…. Matt will be continuing his already successful career with a postdoc at MIT, LIGO lab. From my side, Matt is (actually, was!) my first PhD student and spending 3+ years working with him has been amazing. Thanks, Matt for teaching me Bayesian stats and never letting go when I was saying crap.

It’s student time! Massive congratulations to two of my students who just graduated.

The star of the day is Matteo Boschini, who completed his MSc project with me after a long visit at the AEI (Postdam, Germany) to collaborate with Vijay Varma. Matteo worked out an amazing extension of current numerical-relativity surrogate models… stay tuned for a paper because this is going to be cool!

Daniele Chirico completed his BSc studies with a sweet research project on supernova explosions, orbits, and kicks. He’s staying in Milan for his MSc degree now, so wait a bit for his successes!

Huge congrats to two of my students who graduated today! Matteo Muriano completed a funny BSc project on black-hole merger trees. And Giovanni Cavallotto went all in for his MSc research: he basically “fixed” black-hole binary spin precession at 2PN! (which is pretty cool, stay tuned for these results!). They both defended quite brilliantly, good luck with everything now!

More graduations today! I had the pleasure to see three of my students defending their scientific work. Lorenzo Zanga completed his BSc project on unstable spinning black-hole binaries, Alessandro Carzaniga defended his MSc thesis on gaussianities in the LISA detector, and Alice Spadaro also presented her MSc-thesis work on the LISA mock data challenge. It’s so great to see students reaching the point of defending/arguing/explaining their science… I think it’s actually one of the best things about my job! Thank you all for sharing these months with me, I’ll see you around! (And thanks to Viola De Renzis and Riccardo Buscicchio who co-supervised Lorenzo, Alessandro, and Alice with me).

Wooo! What an amazing performance by two of my students today, who defended their BSc and MSc degrees! Oliver Rossi discussed his BSc project on black holes with large spins completed in collaboration with Viola De Renzis (PhD student in my group). Andrea Geminardi presented the results of his MSc thesis. Andrea studied the stochastic gravitational-wave background with myself, Riccardo Buscicchio (postdoc here in Milan), and Arianna Renzini (postdoc at Caltech). Hope you guys had fun working with us, we certainly did! (and I’m sorry for my pain-in-the-*** comments on your plots…). All the best for what comes next!

Huge congrats to my student Cecilia Fabbri who got her Bachelor’s degree today. Cecilia defended (quite brilliantly!) her project titled “Constraining the black-hole irreducible mass with current gravitational-wave data”. Her work ended up in our recent draft (arxiv:2202.08848). Cecilia is continuing with a Master’s degree in astrophysics at Milano-Bicocca, stay tuned for her future successes!

Huge congrats to Maciej (Max) Dabrowny, who just graduated from the University of Birmingham after a very successful research project with us (Max’s project ended up in a paper!). Well done and all the best for the future.

Congratulations to my Master’s students that graduate this year: **Abdullah Aziz** and Julian Chan from the University of Birmingham, and Beatrice Basset from the University of Lyon. Well done all, and good luck with your future adventures.

Congrats to Alex Toubiana, postdoc with us, who was just awarded an independent fellowship from the Italian Research Ministry. The scheme is called Young Researcher 2024 and will fund Alex and his research for 3 years.

Very happy to report that Arianna Renzini (currently a postdoc at Caltech) was awarded a prestigious Marie Skłodowska-Curie Fellowship from the European Union, to be hosted here with my group. Arianna will bring expertise in modeling the gravitational-wave stochastic background, which is a key target for both current and future experiments. Arianna’s proposal is titled “ Stochastic rewind and fast-forward: calibrating LISA with LIGO’s black holes and stochastic background.” Huge congrats, can’t wait to welcome you here.

The University of Milano-Bicocca (Milan, Italy) invites applications for a tenure-track professorship in Astrophysics.

Milano-Bicocca hosts a vibrant astrophysics group consisting of 11 faculty members, approximately 25 postdocs, and around 15 PhD students. The group has a strong track record of securing national and international funding, with 6 recently awarded ERC grants. We are part of a larger physics department with about 70 faculty members and are situated on a dynamic campus with 40,000 students. Milan is a modern, international city in northern Italy, close to the stunning Alps, offering a lively cultural scene, excellent food, and a high quality of life.

Current interests of the group include gravitational-wave astronomy, formation and evolution of cosmic structures, and experimental cosmology. At the same time, we are open to all strong candidates willing to bring their ambitious research programs in astrophysics to Milan.

The position will be at the assistant professor level (“RTT” in the Italian system), a tenure-track appointment with a well-defined path to tenure within either three or six years, depending on performance. The anticipated start date is fall 2025, though this is negotiable. Responsibilities include conducting research at the highest international standards, teaching BSc and MSc courses, mentoring students, and securing external funding.

Interested candidates are invited to apply by June 12th, 2025:
https://www.unimib.it/ateneo/gare-e-concorsi/2025-rtt-027-dipartimento-fisica-g-occhialinigsd-02phys-05-ssd-phys-05a

Knowledge of the Italian language is not required to apply; the online application portal is available in English. We strive to build a diverse and inclusive environment and welcome applications from traditionally underrepresented groups.

For inquiries, please contact Prof. Michele Fumagalli ([email protected]).

If you’re looking for a PhD in gravitational-wave physics, our 2025 call for PhD scholarships is now available. The procedure is described here (Session 1):

https://en.unimib.it/study/doctoral-research-phd-programmes/applying-doctorate/calls-application

The deadline is April 24th at noon CEST.

For instructions, start from the file “Guide to filling in the online application.” There’s a key step on page 10 where candidates can express interest in some themed scholarships. If you’re interested in working with my group, I encourage you to select PROG.1 and PROG.3.

You will need to submit your research proposal/statement. These are usually 2-3 pages long. It should provide some context about your work in gravitational-wave astronomy (or astrophysics more in general), what you want to do next, your key interests, what you would like to work on here with us, why you want to work with us, and more in general how you plan to integrate with our activities. It should be forward-looking and not just about what you’ve done already. Hope this helps!

For any questions, please do not hesitate to contact me: [email protected]

Happy to share this postdoc opportunity from the Cariplo Foundation, which is a private trust that operates in the Milan area. It’s an independent fellowship for early career researchers, with a duration of 3 years and a total budget of 200k EUR.

https://www.fondazionecariplo.it/static/upload/you/young-researchers-2025.pdf

The deadline is March 24, 2025. If you’re reading this and are interested in applying with us at Milano-Bicocca, please shoot me an email!

FIS (“Fondo Italiano per la Scienza”) is an Italian grant opportunity which is conceptually similar to the ERC. The amount of these grants is >= 1M EUR and grant holders are offered a tenure-track or tenured position. The deadline for this year’s solicitation (FIS 3) is Mar 28, 2025. If you’re interested in applying with Milano-Bicocca as host institution please shoot me an email!

https://www.mur.gov.it/it/atti-e-normativa/decreto-direttoriale-n-1802-del-21-11-2024

The University of Milano-Bicocca welcomes applications for PhD scholarships. This year’s application deadline is May 14th, 2024 (noon CEST) for positions starting in the Fall of 2024:

https://en.unimib.it/education/postgraduates/doctoral-research-phd-programmes/applying-doctorate/calls-application

In particular, we are looking for highly motivated candidates to join our activities in black-hole binary dynamics and gravitational-wave data exploitation. Milano-Bicocca hosts a large group in gravitational-wave physics, covering activities ranging from astrophysical/numerical modeling to data analysis. The group counts 7 faculty members (Bortolas, Colpi, Dotti, Gerosa, Giacomazzo, Sesana, and an upcoming new hire) together with several postdocs (of which two prize fellows) and PhD students. Candidates will also have ample opportunities to work with and visit external collaborators.

Our PhD admission program includes several “open” scholarships, covering all research activities in the department (including ours!). All candidates are considered for those by default. In addition, we are advertising an additional “project” scholarship titled “Gravitational-wave source modeling” which will be supervised by Prof. Davide Gerosa. Candidates wishing to be considered for this opportunity should indicate it explicitly when applying (the number of this position FIS.8). For more information on Gerosa’s group see www.davidegerosa.com/group

We strive to build an inclusive group and welcome applications from all interested candidates. For informal inquiries, expressions of interest, and application tips please do not hesitate to contact [email protected]

The University of Milano-Bicocca (Milan, Italy) will be opening a tenured professorship in astrophysics, with a focus on gravitational-wave data analysis and exploitation. With this notice, we invite expressions of interest from potential candidates.

Milano-Bicocca hosts a large group in gravitational astronomy, with activities covering all bands of the gravitational-wave spectrum and the related experiments (LIGO/Virgo, LISA, ET, PTA). Faculty members with matching interests include Bortolas, Colpi, Dotti, Gerosa, Giacomazzo, and Sesana. The group hosts two large ERC grants and currently counts about 10 PhD students and 15 postdocs. We are part of a wider astrophysics unit at Milano-Bicocca (with activities in large-scale structures and experimental cosmology) as well as a large Physics department with ~70 faculty members.

We are targeting the opening of a faculty position on a timescale of a few months, with a prospective starting date in the early fall of 2024. Onboarding will be at the associate professor level (“professore associato” in the Italian system), which is a tenured appointment. Formal application requirements include holding either the Italian national habilitation (ASN) or a comparable position abroad for at least 3 years. We are happy to assist potential candidates with their ASN application.

Current strategic interests include the development of gravitational-wave data-analysis pipelines for the LISA space mission. At the same time, we are open to all strong candidates willing to bring their ambitious research programs in relativistic astrophysics and/or gravitational-wave astronomy to Milan.

Interested applicants are encouraged to send their CVs and a short cover letter to [email protected] by February 15th, 2024. The CV should include the names and email addresses of three referees who might be approached for references.

The University of Milano-Bicocca (Italy) invites expressions of interest for postdoctoral positions in gravitational-wave astronomy.

Successful candidates will join the group of Prof. Davide Gerosa and will be part of the “GWmining” project funded by the European Research Council, with additional support from national grants. Targeted investigations focus on the astrophysical exploitation of gravitational-wave data. We are particularly interested in candidates with expertise in population-synthesis simulations of compact binaries, gravitational-wave parameter estimation and population studies, as well as applications of statistical and machine-learning tools to gravity (although we are open to all candidates with a strong gravitational-wave and/or high-energy astrophysics background!). Candidates will have ample opportunities to kickstart new projects with group members and will be strongly encouraged to develop their own independent research lines.

We anticipate awarding up to three positions. Appointments will be for 2+1 years and come with a generous research and travel budget. The starting date is negotiable.

The astrophysics unit at Milano-Bicocca provides a vibrant environment with expertise covering all aspects of gravitational-wave astronomy, relativistic astrophysics, and numerical relativity, as well as a wider astronomical context including observational and experimental activities. The group has tight connections with the LISA Consortium, the Virgo Collaboration, the Einstein Telescope Observational Science Board, the Italian National Institute for Nuclear Physics (INFN), and the Italian Center for Supercomputing (ICSC). Faculty members with matching interests include Gerosa, Sesana, Colpi, Giacomazzo, and Dotti. For more information on Gerosa’s group see https://davidegerosa.com/group

Milan is a beautiful, international city in the north of Italy with history, art, and outstanding food. Mountains and lakes are just around the corner.

Successful candidates will have a PhD in Physics or related discipline, strong programming skills, and previous experience in gravitational (astro)physics. Applications should include a CV with a list of publications and a two-page statement covering research interests and plans. These should be sent by November 15th, 2023 using this web form:

https://forms.gle/hnQc3N1xh53YAziH9

Candidates should also arrange for at least two, but preferably three, reference letters to be sent using the same form by November 15th, 2023.

We strive to build a diverse and inclusive environment and welcome expressions of interest from traditionally underrepresented groups.

For inquiries please do not hesitate to contact Davide Gerosa at [email protected]

The University of Milano-Bicocca (Milan, Italy) will be hiring a PhD student under the Italian National PhD program in Space Science and Technology. Candidates with interests in gravitational-wave astronomy, data analysis, and multi-messenger applications are encouraged to apply. For information please see:

The application deadline is July 6th, 2023 at 4pm CEST. For informal enquiries please contact Monica Colpi ([email protected]).

The University of Milano-Bicocca (Italy) invites expressions of interest for postdoctoral positions in gravitational-wave astronomy.

Successful candidates will join the group of Prof. Davide Gerosa and will be part of the “GWmining” project funded by the European Research Council. Targeted investigations focus on the astrophysical exploitation of gravitational-wave data. We are particularly interested in candidates with expertise in population-synthesis simulations of compact binaries, gravitational-wave parameter estimation and population studies, and numerical-relativity surrogate modeling (although we are open to all candidates with a strong gravitational-wave and/or high-energy astrophysics background!). Candidates will have ample opportunities to collaborate and kickstart new projects with group members and will be strongly encouraged to develop their own independent collaborations.

We anticipate awarding up to three positions. Appointments will be for a three-year term and come with generous research and travel budget. The starting date is negotiable.

The astrophysics group at Milano-Bicocca provides a vibrant environment with expertise covering all aspects of gravitational-wave astronomy, relativistic astrophysics, and numerical relativity, as well as a wider astronomical context including observational and experimental activities. The group has tight connections with the LISA Consortium, the Virgo Collaboration, the Einstein Telescope Observational Science Board, the Italian National Institute for Nuclear Physics (INFN), and the newly formed Italian Center for Supercomputing (ICSC). Faculty members with matching interests include Gerosa, Sesana, Colpi, Giacomazzo, and Dotti. For more information on Gerosa’s group see https://davidegerosa.com/group

Milan is a beautiful, international city in the north of Italy with history, art, and outstanding food. Mountains and lakes are just around the corner.

Successful candidates will have a PhD in Physics or related discipline, strong programming skills, and previous experience in gravitational (astro)physics. Applications should include a CV with a list of publications and a two-page statement covering research interests and plans. These should be sent by November 18th, 2022 using this web form:

https://forms.gle/hnQc3N1xh53YAziH9

Candidates should also arrange for at least two, but preferably three, reference letters to be sent using the same form by November 18th, 2022.

We strive to build a diverse and inclusive environment and welcome expressions of interest from traditionally underrepresented groups.

For inquiries please do not hesitate to contact Davide Gerosa at [email protected].

The Italian government has pushed a hiring program dedicated to holders and applicants of Marie Curie Fellowships from the EU. The call targets those that have either (i) completed a successful Marie Curie Fellowship in the past 4 years or (ii) applied unsuccessfully in the past 4 years but were awarded the so-called “Seal of Excellence”.

For both categories, successful candidates will be awarded a 3yr senior researcher position (at the so-called RTDA level in the Italian system). RTDAs are hired as full employees with related benefits and have limited teaching duties. On top of this, candidates in the Marie Curie winners strand (i) will also be offered a substantial startup grant to hire their own PhD students and postdocs.

All Italian institutions can act as hosts, so I encourage you to contact one of us in the country for more information.

In particular, the gravitational-wave group at the University of Milano-Bicocca provides a vibrant environment with activities ranging from relativistic astrophysics. gravitational-wave data analysis, numerical relativity, and gravity theory. The group counts faculty members Gerosa, Sesana, Colpi, Giacomazzo, and Dotti as well as tens of students and postdocs. The city of Milan is a jewel in the north of Italy with a charming international vibe (as well as mountains, history, art, and outstanding food).

The internal application deadline is October 18th. If you’re eligible and/or interested in applying with us, please get in touch asap ([email protected]) and we’ll go from there.

Here are the relevant webpages (scroll down for the English text):

(i) Marie Curie past winners

https://www.unimib.it/ricerca/opportunita/finanziamenti-alla-ricerca/finanziamenti-nazionali/bando-giovani-ricercatori-vincitori-msca-young-researchers-msca-grants-winners

(ii) Seal of Excellence holders:

https://www.unimib.it/ricerca/opportunita/finanziamenti-alla-ricerca/finanziamenti-nazionali/bando-giovani-ricercatori-seal-excellence-msca-call-young-researchers-seal-excellence-msca

The University of Milano-Bicocca welcomes applications for Ph.D. scholarships. The application deadline is May 20th, 2022 for positions starting in the Fall of 2022:

https://en.unimib.it/education/postgraduates/doctoral-research-phd-programmes/applying-doctorate/calls-application

In particular, the theoretical astrophysics group is looking for strong, highly motivated candidates to join our activities in black-hole binary dynamics, gravitational-wave data exploitation, and numerical relativity. Faculty members with matching interests include Gerosa, Sesana, Colpi, Dotti, and Giacomazzo. The candidates will have ample opportunities to work with and visit external collaborators as well.

Our PhD admission program includes a number of “open” scholarships, covering all research activities in the department (including ours!). All candidates are considered for those by default. In addition, our group sponsors two specific positions:

Candidates wishing to be considered for these additional positions should mention it explicitly in their application.

More information on the astrophysics group at Bicocca can be found at astro.fisica.unimib.it. For informal inquiries please do not hesitate to contact [email protected] or [email protected].

The University of Milano-Bicocca (Italy) invites expressions of interest for a 3+2 year research position in HPC applications to astrophysics.

The astrophysics group at Milano-Bicocca provides a vibrant environment with expertise covering all aspects of gravitational-wave astronomy, relativistic astrophysics, galactic dynamics, and numerical relativity. This is embedded in a wider astronomical context including both observational and experimental activities. Our group has tight connections with the LISA Consortium, the Virgo Collaboration, the Einstein Telescope Science Board, the European Pulsar Timing Array, and the Italian National Institute for Nuclear Physics (INFN) via the TEONGRAV national initiative. Staff members with matching interests include Colpi, Dotti, Gerosa, Giacomazzo, Lupi, and Sesana.

Milan is a beautiful, international city in the north of Italy. Mountains and lakes are just around the corner. Art, culture, and food are outstanding. The city hosts three international airports with worldwide connections.

This recruitment campaign is part of a wider national initiative supporting HPC-related computational activities throughout the country. This is a major investment program directly supported by the European Union. It will provide the most ideal context for ambitious candidates wishing to develop and apply state-of-the-art computational and machine-learning tools to current astrophysical and gravitational-wave modeling issues.

The researcher will be appointed at the so-called “RTDA” level for 3 years. The contract can also be extended for 2 more years depending on funding availability. The starting date is negotiable, with the earliest and latest dates on January 1st, 2023 and May 1st, 2023, respectively. RTDA researchers are full-time university employees (with full benefits, such as health insurance and pension plan), have limited teaching duties, and are eligible to fully supervise research MSc student projects. This is an ideal setup for early-career researchers wishing to transition toward research independence and start developing their own group.

The successful candidate will have a PhD in Physics, Astronomy, Computer Science, or related discipline, strong programming skills, and previous experience in one or more of the following topics: HPC workflows, GPU software development, computational astrophysics, gravitational-wave astronomy, numerical relativity, statistical data analysis, machine learning.

Applications should include a CV with a list of publications and a two-page statement covering research interests and plans. These should be sent to [email protected] by June 15th, 2022 for full consideration. Candidates should also arrange for two reference letters to be sent to [email protected] by June 15th, 2022.

We strive to build a diverse and inclusive environment and welcome expressions of interest from traditionally underrepresented groups. Women are especially encouraged to apply. For inquiries please do not hesitate to contact Bruno Giacomazzo ([email protected]) or Davide Gerosa ([email protected]).

The University of Milan-Bicocca (Italy) invites expressions of interest for postdoctoral positions in gravitational-wave astronomy.

Successful candidates will join Prof. Davide Gerosa and will constitute the core team of the “GWmining” project funded by the European Research Council. Targeted investigations include applications of machine-learning techniques to gravitational-wave physics, modeling of black-hole binary populations from their stellar progenitors, relativistic dynamics, and statistical inference. Candidates will have ample opportunities to explore other areas of gravitational-wave astronomy and will be encouraged to develop independent collaborations.

We anticipate awarding two positions. Appointments will be for a three-year term and come with generous research and travel budget. The starting date is negotiable.

The astrophysics group at Milan-Bicocca provides a vibrant environment with expertise covering all aspects of gravitational-wave astronomy, relativistic astrophysics, and numerical relativity, as well as a wider astronomical context including observational and experimental activities. The group has tight connections with the LISA Consortium, the Virgo Collaboration, and the Italian National Institute for Nuclear Physics (INFN) via the TEONGRAV national initiative. Faculty members with matching interests include Gerosa, Sesana, Colpi, Giacomazzo, and Dotti.

Milan is a beautiful, international city in the north of Italy. Mountains and lakes are just around the corner.

Successful candidates will have a PhD in Physics or related discipline, strong programming skills, and previous experience in one or more of the following topics: gravitational-wave astronomy, stellar evolution, relativistic astrophysics, general relativity, machine learning, statistical inference.

Applications should include a CV with a list of publications and a two-page statement covering research interests and plans. These should be sent to [email protected] by December 1st, 2021 for full consideration. Candidates should also arrange for at least two, but preferably three, reference letters to be sent to the same address by December 1st, 2021. We strive to build a diverse and inclusive environment and welcome expressions of interest from traditionally underrepresented groups.

For inquiries please do not hesitate to contact Davide Gerosa at [email protected].

The University of Milano-Bicocca welcomes applications for Ph.D. scholarships. The application deadline is June 16th, 2021 for positions to start later in 2021:

https://en.unimib.it/education/doctoral-research-phd-programmes/how-apply-phd-programme

In particular, I am looking for a strong, highly motivated candidate to join my newly established research group supported by the European Research Council. The candidate will work toward interpreting the phenomenology and the astrophysics of gravitational-wave sources using innovative machine-learning techniques. My activities are embedded within the wider Astrophysics group at the University of Milano-Bicocca –a world-leading research environment in strong gravity and relativistic astrophysics. Faculty members with matching interests include Colpi, Sesana, Dotti, and Giacomazzo. The candidate will have ample opportunities to work with and visit external collaborators as well.

This specific position is titled “Large catalogs of gravitational-wave events with machine learning”. Interested candidates should mention it explicitly in their application.

Milan is a beautiful, international city in the north of Italy. Mountains and lakes are just around the corner. For further information and informal inquiries please do not hesitate to contact me ([email protected]).

The Institute for Gravitational Wave Astronomy at the University of Birmingham, UK, invites applications for postdoctoral positions.

The Institute provides a vibrant and diverse environment with expertise covering theoretical and experimental gravitational-wave research, with applications to present and future-generation detectors, theoretical astrophysics, transient astronomy, gravitational-wave source modeling, and general relativity theory. Applications from top researchers in all areas related to gravitational-wave and transient astronomy are encouraged.

Institute faculty members include Andreas Freise, Davide Gerosa, Denis Martynov, Haixing Miao, Christopher Moore, Conor Mow-Lowry, Matt Nicholl, Patricia Schmidt, Silvia Toonen, and Alberto Vecchio.

One postdoctoral appointment is funded by the UK Leverhulme Trust (PI Dr. Davide Gerosa) and is focused on developing astrophysical and statistical predictions for the LISA space mission. The successful candidate will have ample opportunities to explore other areas of gravitational-wave astronomy as well.

Appointments will be for a three-year term starting in the Fall of 2020 and come with generous research and travel budget.

Applications should include a CV with a list of publications, and a two-page statement covering research interests and plans. Complete applications should be received by 27 January 2020 for full consideration. Applications should be sent to Ms. Joanne Cox at: [email protected].
Applicants should also arrange for 3 reference letters to be sent by 27 January 2020 to the same email address.

For further information and informal inquiries please contact Dr. Davide Gerosa ([email protected]) and Prof. Alberto Vecchio ([email protected]).

We’re accepting applications from prospective PhD students. The deadline is Dec 31, 2019 for positions starting in the Fall of 2020.

Here below is my project description:

_Astrophysics and phenomenology of gravitational-wave sources with LIGO and LISA
This project concentrates on developing theoretical and astrophysical prediction s of gravitational-wave sources. The first observations of gravitational waves by LIGO have ushered us into the golden age of gravitational-wave discoveries. Thousands of new events are expected to be observed in the next few years as detectors reach their design sensitivities. Such large catalogs of gravitational-wave observations will open new, unprecedented opportunities in terms of both fundamental physics and astrophysics. Crucially, they will need to be faced with increasingly accurate predictions. First, among large catalogs, there will be “golden” events. We expect systems that, because of their properties, are particularly interesting to carry out some specific measurements (perhaps because of their favorable orientations, or because they are very massive, or very rapidly rotating, etc). Second, large catalogs need to be exploited with powerful statistical techniques. In the long run, future facilities like LISA will deliver new kinds of sources providing access to a whole new set of phenomena in both astrophysics and fundamental physics. New theoretical tools and techniques need to be developed (and immediately applied!) to maximize the scientific payoff of current and future gravitational-wave observatories. _

And the second paper on the arxiv today is Daria’s masterpiece! pAGN (which Daria says you should read “pagan”) is a brand new, super cool code that implements the hydrodynamics of AGN disks, at least in their most popular one-dimensional fashion. Those solutions have been around for a long time but their details were, well, let’s say unclear. Daria went through everything from beginning to end, coming up with the “one-stop solution for your AGN disc needs” (that was actually the working title of the paper…). So pip install pAGN and have fun.

Daria Gangardt, Alessandro Alberto Trani, Clément Bonnerot, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society, 530 (2024) 3986–3997.
arXiv:2403.00060 [astro-ph.HE].

We do population analysis in gravitational waves all the time now. That is: we compare many observations from GW experiments against many simulated datapoints from simulations. But what if you only have one observation? That could be a LIGO guy that is kind of an outlier (think GW190521) or maybe a datapoint from a future detector (think LISA) that feels lonely in his parameter space. Don’t look further, this is stats for you (and Matt’s last paper as a grad student…)

Matthew Mould, Davide Gerosa , Marco Dall’Amico, Michela Mapelli.
Monthly Notices of the Royal Astronomical Society, 525 (2023) 3986–3997
arXiv:2305.18539 [astro-ph.HE].

We want more! With gravitational-wave data, some quantities like the masses of the black holes are much easier to see than others. But those others are very interesting, notably spins that process and orbits that are eccentric, because they would tell us how black hole binaries came to be in the first place. So while it would be great to see those, it’s also being very hard. Some tentative claims have been made with current data, but nothing unambiguous so far. In this paper led by Isobel from Cambridge, we show that (surprise surprise…) the signals needs to be long enough before one can tell eccentricity and spin precession apart.

Isobel Romero-Shaw, Davide Gerosa , Nicholas Loutrel.
Monthly Notices of the Royal Astronomical Society 519 (2023) 5352–5357.
arXiv:2211.07528 [astro-ph.HE].

Together with my postdoc Nate, we’re proceeding our investigations on supermassive, spinning binary black holes surrounded by accretion disks (that is: a ton of gas around big monsters at the center of galaxies!). In today’s paper, we dig a bit deeper into what happens when the disk breaks. That presumably stops the interactions between the gas and the black-hole spins which could make all this funky astrophysics (spins that moves, disks that breaks, etc) actually observable with future gravitational-wave detectors. More needs to be done of course, but here we are.

Nathan Steinle, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society 519 (2023) 5031–5042.
arXiv:2211.00044 [astro-ph.HE].

Big stars burn everything they have, die fast, and produce big black holes. So when you see two black holes together, it’s likely that the big black hole comes from the big star. Or maybe not? Before dying, the big star can drop some mass onto the other guy, making it bigger! So now, the initially big star still produces the first black hole, but, at the end of the day, that might not be the more massive black hole anymore! This scenario is called “mass-ratio reversal” and our astrophysics friends have put together many models out there showing this is indeed possible for a good fraction of the black holes that produce gravitational-wave events. So here we ask the data: given the events LIGO and Virgo have seen so far, what’s the evidence for mass-ratio reversal in binary stars? Read Matt’s paper to find out.

Matthew Mould, Davide Gerosa , Floor S. Broekgaarden, Nathan Steinle.
Monthly Notices of the Royal Astronomical Society 517 (2022) 2738–2745.
arXiv:2205.12329 [astro-ph.HE].

Breaking things is fun! In the previous paper of this series, we looked at accretion disks around massive black-hole binaries and found things were going awry. We kept on finding configurations that our implementation could not handle… And now we know this is real! Finding disk solutions when the spin of the black hole has a large misalignment is just not possible! And that’s because the disk really breaks into different sections. We’ve now checked it with state-of-the-art hydrodynamical numerical simulations that not only confirm what we suspected but also show some funny things (like breaking being prevented by disk spirals, etc). I was serious, breaking things is real fun!

Check out Rebecca’s beautiful movies!

Rebecca Nealon, Enrico Ragusa, Davide Gerosa , Giovanni Rosotti, Riccardo Barbieri.
Monthly Notices of the Royal Astronomical Society 509 (2022) 5608–5621.
arXiv:2111.08065 [astro-ph.HE].

Supermassive black hole inspiral and spin evolution are deeply connected. In the early stages when black holes are brought together by star scattering and accretion, spin orientations can change because of interactions with the environment. Later on, when gravitational waves are driving the mergers, spins change because of relativistic couplings. In this paper we try to follow this complicated evolution in a full cosmological framework, using products of the Illustris simulation suite, a new sub-resolution model, and post-Newtonian integrations.

Mohammad Sayeb, Laura Blecha, Luke Zoltan Kelley, Davide Gerosa , Michael Kesden, July Thomas.
Monthly Notices of the Royal Astronomical Society 501 (2020) 2531–2546.
arXiv:2006.06647 [astro-ph.GA].

New paper today! We’ve been working on this for a very long time but three weeks of lockdown forced us to finish it. It’s about distorted (aka warped) accretion discs surrounding black holes. If the black hole is spinning and part of a binary system, the disc behaves in a funny way. First, it’s not planar but warped to accomodate these external disturbances. Second, disc and black hole interacts and tend to reach some mutual agreement where the disc is flat and the black-hole spin is aligned. We find it’s not that easy and things are actually much more complicated: read the paper to know more about non-linear fluid viscosities, critical obliquity, mass depletion, etc.

Davide Gerosa , Giovanni Rosotti, Riccardo Barbieri.
Monthly Notices of the Royal Astronomical Society 496 (2020) 3060-3075.
arXiv:2004.02894 [astro-ph.GA].

ps. Here is a Twitter thread by P. Armitage.

Spoiler alert: this paper is a bit sad.

Stellar-mass black-hole binaries are now detected by LIGO on a weekly basis. It would be really cool if LISA (a future space mission targeting low-frequencies gravitational waves) could see them as well. We could do a lot of cool stuff, in both the astro and the theory side of things. In today’s paper, we try to figure out how easy or hard it will be to extract these signals from the LISA noise. Well, it’s hard. In terms of the minimum signal-to-noise ratio required, we find that this is as high as 15. The number of expected detection becomes discouragingly low unless the detector behaves a bit better at high frequencies or black holes with 100 solar masses start floating around.

Christopher J. Moore, Davide Gerosa , Antoine Klein.
Monthly Notices of the Royal Astronomical Society Letters 488 (2019) L94–L98.
arXiv:1905.11998 [astro-ph.HE].

Supermassive black holes in binaries and their accretion discs… Spins align on some timescale, but migration also takes place. Do gas discs have enough time to align the spins? Well, the secret is the mass ratio: light secondaries might prevent primaries from aligning. A great collaboration between gravitational-wave and planet researchers!

Davide Gerosa , Benedetta Veronesi, Giuseppe Lodato, Giovanni Rosotti.
Monthly Notices of the Royal Astronomical Society 451 (2015) 3941-3954.
arXiv:1503.06807 [astro-ph.GA].

Black-hole kicks are powerful. I mean, really powerful. They can even eject supermassive black holes from the heavier galaxies in our Universe. And then these galaxies are left “empty”…

Davide Gerosa , Alberto Sesana.
Monthly Notices of the Royal Astronomical Society 446 (2015) 38-55.
arXiv:1405.2072 [astro-ph.GA].

My very first paper was on alignment between supermassive black holes and their accretion disks. Maybe it takes a bit longer…

Giuseppe Lodato, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society Letters 429 (2013) L30-L34.
arXiv:1211.0284 [astro-ph.CO].

Welcome to the beautiful world of SBI, with this terrific piece of work by Pippa Cole. Here we’re looking at extreme mass-ratio inspirals (EMRIs), that is, a small black hole orbiting a big black hole, which will be (one day) detected by LISA. These signals are nasty (long and of a very complicated morphology). We’re trying something new here – a deep learning called “truncated marginal neural ratio estimation” that does not even require writing down the likelihood of the problem. Just simulate all you can. The answer, this thing is great for narrowing down the parameter space where EMRIs will be, kind of like searches do with current gravitational-wave data, but in a very different way.

Philippa S. Cole, James Alvey, Lorenzo Speri, Christoph Weniger, Uddipta Bhardwaj, Davide Gerosa , Gianfranco Bertone.
arXiv:2505.16795 [gr-qc].

The University of Milano-Bicocca (Milan, Italy) invites applications for a tenure-track professorship in Astrophysics.

Milano-Bicocca hosts a vibrant astrophysics group consisting of 11 faculty members, approximately 25 postdocs, and around 15 PhD students. The group has a strong track record of securing national and international funding, with 6 recently awarded ERC grants. We are part of a larger physics department with about 70 faculty members and are situated on a dynamic campus with 40,000 students. Milan is a modern, international city in northern Italy, close to the stunning Alps, offering a lively cultural scene, excellent food, and a high quality of life.

Current interests of the group include gravitational-wave astronomy, formation and evolution of cosmic structures, and experimental cosmology. At the same time, we are open to all strong candidates willing to bring their ambitious research programs in astrophysics to Milan.

The position will be at the assistant professor level (“RTT” in the Italian system), a tenure-track appointment with a well-defined path to tenure within either three or six years, depending on performance. The anticipated start date is fall 2025, though this is negotiable. Responsibilities include conducting research at the highest international standards, teaching BSc and MSc courses, mentoring students, and securing external funding.

Interested candidates are invited to apply by June 12th, 2025:
https://www.unimib.it/ateneo/gare-e-concorsi/2025-rtt-027-dipartimento-fisica-g-occhialinigsd-02phys-05-ssd-phys-05a

Knowledge of the Italian language is not required to apply; the online application portal is available in English. We strive to build a diverse and inclusive environment and welcome applications from traditionally underrepresented groups.

For inquiries, please contact Prof. Michele Fumagalli ([email protected]).

This paper came out of some discussions from our “Gravitational-wave snowballs” workshop in Sexten (Italy). We were discussing the good old problem of separating black-hole binary formation channels with spin measurements. Usually one says “aligned=isolated”, “isotropic=dynamical”. But then, some binaries that formed dynamically should also be eccentric. What we then realized is that, for those eccentric binaries and only for those, spin measurements can actually tell which of the dynamical channel (because there are many…) is at play.

Jakob Stegmann, Davide Gerosa , Isobel Romero-Shaw, Giulia Fumagalli, Hiromichi Tagawa, Lorenz Zwick.
arXiv:2505.13589 [astro-ph.HE].

We’re back to predicting the excitation amplitude of black hole merger ringdowns. We already looked into the simpler case of binaries with aligned spins, and now tried to study the full problem of binaries with misaligned (i.e. processing) spins. Well, this is a hard problem! It’s not even clear which mode is the stronger one anymore, and finding suitable coordinates is not at all trivial. While this is just a first exploration, there’s so much interesting phenomenology here! Do it yourself with the postmerger package.

Francesco Nobili, Swetha Bhagwat, Costantino Pacilio, Davide Gerosa.
arXiv:2504.17021 [gr-qc].

The 2017 paper “Are merging black holes born from stellar collapse or previous mergers? ” that I wrote with Emanuele Berti was selected 2025 Frontiers of Science Award. These prizes are awarded by the International Congress of Basic Science (ICBS), sponsored by the City of Beijing and the Yanqi Lake Beijing Institute of Mathematical Sciences and Application (BIMSA). Every year, they select influential recent papers in Physics, Maths, and Computer Science.

The complete list of Physics papers selected for awards is available here. Ours is one of only three papers that were selected in the category Astrophysics and Cosmology – Theory. We’ll collect the award in July at the Great Hall of the People of China in Beijing.

I’m so happy to see how a seemingly simple idea we had (“What if LIGO’s black holes merge multiple times?”) went so far! Our paper was published in Physical Review D in 2017, selected as an Editor’s Suggestion back then… and now got an award!

Great paper led by our former MSc student Alessandro today! This is about combining the distributions of gravitational waves and galaxies to do cosmology. These two probes measure different things (distance and redshift, respectively), so their distributions will “match” only if the cosmological model is right. You can actually use this to measure the cosmological model itself. Short answer: putting together 3G detectors and Euclid is a great idea.

Alessandro Pedrotti, Michele Mancarella, Julien Bel, Davide Gerosa.
arXiv:2504.10482 [astro-ph.CO].

Four BSc+MSc students just graduated with projects in our group!

… congrats all!

If you’re looking for a PhD in gravitational-wave physics, our 2025 call for PhD scholarships is now available. The procedure is described here (Session 1):

https://en.unimib.it/study/doctoral-research-phd-programmes/applying-doctorate/calls-application

The deadline is April 24th at noon CEST.

For instructions, start from the file “Guide to filling in the online application.” There’s a key step on page 10 where candidates can express interest in some themed scholarships. If you’re interested in working with my group, I encourage you to select PROG.1 and PROG.3.

You will need to submit your research proposal/statement. These are usually 2-3 pages long. It should provide some context about your work in gravitational-wave astronomy (or astrophysics more in general), what you want to do next, your key interests, what you would like to work on here with us, why you want to work with us, and more in general how you plan to integrate with our activities. It should be forward-looking and not just about what you’ve done already. Hope this helps!

For any questions, please do not hesitate to contact me: [email protected]

Happy to share this postdoc opportunity from the Cariplo Foundation, which is a private trust that operates in the Milan area. It’s an independent fellowship for early career researchers, with a duration of 3 years and a total budget of 200k EUR.

https://www.fondazionecariplo.it/static/upload/you/young-researchers-2025.pdf

The deadline is March 24, 2025. If you’re reading this and are interested in applying with us at Milano-Bicocca, please shoot me an email!

I’m on the hook for teaching this semester (can’t complain really with such fun classes!). I’m down for “Astrostatistics and Machine Learning” for our MSc degree in Astrophysics and “Machine Learning for Physics and Astronomy” for our BSc degree in Artificial Intelligence. Here is my material for both, and thanks to all the students who will be engaging with this!

github.com/dgerosa/astrostatistics_bicocca_2025
github.com/dgerosa/machinelearning4physics_bicocca_2025

The Italian Society of General Relativity and Gravitation (SIGRAV) announces the 26th SIGRAV Conference, hosted by the University of Milano-Bicocca, to be held in Milan, Italy, from September 8-12, 2025.

https://sites.google.com/unimib.it/sigrav2025

The conference will cover various aspects of classical and quantum gravity, including tests of General Relativity, cosmology, gravity experiments, and gravitational waves from experimental, theoretical, and data-analysis perspectives.

Participation is open to SIGRAV members and non-members alike, both nationally and internationally. The program will feature a series of broad review talks on various aspects of gravitational physics, as well as contributed talks. The SIGRAV Amaldi medals, the SIGRAV prizes for young researchers, and the Giulio Rampa PhD thesis prize will be awarded during the conference. There will also be a public event dedicated to the 10-year anniversary of the first direct detection of gravitational waves, GW150914.

Abstracts for contributed talks should submitted by May 31, 2025. We aim to announce the full conference program by the end of June. Registrations will be accepted until July 15, 2025.

Milan is a beautiful, international city in the north of Italy and is served by three major airports with worldwide connections. The city is home to art, history, and great food; you can also explore nearby lakes or venture into the stunning Alps.

Long gravitational-wave signals are, well, long. And long often means painful, as more data need to be stored and processed. Kind of intuitively, the solution might be that of cutting things into chunks, so that long becomes short. Here we apply this idea to the popular inner product entering all gravitational-wave pipelines; this is a key building block of everything we do. The answer is that using SFTs, “Short-time Fourier Transforms”, can make things faster by more than 3 orders of magnitudes, sometimes 5. We think this is the solution to future gravitational-wave data analysis problems (think LISA and 3G…).

Rodrigo Tenorio, Davide Gerosa.
arXiv:2502.11823 [gr-qc].

When inferring black holes from gravitational-wave data, we tend to do two things, one after the other. First, we consider each event individually and measure its parameters (masses, spins, etc). Then we consider all the events together and measure the population properties. This is what we do all the time, but, actually, if objects are now part of a population, those parameters should be looked at again in light of all the others. This full problem (all parameters of all the events plus the population parameters) is daunting, and in the past we used an indirect and somewhat convoluted approach. We got back to it now, and this time, we managed to do it head-on. Let me introduce this giant 500-dimensional sampling of the full population problem!

Michele Mancarella, Davide Gerosa.
arXiv:2502.12156 [gr-qc].

General relativity has this beautiful property that coordinates are meaningless. You can change them at will, which means they don’t contain any physics. And, believe it or not, some of the popular formulations we use to write down the dynamics of eccentric binary black holes still have coordinates in them. They go away if you take an average of an orbit (Peters, the man!) but that’s killing some information. In this paper we go back to those old results and show how those gauges can actually be absorbed into the formulation itself. The paper is on the maths-heavy side of things, but the results are great. Peters, you were basically right, but not quite.

Giulia Fumagalli, Nicholas Loutrel, Davide Gerosa , Matteo Boschini.
arXiv:2502.06952 [gr-qc].

We’re going to have quite a few visitors in the next few months. They will be giving amazing seminars, with lots of research ideas floating around: Stephen Green from Nottingham, Cecilia Sgalletta from Trieste, Francisco Duque from the AEI, Angela Borchers from the other AEI, Lorenzo Pompili also from AEI (!), Ilaria Caporali from Pisa, Aleksandra Olejak from the MPA at Garching, Pantelis Pnigouras from Alicante, Lucy McNeill from Kyoto, and James Alvey from Cambridge. Hope I didn’t forget anyone… This is going to be exciting 🙂

Quasar 3C 186 strikes back! Matteo and I got interested in this funny quasar last year (see this one). When our paper hit the arxiv, we got contacted by the real astronomers who take actual data, who told us they had even more beautiful data. We ended up contributing with our relativistic model and… well… everything seems to work. 3C 186 is indeed a recoiling black hole (it might be a rare one, but we’ve observed it nonetheless). The abstract says “decisive,” and this is indeed the right word.

Marco Chiaberge, Takahiro Morishita, Matteo Boschini, Stefano Bianchi, Alessandro Capetti, Gianluca Castignani, Davide Gerosa , Masahiro Konishi, Shuhei Koyama, Kosuke Kushibiki, Erini Lambrides, Eileen T. Meyer, Kentaro Motohara, Massimo Stiavelli, Hidenori Takahashi, Grant R. Tremblay, Colin Norman.
arXiv:2501.18730 [astro-ph.GA].

Gravitational-wave population people talk all the time about parametric vs non-parametric methods. Parametric methods mean imposing our astrophysical knowledge on how we look at GW data. This is great, we do want to extract astrophysical knowledge, but what if we don’t know what to look for? The statisticians tell us to go non-parametric, which means using a flexible model that can fit whatever you want. That’s great, but what do we learn then? In other words, where’s the boundary between flexibility and interpretability? Today’s paper shows that one can conceptually separate these two processes and extract parametric results from non-parametric fits. I’m very proud of this piece of work, which was Cecilia Fabbri‘s MSc thesis project and was actually kickstarted by one of my previous students, Alessandro Santini. We even wrote a poem about this!

Cecilia Maria Fabbri, Davide Gerosa , Alessandro Santini, Matthew Mould, Alexandre Toubiana, Jonathan Gair.
arXiv:2501.17233 [astro-ph.HE].

I’m so so proud to see my PhD student Viola De Renzis defending her PhD thesis today. Viola’s thesis is titled “Gravitational-wave astronomy at the crossroads: from current to future detectors, from single events to populations” and was examined by Maya Fishbach (Toronto), Laura Sberna (Nottingham) as external referees, as well as Walter Del Pozzo (Pisa), Stephen Green (Nottingham) and Alberto Sesana (Milano-Bicocca) as defense committee members. What should I say, from the first “off you go and learn Bilby” meeting we had, to all those discussions at the board, learning how to ski, those codes that did (not) work, and that distinctive laughter across the corridor. Our group will not be the same without Viola. You turned into a great scientist: now “spacca tutti” in Marseille!

That’s me, Steve, Walter, Viola, and Alberto…

Congrats to Alex Toubiana, postdoc with us, who was just awarded an independent fellowship from the Italian Research Ministry. The scheme is called Young Researcher 2024 and will fund Alex and his research for 3 years.

In 2024…. We welcomed Tristan, Chiara, Caroline, Rodrigo, Alex, Federico, and Zachos (group accretion at the Eddington limit). Michele started a faculty in Marseille, Daria graduated, Viola almost graduated and is fighting the paperwork in Marseille, Giulia went to Cambridge, Alice went to the AEI, Cecilia went to Nottingham, Costantino went to Novara. Ringdowns, EMRIs, stochastic backgrounds, p_det, catastrophes, SBI, and 3G detectors don’t have secrets for us. I think 13 BSc and 3 MSc students defended their projects with us, not sure. Arianna and Nick are two Giovani Talenti, Alex is a Young Researcher. We went to the lake together, got risotto together, and organized a conference named after Inter’s striker. If you don’t know what to eat for dinner, define a likelihood and sample it (Loutrel et al. 2024). Or put pins on google maps (Borhanian et al. 2024). You look at data, I look at the physics (Bruel et al. 2024).

FIS (“Fondo Italiano per la Scienza”) is an Italian grant opportunity which is conceptually similar to the ERC. The amount of these grants is >= 1M EUR and grant holders are offered a tenure-track or tenured position. The deadline for this year’s solicitation (FIS 3) is Mar 28, 2025. If you’re interested in applying with Milano-Bicocca as host institution please shoot me an email!

https://www.mur.gov.it/it/atti-e-normativa/decreto-direttoriale-n-1802-del-21-11-2024

This semester I’m back teaching my “Scientific computing with Python” module to PhD students. It’s a really really fun class to teach (and it’s just 8 lectures, short and sweet!). If you’re curious:

github.com/dgerosa/scientificcomputing_bicocca_2024

Black holes on eccentric orbits… what does it even mean? The hard (but fun) thing is that we work in General Relativity, where coordinates don’t have a physics inside. One can always change the coordinates as they want, so they can’t be used to define observables. The eccentricity of an orbit has to do, indeed, with the shape of the orbit itself, and that can be transformed away with suitable coordinates. So, does it even sense to measure the orbital eccentricity of black-hole binaries? The one thing we are allowed to do is to find a coordinate-free estimator in General Relativity that reduces to the eccentricity we all know and love in the Newtonian limit. This is possible! The right mathematical framework for this is something called “catastrophe theory”, a funny name, but Nick likes it.

Matteo Boschini, Nicholas Loutrel, Davide Gerosa , Giulia Fumagalli.
Physical Review D 111 (2025) 024008.
arXiv:2411.00098 [gr-qc].

Our “popfisher” paper is finally out! (and now Viola can submit her PhD thesis). This is about next-generation (aka 3G) gravitational wave detectors. Those beasts will measure millions of black holes… and with so many of them who cares about each source individually. The important thing will be the population of objects, i.e. how those black holes are distributed as a whole. Measuring populations is an interesting but convoluted statistical problem. Here we implement a quick shortcut (the Fisher matrix) and show that yes, 3G detectors will be amazing… but more amazing for some things than for others.

Viola De Renzis, Francesco Iacovelli, Davide Gerosa , Michele Mancarella, Costantino Pacilio.
Physical Review D 111 (2025) 044048.
arXiv:2410.17325 [astro-ph.HE].

Four BSc students and one MSc student defended their research project with us this month.

Thanks all for spending some time in our research group!

Huge congrats to Arianna Renzini and Nick Loutrel who won two of this year’s “Giovani Talenti” (Young Talents) prizes from the University of Milano-Bicocca. These are internal grants for postdocs: there were four grants awarded in Physics in total and two of them are from our group! Let’s gooooooooooo

LISA will see a gazillion white dwarfs, but we won’t, or at least not individually. Those signals will actually pile up together in a mashed potato thing called foreground. But this mashed potato won’t be smooth (translate: the gravitational-wave signal won’t be stationary and Gaussian) and this structure can indeed be precious for extracting more information from LISA. But first, let’s taste this with today’s paper, i.e. characterize the foreground.

Riccardo Buscicchio, Antoine Klein, Valeriya Korol, Francesco Di Renzo, Christopher J. Moore, Davide Gerosa , Alessandro Carzaniga.
arXiv:2410.08263 [astro-ph.HE].

ps. This started as the student project of Alessandro Carzaniga, great it’s finally out!

Our group is accreting people at the Eddington rate! There are 5 new postdocs and 2 PhD students who have just started or are about to start:

Group meetings are funny and busy these days, with too many ideas going around.

This is a fun IMBH story we worked out when Kostas and Luca were visiting last summer from JHU. What if (one day, who knows) we observe a highly spinning intermediate-mass black hole? If that happens, is going to be puzzling because IMBH that grow in clusters by mergers of smaller black holes tend to spin down, not up. This is a funny property of black holes, namely that extracting spins is easier than putting it in, so on average black holes slow down after they have merged many times. So if we see an IMBH with large spins, the spin must come from somewhere else. Where? Maybe gas. The argument then is that one can actually convert an IMBH spin measurement into the minimum amount of gas that must have been accreted to get that spin.

Konstantinos Kritos, Luca Reali,Davide Gerosa , Emanuele Berti.
Physical Review D 110 (2024) 123017.
arXiv:2409.15439 [astro-ph.HE].

To Stars or to gas, that is the question.
Whether ’tis nobler in the hardening to suffer
The slings and arrows of passing stars,
Or to dissipate against a sea of gas
And by disk end them. To inspiral — to merge,
No more; and by LISA to say we end
The models and the thousand PE samples
That gravity is heir to.

Alice Spadaro, Riccardo Buscicchio, David Izquierdo-Villalba, Davide Gerosa , Antoine Klein, Geraint Pratten.
Physical Review D 111 (2025) 023004.
arXiv:2409.13011 [astro-ph.HE].

Four students just graduated with projects in our group…

First, huge congrats to Cecilia Fabbri who got her MSc in Astrophysics. Cecilia (you might remember her) worked on an exciting applied statistics problem (which has already ended up in a poem, but soon in a paper). Her problem got like 10 more people hooked beside us, so we really have to finish it now! From my side, it’s always amazing to see scientists like her growing so much. Cecilia be moving on with a PhD in Nottingham (UK) with Steve Green (and when you come back to visit you’ll tell me everything I don’t understand about simulation-based inference!). Good luck!

We also supervised three BSc students who defended their short projects:

Congrats all, Spritz time now.

All right I think this is great (but it took me a long time to convince myself and the others that’s the case!) In gravitational-wave astronomy we measure binaries, that is, pairs of two objects. Our signals have information about the pair as a whole. At the same time, we care very much about separating those two objects and measuring the properties of individual black holes and neutron stars. We always do that operation without thinking twice, just say that for each posterior sample object “1” is that with the larger mass and object “2” is that with the lower mass. But is that ok? Surely it’s a choice, but is it the best one? What does it even mean to pick the “best” labels? I think machine learning can help us here and that this problem can be framed using the language of semi-supervised clustering. The results? Well, they seem very significant. Measurements of the black-hole spins are more accurate, you can tell more easily if that’s a black hole or a neutron star, and overall the posterior distributions just look nicer (go away nasty multimodalities and non-Gaussianities!).

Davide Gerosa , Viola De Renzis, Federica Tettoni, Matthew Mould, Alberto Vecchio, Costantino Pacilio.
Physical Review Letters 134 (2025) 121402.
arXiv:2409.07519 [gr-qc].
Selected as PRL Editors’ Suggestion.
Press release : Milano-Bicocca.
Other press coverage: ilgiorno, lescienze, ansa.it, adnkronos (1), adnkronos (2), 30science, agenparl.eu, cagliarilivemagazine, ilcentrotirreno, ilgiornaleditalia, laragione, lospecialegiornale, meteoweb, msn.com, occhioche, padovanews, prpchannel, sardegnalive, smartphonology, tgabruzzo24, vetrinatv, unicaradio, altoadige, ecodibergamo, roboreporter, saluteh24, salutedomani.

The ringdown is the final bit of a gravitational-wave signal, after the two black holes have merged. It’s nice because it’s clean; GR is so powerful that all that comes out after a black hole merger has specific frequencies, the fantastic “quasi-normal modes.” While the frequencies only depend on that final BH (thanks Kerr!), the excitations of those frequencies depend on all that happened before, i.e. the merger process itself. In this summer paper by Costantino and the rest of us, we present a new accurate approximant to those amplitudes. Now go home and test GR.

Costantino Pacilio, Swetha Bhagwat, Francesco Nobili, Davide Gerosa.
Physical Review D 110 (2024) 103037.
arXiv:2408.05276 [gr-qc].

Usually my students graduate in Physics, but not this time… Together with Matteo Boschini, I had the pleasure of supervising a student majoring in Computer Science. Alessandro Crespi got his BSc degree with a project on Simulation Design, which is really a computing thing but has lots of physics applications. That was so much fun! It is truly true that putting different expertise/approaches/ideas makes things better.

We’re having a few visitors this summer, with lots of science going around. Welcome Jam Sadiq from SISSA (Italy), Rossella Gamba from Berkeley (USA), Abhishek Chowdhuri from IIT Gandhinagar (India), Luca Reali from JHU (USA), and Kostas Kritos also from JHU (USA), thanks for joining us for a bit.

The orbits of binary black holes could be eccentric, but in practice they’re not. At least when we observe them, and that’s because of a relativistic effect that circularizes the orbit. Even if astrophysics formed black holes eccentric, relativity makes them circular when we observe them with gravitational-wave interferometers. But we’re interested in the astrophysics back then! What we find here is that the tiny residual eccentricity at detection can be crucial. Even eccentricities that are so small that we cannot tell them apart from circular can mess up the astrophysical inference. Unfortunately, this is a new systematic error that needs to be taken into account: inferring the “formation channel” of binary black holes might be even harder than we thought.

Giulia Fumagalli, Isobel Romero-Shaw, Davide Gerosa , Viola De Renzis, Konstantinos Kritos, Aleksandra Olejak.
Physical Review D 110 (2024) 063012.
arXiv:2405.14945 [astro-ph.HE].

… and we’re back to selection effects. That means modeling what you cannot see. The black holes that gravitational-wave detectors observe are not representative of those that are out there in the Universe. Some are easier to see, some are harder. Quantifying how much easier and harder is crucial to properly understand the underlying astrophysics. In this paper (which came out of a BSc student project!), we go back to the basics and work out gravitational-wave selection effects one step after the other, using and refining the most common approximation. Two things to remember: including noise fluctuations is easy, and a signal-to-noise ratio threshold of 11 is probably ok.

Davide Gerosa , Malvina Bellotti.
Classical and Quantum Gravity 41 (2024) 125002.
arXiv:2404.16930 [astro-ph.HE].

This week we’re hosting researchers from the Gran Sasso Science Institute (GSSI) for a joint mini-conference / workshop / group meeting. More here:

davidegerosa.com/lautaro

This is part of a PRIN grant we have together (thanks Italy) with support from other grants as well (thanks Europe). The meeting has the best title ever (that was actually my idea…), the best logo ever (that was Giulia’s idea), and the best organization ever (huge thanks Costantino and Sara!).

Population 3 stars are like “the original” stars. Those formed with material that comes straight from the Big Bang. It would be very (like, a lot!) cool to see them with gravitational-wave detectors. But can we tell them apart? Or do they look like all the other stars? Here is an attempt with a fancy machine-learning classifier.

Filippo Santoliquido, Ulyana Dupletsa, Jacopo Tissino, Marica Branchesi, Francesco Iacovelli, Giuliano Iorio, Michela Mapelli, Davide Gerosa , Jan Harms, Mario Pasquato.
Astronomy & Astrophysics 690 (2024) A362.
arXiv:2404.10048 [astro-ph.HE].

The University of Milano-Bicocca welcomes applications for PhD scholarships. This year’s application deadline is May 14th, 2024 (noon CEST) for positions starting in the Fall of 2024:

https://en.unimib.it/education/postgraduates/doctoral-research-phd-programmes/applying-doctorate/calls-application

In particular, we are looking for highly motivated candidates to join our activities in black-hole binary dynamics and gravitational-wave data exploitation. Milano-Bicocca hosts a large group in gravitational-wave physics, covering activities ranging from astrophysical/numerical modeling to data analysis. The group counts 7 faculty members (Bortolas, Colpi, Dotti, Gerosa, Giacomazzo, Sesana, and an upcoming new hire) together with several postdocs (of which two prize fellows) and PhD students. Candidates will also have ample opportunities to work with and visit external collaborators.

Our PhD admission program includes several “open” scholarships, covering all research activities in the department (including ours!). All candidates are considered for those by default. In addition, we are advertising an additional “project” scholarship titled “Gravitational-wave source modeling” which will be supervised by Prof. Davide Gerosa. Candidates wishing to be considered for this opportunity should indicate it explicitly when applying (the number of this position FIS.8). For more information on Gerosa’s group see www.davidegerosa.com/group

We strive to build an inclusive group and welcome applications from all interested candidates. For informal inquiries, expressions of interest, and application tips please do not hesitate to contact [email protected]

Three more students graduated in March with research projects completed in our group!

On top of “astrostats” for the MSc degree in Astrophysics, this semester I’m excited to start teaching for the new BSc degree in Artificial Intelligence. This course is delivered jointly by the University of Milano-Bicocca (my place), the University of Milano-Statale (“the other” uni in town), and the University of Pavia (south of here…). My class is actually a lab, the full (too long) title is “Laboratory of Machine Learning Applied to Physical Systems.” The class material is available here:

github.com/dgerosa/machinelearning4physics_bicocca_2024

Can’t wait to see what these AI students can do! Hope to learn from you as much as you learn from me.

The teaching semester is back and I’ll be delivering my “Astrostatistics and Machine Learning” class once more. This is designed for our MSc program in Astrophysics.

Astro-data freaks, go here! github.com/dgerosa/astrostatistics_bicocca_2024

This is the first of two papers on the arxiv today: it’s fun when two long, very different projects by different people just happen to be done on the same day! This paper is by my former colleague Nate Steinle (now a postdoc in Manitoba, Canada). Here we connect the dynamics of jets in AGN disks to the spin of black holes observable by LISA. And show the latter is a diagnostic of the former! And it’s nice to see my disk-binary code being used for something I didn’t think of when I wrote it.

Nathan Steinle, Davide Gerosa , Martin G. H. Krause.
Physical Review D 110 (2024) 123034.
arXiv:2403.00066 [astro-ph.HE].

And the second paper on the arxiv today is Daria’s masterpiece! pAGN (which Daria says you should read “pagan”) is a brand new, super cool code that implements the hydrodynamics of AGN disks, at least in their most popular one-dimensional fashion. Those solutions have been around for a long time but their details were, well, let’s say unclear. Daria went through everything from beginning to end, coming up with the “one-stop solution for your AGN disc needs” (that was actually the working title of the paper…). So pip install pAGN and have fun.

Daria Gangardt, Alessandro Alberto Trani, Clément Bonnerot, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society, 530 (2024) 3986–3997.
arXiv:2403.00060 [astro-ph.HE].

Our student Lisa Merlo defended her BSc 3rd year project today! Lisa worked with Pippa Cole and me on computing rates for mergers of primordial black holes, also considering a new detector prototype that the experimental group here is developing (nickname BAUSCIA, from the Milan dialect). Short answer: the rate is low but now is more accurately low. Lisa’s presentation was amazing and working with her has been a real pleasure. Stay tuned for her future astro career!

Huge huge congrats to Zacharias Roupas who was awarded a Marie Curie Fellowship with us! Zachos is currently based at the British University in Egypt and will be joining my group in Milan in the Fall of 2024. The Marie Curie Fellowship program is a prestigious postdoctoral scheme operating at the EU level and, together with Arianna, we’ll now have two Marie Curie grantees in the group. Zachos’ winning proposal is titled “Black hole spin and mass function in gaseous proto-clusters” (nickname: protoBH).

Not sure what happened here, how the hell did I end up writing a paper with actual radio data that needed to be reduced … Call me an ambulance.

The guy here is 3C186 which is not a postcode but a quasar. A funny one because it’s not centered on the galaxy (it’s a bit off) and it’s also going at another velocity (ciao ciao). One of the leading explanations is that 3C186 is a recoiling black hole, the remnant of black-hole merger is being kicked away (yeah these things can happen). 3C186 also has a radio jet, and that should point in the direction of the black-hole spin. The funny thing is that spin and the kick appear perpendicular to each other, and this is fun because theory says they should actually be parallel. We looked into this a bit carefully and discovered it’s all a lie! The spin and the kick both point along the line of sight and appear perpendicular only because of a super strong projection effect. If this is true, the radio jet should also point straight to us! We then tried to test this with whatever ratio data we could grab (where is that ambulance) and found that… mmh, well, it’s a maybe.

Matteo Boschini, Davide Gerosa , Om Sharan Salafia, Massimo Dotti.
Astronomy & Astrophysics 686 (2024) A245.
arXiv:2402.08740 [astro-ph.GA].

The University of Milano-Bicocca (Milan, Italy) will be opening a tenured professorship in astrophysics, with a focus on gravitational-wave data analysis and exploitation. With this notice, we invite expressions of interest from potential candidates.

Milano-Bicocca hosts a large group in gravitational astronomy, with activities covering all bands of the gravitational-wave spectrum and the related experiments (LIGO/Virgo, LISA, ET, PTA). Faculty members with matching interests include Bortolas, Colpi, Dotti, Gerosa, Giacomazzo, and Sesana. The group hosts two large ERC grants and currently counts about 10 PhD students and 15 postdocs. We are part of a wider astrophysics unit at Milano-Bicocca (with activities in large-scale structures and experimental cosmology) as well as a large Physics department with ~70 faculty members.

We are targeting the opening of a faculty position on a timescale of a few months, with a prospective starting date in the early fall of 2024. Onboarding will be at the associate professor level (“professore associato” in the Italian system), which is a tenured appointment. Formal application requirements include holding either the Italian national habilitation (ASN) or a comparable position abroad for at least 3 years. We are happy to assist potential candidates with their ASN application.

Current strategic interests include the development of gravitational-wave data-analysis pipelines for the LISA space mission. At the same time, we are open to all strong candidates willing to bring their ambitious research programs in relativistic astrophysics and/or gravitational-wave astronomy to Milan.

Interested applicants are encouraged to send their CVs and a short cover letter to [email protected] by February 15th, 2024. The CV should include the names and email addresses of three referees who might be approached for references.

Much like Spotify, here is our group “Wrapped”, 2023 edition!

Some of the group highlights include… We welcomed Pippa, Nick, Arianna, Sshorab, and Matteo. We said bye to Matt who moved to MIT and Nate who moved to Canada, while Daria remains our UK stronghold. Michele got a faculty job, Viola got a postdoc, Davide got a PRIN grant, and Giulia got a SigmaXi grant. We graduated something like 12 BSc students and 4 MSc students (and all 4 of them now have PhD positions). A few long-term visitors (Francesco, Giulia, Harrison) made the group even better for a while. We wrote lots of papers, gave lots of talks, and ate lots of cakes. LIGO is taking data, LISA is being adopted, Virgo has seen better days, and GR is still true. Arianna was in the newspaper, Sshorab broke Davide’s ribs, Alice danced Greek dances, and Costantino got his first American coffee ever. Our gwpopnext conference was a blast and we discussed too much, thunderstorms included.

… now get ready for all the 2024 surprises!

In the gravitational-wave world, we usually say a binary merger is detected if it has a sufficiently large SNR (signal-to-noise ratio). But is that true? Detection pipelines are far more complicated than that. Here we try to figure out a section threshold from what’s detected. That is: (some) people agree that these guys are GWs, so what’s your SNR threshold for detectability? It’s like reading in the minds of a GW data analyst…

Matthew Mould, Christopher J. Moore, Davide Gerosa.
Physical Review D 109 (2023) 063013.
arXiv:2311.12117 [gr-qc].

I’m teaching the first lecture of a new class today. This is “Scientific computing with Python,” a 16h module for PhD students. To the (many) PhD students who signed up: thanks for your interest, hope you’ll like this. BTW the title says Python but there will also be some Mathematica and some git, just for fun. My material is online at

github.com/dgerosa/scientificcomputing_bicocca_2023

Have a look if you want and please do give feedback if you do 🙂

We had another graduation session in November, and a whopping 4 people graduated with research projects in our group. Here are the new BSc physicists who just defended:

Congrats all (and twice congrats to Marco and Serena, who graduated with full marks and honors). It was great working with you. Matteo and Martin are now enrolled in an MSc degree in Artificial Intelligence (good luck!), while Marco and Serena are starting our MSc degree in Astrophysics.

Looks like my name is on a list of the 2% top scientists worldwide. Take these rankings with a grain (or a block) of salt… but this is kind of cool! The list was compiled by Stanford University and bounced by our press office.

This is the latest of the amazing reviews coming out on the LISA space mission. Short post to say that the LISA signal modeling is challenging but so fascinating. Everything you ever wanted to know it’s in here.

Niaesh Afshordi, et al. (105 authors incl. Davide Gerosa).
arXiv:2311.01300 [gr-qc].

I’m obsessed with spinning black-hole binaries but, guys, spinning and eccentric black holes are even better! This is the first first-author paper by Giulia, who is not only a rising GW astronomer but also a semi-professional baker… So take two spoons of black holes, one spoon of spin dynamics, some eccentricity (but less than 0.6 ounces), and a pinch of maths. Put this in a bowl, mix it thoroughly with numerical integrations …and the result is very tasty! Spins and eccentricity shape the dynamics of black-hole binaries together , which means one can hope to measure eccentricity indirectly from the spins, but also that if you forget about eccentricity then your spin inference will be crap. Buon appetito.

Giulia Fumagalli, Davide Gerosa.
Physical Review D 108 (2023) 124055.
arXiv:2310.16893 [gr-qc].

We’ve had four amazing research students graduating with us in October!

After the Master’s defenses, students turned the graduation party into a football supporter thing, with chants and all the rest!

Arianna Renzini (Marie Curie Fellow in my group) is on the Italian national newspaper “Corriere della Sera” today!

Here is the online version: Arianna Renzini, la studiosa di Astrofisica corteggiata da tutto il mondo: «Basta cervelli in fuga, lavorerò a Milano»

But it was also on the printed thing distributed everywhere!

(That picture was taken in my office… So my office is also in the national newspaper today! Wooo!)

…and we’re back to testing GR. We’ve got many gravitational-wave events and would like to use them all together to figure out if our equations for gravity are correct. And here is the issue: there’s only one set (aka catalog) of black holes that contains all the black holes we’ve observed. Now that’s obvious you’d say, and you would be right!, much like we have a single Universe to observe (I’m not a language guy but indeed “Universe” means like “the whole thing”). This effect is known in cosmology (think those low-order multiples in the usual CMB plot), so we called it “the catalog variance of testing GR”. It’s bad, but the Baron Munchauseen tells us we can bootstrap.

Costantino Pacilio, Davide Gerosa , Swetha Bhagwat.
Physical Review D Letters 109 (2024) L081302.
arXiv:2310.03811 [gr-qc].

Our group is getting some tremendous additions, with 5 people joining in the fall of 2023! The scope of our research is getting broader and broader 🙂

We’re soon going to have Giulia Capurri who will be visiting us for a few months from Trieste. Welcome aboard all! There are like 13 people at group meetings now…

Three of our BSc students graduated today.

And, last but not least, let me add Simone Piscitelli, who last week defended his MSc degree at Milano Statale (“the other” University of Milan) supervised by Costantino Pacilio and myself. Simone worked on a cool test of GR. Stay tuned…

Congrats all!

The University of Milano-Bicocca (Italy) invites expressions of interest for postdoctoral positions in gravitational-wave astronomy.

Successful candidates will join the group of Prof. Davide Gerosa and will be part of the “GWmining” project funded by the European Research Council, with additional support from national grants. Targeted investigations focus on the astrophysical exploitation of gravitational-wave data. We are particularly interested in candidates with expertise in population-synthesis simulations of compact binaries, gravitational-wave parameter estimation and population studies, as well as applications of statistical and machine-learning tools to gravity (although we are open to all candidates with a strong gravitational-wave and/or high-energy astrophysics background!). Candidates will have ample opportunities to kickstart new projects with group members and will be strongly encouraged to develop their own independent research lines.

We anticipate awarding up to three positions. Appointments will be for 2+1 years and come with a generous research and travel budget. The starting date is negotiable.

The astrophysics unit at Milano-Bicocca provides a vibrant environment with expertise covering all aspects of gravitational-wave astronomy, relativistic astrophysics, and numerical relativity, as well as a wider astronomical context including observational and experimental activities. The group has tight connections with the LISA Consortium, the Virgo Collaboration, the Einstein Telescope Observational Science Board, the Italian National Institute for Nuclear Physics (INFN), and the Italian Center for Supercomputing (ICSC). Faculty members with matching interests include Gerosa, Sesana, Colpi, Giacomazzo, and Dotti. For more information on Gerosa’s group see https://davidegerosa.com/group

Milan is a beautiful, international city in the north of Italy with history, art, and outstanding food. Mountains and lakes are just around the corner.

Successful candidates will have a PhD in Physics or related discipline, strong programming skills, and previous experience in gravitational (astro)physics. Applications should include a CV with a list of publications and a two-page statement covering research interests and plans. These should be sent by November 15th, 2023 using this web form:

https://forms.gle/hnQc3N1xh53YAziH9

Candidates should also arrange for at least two, but preferably three, reference letters to be sent using the same form by November 15th, 2023.

We strive to build a diverse and inclusive environment and welcome expressions of interest from traditionally underrepresented groups.

For inquiries please do not hesitate to contact Davide Gerosa at [email protected]

Gravitational-wave data keep on giving us surprises. The most outstanding one IMO is an observed correlation between mass ratios and spins of the black holes, which was first found by Tom Callister and friends. That is so, so weird… to the point that virtually zero astrophysical models so far can explain it fully and consistently. Well, we can’t either (at least not fully and consistently) but we think this paper is a nice attempt. The secret seems to be the symmetry of the astrophysical environment one considers, and data tends to prefer black holes assembled in cylindrical symmetry. That’s also weird to be honest, but there’s a candidate for this setup, namely accretion disks and their migration traps. Who knows, more data will tell.

… and huge congrats to my MSc student Alessandro who managed to publish a paper even before graduating!

Alessandro Santini, Davide Gerosa , Roberto Cotesta, Emanuele Berti
arXiv:2308.12998 [astro-ph.HE].
Physical Review D 108 (2023) 083033.
Other press coverage: astrobites.

Two students just completed their Bachelor’s degree with research projects in our group.

I had the honor of heading their graduation committee and could call them “physicists” for the very first time (and the Italian ceremonial sentence is quite imposing: “ coi poteri conferitami… “). Congrats Simone and Leonardo!

Last week my group and I hosted the international workshop “Gravitational-wave populations: what’s next?.” It’s been a blast!

An unconventional conference, with almost zero talks and the vast majority of the time dedicated to discussions. I report the program here below, just to give you a feeling of what we discussed. The conference started with the question “ How many of you entered the field after GW150914? ” and virtually everyone raised their hand! It was so refreshing to see our field is alive.

We then went through population synthesis simulations, fancy statistical methods (I promise I’ll understand nonparametric methods one day!), intricacies of injections, catalogs, and overlap with our EM observer friends. We took a break on Wednesday for a social activity on Lake Como, with some folks diving into the lake and others hiking up to a small castle. All before dinner with a fascinating lake (and thunderstorm!) view.

Thanks all for joining and participating so actively. Huge thanks to Emanuele Berti and Salvo Vitale for co-organizing this with me, as well as the local GW group for assistance. Finally, congrats to Amanda Farah and Alex Criswell who won our SIGRAV early career prize.

And if you couldn’t make it for whatever reason no worries, we’ll do it again!

Conference program in a nutshell. These are our discussion sessions

New paper from a new student! Here is Matteo Boschini’s first piece of work, where we look at predictions for the final mass and spins of black-hole remnants. That is, after two black hole merge, what’s the mass and spin of the guy they left behind? These predictions are typically done by fitting (in various ways) outputs from numerical-relativity simulations but those, unfortunately, can only handle black holes of similar masses. On the other hand, black holes with masses that are very different from each other can be handled analytically. Here we show how to put the two together with a single machine-learning fit.

Matteo Boschini, Davide Gerosa , Vijay Varma, Cristobal Armaza, Michael Boyle, Marceline S. Bonilla, Andrea Ceja, Yitian Chen, Nils Deppe, Matthew Giesler, Lawrence E. Kidder, Guillermo Lara, Oliver Long, Sizheng Ma, Keefe Mitman, Peter James Nee, Harald P. Pfeiffer, Antoni Ramos-Buades, Mark A. Scheel, Nils L. Vu, and Jooheon Yoo.
Physical Review D 108 (2023) 084015.
arXiv:2307.03435 [gr-qc].

The University of Milano-Bicocca (Milan, Italy) will be hiring a PhD student under the Italian National PhD program in Space Science and Technology. Candidates with interests in gravitational-wave astronomy, data analysis, and multi-messenger applications are encouraged to apply. For information please see:

The application deadline is July 6th, 2023 at 4pm CEST. For informal enquiries please contact Monica Colpi ([email protected]).

The advanced class “Big data within science and industry” will take place on September 22nd at the University of Milano-Bicocca (Milan, Italy).

https://sites.google.com/unimib.it/bigdatamasterclass

Data are everywhere. Exploring scientific data is now at the heart of both scientific advances as well as industrial applications. This one-day master class provides a “learn by example” introduction to the fascinating world of big data, namely pieces of information that are so rich and structured that require targeted analysis techniques loosely referred to as machine learning or artificial intelligence.

The class is suitable for advanced MSc students, PhD students, and postdocs who wish to expand their proficiency in handling scientific data. The program features the participation of three world-leading experts from both academia and the private sector, as well as a hands-on experience for all participants.

For students enrolled in the Physics and Astronomy PhD program here at Milano-Bicocca, this 8-hour program will be recognized with 1 CFU. In any case, we are happy to provide attendance certificates.

Interested students should register by ** September 8th, 2023**. Participation is free of charge. We hope to accommodate everyone, but depending on the number of people registering, participants might need to be selected.

Davide Gerosa, Michele Fumagalli (Milano-Bicocca)

All right, this is kind of far from my day-to-day topics but working on this paper with Alice and Riccardo was super fun. Think LISA and supermassive binary black holes. And… the detector does what it wants. That’s not true of course because the experimentalists are amazing, but there will be noise transients: unexpected blips when the gravitational-wave signal will be corrupted. Here we look at what would happen in a realistic setting when a LISA glitch happens on top of a gravitational wave from a supermassive black hole.

Alice Spadaro, Riccardo Buscicchio, Daniele Vetrugno, Antoine Klein, Davide Gerosa , Stefano Vitale, Rita Dolesi, William Joseph Weber, Monica Colpi.
Physical Review D 108 (2023) 123029.
arXiv:2306.03923 [gr-qc].

Happy to report we got a grant from the Italian PRIN program! This is in collaboration with Andrea Maselli from GSSI in L’Aquila. The title is “Gravitational-wave astronomy as a mature field: characterizing selection biases and environmental effects”. Stay tuned for more research (and more positions to join our group!).

Welcome Harrison Blake! My group is hosting a student from the IREU program in Gravitational Physics, which is administered by the University of Florida. Harrison is visiting from Ohio State University and will be working with Michele Mancarella on forecasting the science with can do with gravitational waves from the Moon…

We do population analysis in gravitational waves all the time now. That is: we compare many observations from GW experiments against many simulated datapoints from simulations. But what if you only have one observation? That could be a LIGO guy that is kind of an outlier (think GW190521) or maybe a datapoint from a future detector (think LISA) that feels lonely in his parameter space. Don’t look further, this is stats for you (and Matt’s last paper as a grad student…)

Matthew Mould, Davide Gerosa , Marco Dall’Amico, Michela Mapelli.
Monthly Notices of the Royal Astronomical Society, 525 (2023) 3986–3997
arXiv:2305.18539 [astro-ph.HE].

We’ve got the best name ever for a numerical code! Let me introduce QLUSTER which, guess what, simulates clusters. We finally put out a piece of code that was originally developed in 2019 and later used in several papers. It’s a very very simple treatment of black-hole binary formation in dense stellar environments, with the goal of predicting gravitational waves from repeated mergers. The code is available at github.com/mdmould/qluster and a short description is provided in the proceedings of the 2023 edition of the amazing Moriond conference.

Davide Gerosa , Matthew Mould.
Proceedings of the 57th Rencontres de Moriond.
arXiv:2305.04987 [astro-ph.HE].

This paper is episode four in the up-down instability series. We first figured out the instability exists (episode 1), then computed when binaries go after the instability (i.e. the endpoint, episode 2), and also checked binaries are really unstable in numerical relativity (episode 3). Now we look at the inference problem with LIGO/Virgo: if unstable up-down binaries enter the sensitivity window of the detector, will we be able to tell? We phrased the problem with some fancy stats using the so-called Savage Dickey density ratio, which is the right tool to answer this question. As is too often the case, current data are not informative enough but the future is bright and loud.

Viola De Renzis, Davide Gerosa , Matthew Mould, Riccardo Buscicchio, Lorenzo Zanga.
Physical Review D 108 (2023) 024024.
arXiv:2304.13063 [gr-qc].

It’s out! The notorious (ask my students…) “ precession v2 ” paper is finally out! This took a veeeery long time; we checked and the first commit for this paper is from May 2020 (!). But the result is an exhilarating tour of spin precession at 2PN with 27 pages and 183 (!!!) numbered equations. We rewrote the entire formalism, change how we parametrize things, compute all we could in closed forms, and speed up the computational implementation. It’s cool, now performing a precession-averaged evolution is a <0.1s operation. If you’re into BH binary spin precession, this is the paper for you. All of this is now part v2 of our PRECESSION python module. So long, and thanks for all the spin.

Davide Gerosa , Giulia Fumagalli, Matthew Mould, Giovanni Cavallotto, Diego Padilla Monroy, Daria Gangardt, Viola De Renzis.
Physical Review D 108 (2023) 024042.
arXiv:2304.04801 [gr-qc].
Open-source code: homepage, repository, documentation.

Third-generation gravitational wave detectors are going to see all stellar-mass black-hole mergers in the Universe. Wooooooooo. But hang on, is this enough? Observing the sources is great, but then we need to measure them. Here we try to focus on the latter and quantify how well we will be able to measure the distance of black holes. Read the paper now, but the short answer is that 3G detectors are going to be awesome but not that awesome…

Michele Mancarella, Francesco Iacovelli, Davide Gerosa.
Physical Review D Letters 107 (2023) L101302.
arXiv:2303.16323 [gr-qc].

The University of Milano-Bicocca welcomes applications for PhD scholarships. The application deadline is April 19th, 2023 for positions starting in the Fall of 2023:

https://en.unimib.it/education/postgraduates/doctoral-research-phd-programmes/applying-doctorate/calls-application

In particular, the theoretical astrophysics group is looking for highly motivated candidates to join our activities in black-hole binary dynamics, gravitational-wave data exploitation, and numerical relativity. Faculty members with matching interests include Gerosa, Sesana, Colpi, Dotti, and Giacomazzo. Candidates will have ample opportunities to work with and visit external collaborators as well.

Our PhD admission program includes a number of “open” scholarships, covering all research activities in the department (including ours!). All candidates are considered for those by default. In addition, our group this year is advertising an additional “project” scholarship titled “Gravitational-wave source modeling” and supervised by Gerosa. Candidates wishing to be considered for this additional opportunity should indicate it explicitly when applying (the number of this position FIS.3).

We strive to build an inclusive group and welcome applications from all interested candidates. More information on the astrophysics group at Bicocca can be found at astro.fisica.unimib.it. For informal inquiries and expressions of interest please do not hesitate to contact [email protected]

It’s student time! Massive congratulations to two of my students who just graduated.

The star of the day is Matteo Boschini, who completed his MSc project with me after a long visit at the AEI (Postdam, Germany) to collaborate with Vijay Varma. Matteo worked out an amazing extension of current numerical-relativity surrogate models… stay tuned for a paper because this is going to be cool!

Daniele Chirico completed his BSc studies with a sweet research project on supernova explosions, orbits, and kicks. He’s staying in Milan for his MSc degree now, so wait a bit for his successes!

I’m about to start teaching this year’s edition of “Astrostatistics and Machine Learning” for the MSc degree in Astrophysics here at Milano-Bicocca. The material is available at

github.com/dgerosa/astrostatistics_bicocca_2023

Feel free to have a look if you fancy some stats… and please do send me feedback if you work through the material.

The student reps of our department (codename: redshift) have organized a stellar event today. Curiosity and interest in the LISA space mission brought them to design a full day of talks from leading experts in the field. They put Stefano Vitale, Alessandra Buonanno, and Bernard Schutz in the same room with the (astro)physics students and, well, a few of us who tagged along. The result was an amazing rollercoaster called “LISA and beyond” across the wonders of the experimental design by Stefano (is this truly going to work?!?), some amazing order-of-magnitude calculations that Bernard pulled off (wish I could do that!), and a broad vision by Alessandra across the discoveries we had and those we will soon be seeing (can’t wait, can’t wait!). Our students engaged with the speakers, asked questions, and organized a round table touching topics like the carbon footprint of space missions, gender equality, and how to manage a research group. Such ingenuity and enthusiasm are what keeps science alive! We should learn from our students and do science like that.

It is a pleasure to announce the workshop “Gravitational-wave populations: what’s next?” which we are currently organizing for next summer:

https://sites.google.com/unimib.it/gwpopnext

As the catalog of detected gravitational-wave events grows from O(10) to O(100) sources (but think millions in a few decades!), such increasingly detailed information is allowing us to dig deeper into the (astro)physics of compact objects. At the same time, new and more data require appropriately powerful statistical tools to be fully exploited. This highly interactive workshop (fewer talks, more working together!) will be the opportunity to share recent progress, identify what new steps are now needed, and hopefully set the stage for substantial progress in the field.

The workshop will take place on July 10-14, 2023 at the University of Milano-Bicocca, which is located near the city center of Milan, Italy. Milan is a beautiful, international city in the north of Italy and is served by three major airports with worldwide connections. The city is home to art, history, and great food; nearby excursions will take you to the Italian lakes and the stunning Alps.

While we are unable to provide travel support, the workshop will have no registration fee. The workshop will be in person without remote options.

Interested participants should register on the conference website by March 1st, 2023. Depending on the number of people registering, participants might need to be selected. We will be in touch soon after the registration deadline, so please do not make travel plans until you hear back from us. When registering please indicate which of the discussion session(s) you would like to contribute to. Early career scientists will have the opportunity to give flash talks highlighting their science.

Davide Gerosa (Milano-Bicocca), Emanuele Berti (Johns Hopkins), Salvatore Vitale (MIT)

Welcome to 2023… and what better way to start the new year than welcoming a new friend! Alice Spadaro (who has recently graduated with an MSc degree here in Milan) is now officially starting her PhD in my group. Alice always smiles, likes surfing, and of course is into gravitational waves 🙂 .

Huge congrats to two of my students who graduated today! Matteo Muriano completed a funny BSc project on black-hole merger trees. And Giovanni Cavallotto went all in for his MSc research: he basically “fixed” black-hole binary spin precession at 2PN! (which is pretty cool, stay tuned for these results!). They both defended quite brilliantly, good luck with everything now!

We want more! With gravitational-wave data, some quantities like the masses of the black holes are much easier to see than others. But those others are very interesting, notably spins that process and orbits that are eccentric, because they would tell us how black hole binaries came to be in the first place. So while it would be great to see those, it’s also being very hard. Some tentative claims have been made with current data, but nothing unambiguous so far. In this paper led by Isobel from Cambridge, we show that (surprise surprise…) the signals needs to be long enough before one can tell eccentricity and spin precession apart.

Isobel Romero-Shaw, Davide Gerosa , Nicholas Loutrel.
Monthly Notices of the Royal Astronomical Society 519 (2023) 5352–5357.
arXiv:2211.07528 [astro-ph.HE].

Together with my postdoc Nate, we’re proceeding our investigations on supermassive, spinning binary black holes surrounded by accretion disks (that is: a ton of gas around big monsters at the center of galaxies!). In today’s paper, we dig a bit deeper into what happens when the disk breaks. That presumably stops the interactions between the gas and the black-hole spins which could make all this funky astrophysics (spins that moves, disks that breaks, etc) actually observable with future gravitational-wave detectors. More needs to be done of course, but here we are.

Nathan Steinle, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society 519 (2023) 5031–5042.
arXiv:2211.00044 [astro-ph.HE].

More graduations today! I had the pleasure to see three of my students defending their scientific work. Lorenzo Zanga completed his BSc project on unstable spinning black-hole binaries, Alessandro Carzaniga defended his MSc thesis on gaussianities in the LISA detector, and Alice Spadaro also presented her MSc-thesis work on the LISA mock data challenge. It’s so great to see students reaching the point of defending/arguing/explaining their science… I think it’s actually one of the best things about my job! Thank you all for sharing these months with me, I’ll see you around! (And thanks to Viola De Renzis and Riccardo Buscicchio who co-supervised Lorenzo, Alessandro, and Alice with me).

My group is hosting quite a few visitors this semester. We’re alive!

So many new people are joining us this Fall!

Welcome everybody, it’s an honor you decided to do science with us! You can read their profiles here. And if you’re also interested in my group, we have multiple openings right now. Consider applying!

The University of Milano-Bicocca (Italy) invites expressions of interest for postdoctoral positions in gravitational-wave astronomy.

Successful candidates will join the group of Prof. Davide Gerosa and will be part of the “GWmining” project funded by the European Research Council. Targeted investigations focus on the astrophysical exploitation of gravitational-wave data. We are particularly interested in candidates with expertise in population-synthesis simulations of compact binaries, gravitational-wave parameter estimation and population studies, and numerical-relativity surrogate modeling (although we are open to all candidates with a strong gravitational-wave and/or high-energy astrophysics background!). Candidates will have ample opportunities to collaborate and kickstart new projects with group members and will be strongly encouraged to develop their own independent collaborations.

We anticipate awarding up to three positions. Appointments will be for a three-year term and come with generous research and travel budget. The starting date is negotiable.

The astrophysics group at Milano-Bicocca provides a vibrant environment with expertise covering all aspects of gravitational-wave astronomy, relativistic astrophysics, and numerical relativity, as well as a wider astronomical context including observational and experimental activities. The group has tight connections with the LISA Consortium, the Virgo Collaboration, the Einstein Telescope Observational Science Board, the Italian National Institute for Nuclear Physics (INFN), and the newly formed Italian Center for Supercomputing (ICSC). Faculty members with matching interests include Gerosa, Sesana, Colpi, Giacomazzo, and Dotti. For more information on Gerosa’s group see https://davidegerosa.com/group

Milan is a beautiful, international city in the north of Italy with history, art, and outstanding food. Mountains and lakes are just around the corner.

Successful candidates will have a PhD in Physics or related discipline, strong programming skills, and previous experience in gravitational (astro)physics. Applications should include a CV with a list of publications and a two-page statement covering research interests and plans. These should be sent by November 18th, 2022 using this web form:

https://forms.gle/hnQc3N1xh53YAziH9

Candidates should also arrange for at least two, but preferably three, reference letters to be sent using the same form by November 18th, 2022.

We strive to build a diverse and inclusive environment and welcome expressions of interest from traditionally underrepresented groups.

For inquiries please do not hesitate to contact Davide Gerosa at [email protected].

Wooo! What an amazing performance by two of my students today, who defended their BSc and MSc degrees! Oliver Rossi discussed his BSc project on black holes with large spins completed in collaboration with Viola De Renzis (PhD student in my group). Andrea Geminardi presented the results of his MSc thesis. Andrea studied the stochastic gravitational-wave background with myself, Riccardo Buscicchio (postdoc here in Milan), and Arianna Renzini (postdoc at Caltech). Hope you guys had fun working with us, we certainly did! (and I’m sorry for my pain-in-the-*** comments on your plots…). All the best for what comes next!

The Italian government has pushed a hiring program dedicated to holders and applicants of Marie Curie Fellowships from the EU. The call targets those that have either (i) completed a successful Marie Curie Fellowship in the past 4 years or (ii) applied unsuccessfully in the past 4 years but were awarded the so-called “Seal of Excellence”.

For both categories, successful candidates will be awarded a 3yr senior researcher position (at the so-called RTDA level in the Italian system). RTDAs are hired as full employees with related benefits and have limited teaching duties. On top of this, candidates in the Marie Curie winners strand (i) will also be offered a substantial startup grant to hire their own PhD students and postdocs.

All Italian institutions can act as hosts, so I encourage you to contact one of us in the country for more information.

In particular, the gravitational-wave group at the University of Milano-Bicocca provides a vibrant environment with activities ranging from relativistic astrophysics. gravitational-wave data analysis, numerical relativity, and gravity theory. The group counts faculty members Gerosa, Sesana, Colpi, Giacomazzo, and Dotti as well as tens of students and postdocs. The city of Milan is a jewel in the north of Italy with a charming international vibe (as well as mountains, history, art, and outstanding food).

The internal application deadline is October 18th. If you’re eligible and/or interested in applying with us, please get in touch asap ([email protected]) and we’ll go from there.

Here are the relevant webpages (scroll down for the English text):

(i) Marie Curie past winners

https://www.unimib.it/ricerca/opportunita/finanziamenti-alla-ricerca/finanziamenti-nazionali/bando-giovani-ricercatori-vincitori-msca-young-researchers-msca-grants-winners

(ii) Seal of Excellence holders:

https://www.unimib.it/ricerca/opportunita/finanziamenti-alla-ricerca/finanziamenti-nazionali/bando-giovani-ricercatori-seal-excellence-msca-call-young-researchers-seal-excellence-msca

The Italian government is pushing a major inverstment program in High-Performance Computing, and we’re part of it! The new ICSC (Italian Center for Supercomputing) will manage >300M Euros going towards early-career researchers, PhD scholarships, and computing infrastructure. The University of Milano-Bicocca is part of the founding member of ICSC, with our research group providing some core activities for the Bicocca contribution. If you’re interested in computational (astro)physics, stay tuned for several upcoming opportunities!

Lots of “firsts” today! My first -year PhD student Viola just put out her first first -author paper. This is about measuring black holes with not one, but two precessing spins. People have been trying to figure out how to tell if at least one of the two spins of a merging black-hole binary is precessing for quite some time now. And maybe we’ve even done it already for one or two of the current LIGO-Virgo events. But here I must quote that epic Italian commercial from the 90s: “two gust is megl che one” (which is a terrible Italian-English mishmash on a terrible joke to say that when you eat a Maxibon “two flavors are better than one”). In this paper we propose a strategy to identify sources that have the strongest evidence of two processing spins. Viola has been putting together simulated data for the next LIGO/Virgo data-taking period, and the result is pretty cool. If these binaries are out there in the Universe, we will be able to tell they have two spins going around!

Viola De Renzis, Davide Gerosa, Geraint Pratten, Patricia Schmidt, Matthew Mould.
Physical Review D 106 (2022) 084040.
arXiv:2207.00030 [gr-qc].

Very happy to report that Arianna Renzini (currently a postdoc at Caltech) was awarded a prestigious Marie Skłodowska-Curie Fellowship from the European Union, to be hosted here with my group. Arianna will bring expertise in modeling the gravitational-wave stochastic background, which is a key target for both current and future experiments. Arianna’s proposal is titled “ Stochastic rewind and fast-forward: calibrating LISA with LIGO’s black holes and stochastic background.” Huge congrats, can’t wait to welcome you here.

We’re having four (!) summer students joining the group this year!

Welcome all! We look forward to seeing your summer discoveries!

Big stars burn everything they have, die fast, and produce big black holes. So when you see two black holes together, it’s likely that the big black hole comes from the big star. Or maybe not? Before dying, the big star can drop some mass onto the other guy, making it bigger! So now, the initially big star still produces the first black hole, but, at the end of the day, that might not be the more massive black hole anymore! This scenario is called “mass-ratio reversal” and our astrophysics friends have put together many models out there showing this is indeed possible for a good fraction of the black holes that produce gravitational-wave events. So here we ask the data: given the events LIGO and Virgo have seen so far, what’s the evidence for mass-ratio reversal in binary stars? Read Matt’s paper to find out.

Matthew Mould, Davide Gerosa , Floor S. Broekgaarden, Nathan Steinle.
Monthly Notices of the Royal Astronomical Society 517 (2022) 2738–2745.
arXiv:2205.12329 [astro-ph.HE].

Another short post today, but it’s not just astrophysics that will be awesome with LISA, but fundamental physics too! Here is the white paper of the relevant LISA working group. Get ready to test your wildest ideas, my theory friend!

K. G. Arun, et al. (141 authors incl. Davide Gerosa)
arXiv:2205.01597 [gr-qc].

The University of Milano-Bicocca welcomes applications for Ph.D. scholarships. The application deadline is May 20th, 2022 for positions starting in the Fall of 2022:

https://en.unimib.it/education/postgraduates/doctoral-research-phd-programmes/applying-doctorate/calls-application

In particular, the theoretical astrophysics group is looking for strong, highly motivated candidates to join our activities in black-hole binary dynamics, gravitational-wave data exploitation, and numerical relativity. Faculty members with matching interests include Gerosa, Sesana, Colpi, Dotti, and Giacomazzo. The candidates will have ample opportunities to work with and visit external collaborators as well.

Our PhD admission program includes a number of “open” scholarships, covering all research activities in the department (including ours!). All candidates are considered for those by default. In addition, our group sponsors two specific positions:

Candidates wishing to be considered for these additional positions should mention it explicitly in their application.

More information on the astrophysics group at Bicocca can be found at astro.fisica.unimib.it. For informal inquiries please do not hesitate to contact [email protected] or [email protected].

The University of Milano-Bicocca (Italy) invites expressions of interest for a 3+2 year research position in HPC applications to astrophysics.

The astrophysics group at Milano-Bicocca provides a vibrant environment with expertise covering all aspects of gravitational-wave astronomy, relativistic astrophysics, galactic dynamics, and numerical relativity. This is embedded in a wider astronomical context including both observational and experimental activities. Our group has tight connections with the LISA Consortium, the Virgo Collaboration, the Einstein Telescope Science Board, the European Pulsar Timing Array, and the Italian National Institute for Nuclear Physics (INFN) via the TEONGRAV national initiative. Staff members with matching interests include Colpi, Dotti, Gerosa, Giacomazzo, Lupi, and Sesana.

Milan is a beautiful, international city in the north of Italy. Mountains and lakes are just around the corner. Art, culture, and food are outstanding. The city hosts three international airports with worldwide connections.

This recruitment campaign is part of a wider national initiative supporting HPC-related computational activities throughout the country. This is a major investment program directly supported by the European Union. It will provide the most ideal context for ambitious candidates wishing to develop and apply state-of-the-art computational and machine-learning tools to current astrophysical and gravitational-wave modeling issues.

The researcher will be appointed at the so-called “RTDA” level for 3 years. The contract can also be extended for 2 more years depending on funding availability. The starting date is negotiable, with the earliest and latest dates on January 1st, 2023 and May 1st, 2023, respectively. RTDA researchers are full-time university employees (with full benefits, such as health insurance and pension plan), have limited teaching duties, and are eligible to fully supervise research MSc student projects. This is an ideal setup for early-career researchers wishing to transition toward research independence and start developing their own group.

The successful candidate will have a PhD in Physics, Astronomy, Computer Science, or related discipline, strong programming skills, and previous experience in one or more of the following topics: HPC workflows, GPU software development, computational astrophysics, gravitational-wave astronomy, numerical relativity, statistical data analysis, machine learning.

Applications should include a CV with a list of publications and a two-page statement covering research interests and plans. These should be sent to [email protected] by June 15th, 2022 for full consideration. Candidates should also arrange for two reference letters to be sent to [email protected] by June 15th, 2022.

We strive to build a diverse and inclusive environment and welcome expressions of interest from traditionally underrepresented groups. Women are especially encouraged to apply. For inquiries please do not hesitate to contact Bruno Giacomazzo ([email protected]) or Davide Gerosa ([email protected]).

I was just awarded a large allocation on the Italian national supercomputer at CINECA. My PhD student Viola De Renzis (our parameter-estimation expert!) is the co-I on our proposal. Our award is part of the so-called ISCRA Class B program (which is their medium-size allocation scheme) and amounts to 1.2M CPUh on the Galileo cluster (that is: we’re going to have to crunch a ton of numbers now!). Viola and I will study the extraction of spin-spin couplings from black-hole binaries using gravitational-wave data and stochastic sampling techniques. Stay tuned!

Observing gravitational waves from the ground (i.e. LIGO, Virgo, etc) give us a unique view on “the last three minutes” of the life of compact objects before they merge with each other. Going to space (I’m talking to you, LISA!) will instead give us “the last three years”. Completed together with the rest of the Birmingham crowd, this paper provides a realistic view of this truly amazing landscape. LISA observations at low frequencies in the 2030s will be paired with high-frequency data from LIGO’s successors (the so-called 3rd generation detectors). Together (and that’s crucial, together!) LISA and 3g detectors will tell us the full story of the life of merging black holes. LIGO alone is like catching up with a movie because you were late at the theatre, LISA alone is like a huge cliffhanger before the series finale… multiband observations are a bingewatching experience!

Antoine Klein, Geraint Pratten, Riccardo Buscicchio, Patricia Schmidt, Christopher J. Moore, Eliot Finch, Alice Bonino, Lucy M. Thomas, Natalie Williams, Davide Gerosa , Sean McGee, Matt Nicholl and Alberto Vecchio.
arXiv:2204.03423 [gr-qc].

Daria’s new paper is out! (With key contributions from others in the group… This is also Viola’s first paper!).

Here we look at sub-dominant black-hole spin effects in current data from LIGO and Virgo (yeah sorry guys… our black-hole spin obsession goes on). People have looked at spin precession before, but we’re interested in even more subtle things, namely disentangling precession and nutation. This is a tricky business, which is made complicated by the fact that this piece of information is hidden behind other parameters that are easier to measure (say the masses of the two black holes). Our paper is an attempt to formulate and systematically exploit something we called “sequential prior conditioning” (which is: mix&match priors and posteriors in Bayesian stats…). Results are weak today but strong tomorrow.

Daria Gangardt, Davide Gerosa , Michael Kesden, Viola De Renzis, Nathan Steinle.
Physical Review D 106 (2022) 024019.
arXiv:2204.00026 [gr-qc].

LISA astrophysics is awesome and everything you might ever want to know is written this paper. [Sorry for the short blog post, but there isn’t much else to say really…] A huge thanks to all the captains that put this massive community-wide effort together.

Pau Amaro-Seoane, et al. (155 authors incl. Davide Gerosa).
arXiv:2203.06016 [gr-qc].

It took a while (so many technical challenges…) but we made it! Matt‘s monster paper is finally out!

Let me introduce a fully-fledged pipeline to study populations of gravitational-wave events with deep learning. If it sounds cool, well, it is cool (just look at the flowchart in Figure 1!). We can now perform a hierarchical Bayesian analysis on GW data but, unlike current state-of-the-art applications that rely on simple functional form, we can use populations inferred from numerical simulations. This might sound like a detail but it’s not: it’s necessary to compare GW data directly against stellar physics. While we don’t do that yet here (our simulations are admittedly too simple), there’s a ton of astrophysics already in this paper. Whether you care about neural networks or hierarchical black-hole mergers (or, why not, both!), sit tight, fasten your seatbelt, and read Matt’s paper.

Matthew Mould, Davide Gerosa , Stephen R. Taylor.
Physical Review D 106 (2022) 103013.
arXiv:2203.03651 [astro-ph.HE].

I just had the first lectures of a class I’m teaching for the first time: Astrostatistics and Machine Learning (sounds exciting? Well, it is!). This is an advanced course for the MSc degree in Astrophysics and Space Science at the University of Milano-Bicocca. My students and I will travel across data inference, Bayesian wonders, sampling, regression, classification, and become best friends with deep learning. All of this is applied to astrophysical datasets.

The entire class is available under the form of jupyter notebooks at github.com/dgerosa/astrostatistics_bicocca_2022. The repository is hooked up with the mybinder service.

Huge congrats to my student Cecilia Fabbri who got her Bachelor’s degree today. Cecilia defended (quite brilliantly!) her project titled “Constraining the black-hole irreducible mass with current gravitational-wave data”. Her work ended up in our recent draft (arxiv:2202.08848). Cecilia is continuing with a Master’s degree in astrophysics at Milano-Bicocca, stay tuned for her future successes!

Spinning black holes are weird (well, all black holes are weird but those that spin are the worse!). They have a funny thing called ergoregion where orbiting particles can have negative energy. Penrose was the first to realize that this can be exploited to extract energy from the black hole itself. The thing is, even if you figure out how to do it, you’re inevitably going to spin the black hole down. At the end of the day, you’re left with a fossil black hole that does not have any spin. The mass of that leftover black hole (“ What’s for lunch dear? Fancy some sushi or prefer a black hole?”) is called irreducible mass. Hawking (another giant!) figured out this has to do with thermodynamics.

Long story short, in this paper we compute the irreducible mass of the black holes detected in gravitational waves by LIGO. It was funny to re-discover that gravitational wave detection was indeed the motivation behind Hawking original proof of the area theorem (he had Weber‘s claimed detection in mind at the time). The story behind our paper starts as a toy calculation with my undergraduate student Cecilia and ended up in a neat, hopefully informative exploitation of LIGO data. We reparametrized LIGO’s black-hole properties using the rotational and rotational contributions to their total energy, we ranked current gravitational-wave events according to their “irreversibility”, and we compute a sort of population version of the area law. Enjoy!

Davide Gerosa , Cecilia Maria Fabbri, Ulrich Sperhake.
Classical and Quantum Gravity 39 (2020) 175008.
arXiv:2202.08848 [gr-qc].

Traveling is (kind of) coming back, and we’re having lots of visitors around, all supported by external research grants (congrats folks, you’re great!)

Safe travel everyone, it’s time we move our group meetings to a larger room.

I’m very happy to share that I was awarded a 330k EUR grant from the Cariplo Foundation. Cariplo is a trust that supports scientific research in the Lombardy region of Italy. Our proposal is titled “Deep into the relativistic two-body problem“, though arguably a better title could have been “more black-hole fun coming our way”!

My group and I are now part of TEONGRAV, which is the Italian national initiative dedicated to gravitational theory and phenomenology. TEONGRAV is run by the INFN (National Institute for Nuclear Physics) and, besides the other folks here in Milan, it counts members from Florence, Rome, Naples, Padua, Trento, and Trieste. Looking forward to new exciting collaborations, all surrounded by good Italian coffee of course!

Breaking things is fun! In the previous paper of this series, we looked at accretion disks around massive black-hole binaries and found things were going awry. We kept on finding configurations that our implementation could not handle… And now we know this is real! Finding disk solutions when the spin of the black hole has a large misalignment is just not possible! And that’s because the disk really breaks into different sections. We’ve now checked it with state-of-the-art hydrodynamical numerical simulations that not only confirm what we suspected but also show some funny things (like breaking being prevented by disk spirals, etc). I was serious, breaking things is real fun!

Check out Rebecca’s beautiful movies!

Rebecca Nealon, Enrico Ragusa, Davide Gerosa , Giovanni Rosotti, Riccardo Barbieri.
Monthly Notices of the Royal Astronomical Society 509 (2022) 5608–5621.
arXiv:2111.08065 [astro-ph.HE].

Great Scott, a new paper! When analyzing gravitational-wave data, looking at one black hole at a time is not enough anymore, the fun part is looking at them all together. The issue Matt and I are tackling here is that one needs to be consistent with putting together different events when fitting the entire population. This is obvious for things that do not change (say the masses of the black holes, those are what they are), but becomes a very tricky business for varying quantities (say the spin directions, which is what we look at here). In that case, it’s dangerous to put together events taken at different stages of their evolution. And the solution to this problem is…. time travel! We show that but propagating binaries backward in time, one can put all sources on the same footing. After that, estimating the impact of the detector requires traveling forward in time, so going “back to the future”. After all, we all know that post-Newtonian black-hole binary integrations look like this:

Matthew Mould, Davide Gerosa.
Physical Review D 105 (2022) 024076.
arXiv:2110.05507 [astro-ph.HE].

The University of Milan-Bicocca (Italy) invites expressions of interest for postdoctoral positions in gravitational-wave astronomy.

Successful candidates will join Prof. Davide Gerosa and will constitute the core team of the “GWmining” project funded by the European Research Council. Targeted investigations include applications of machine-learning techniques to gravitational-wave physics, modeling of black-hole binary populations from their stellar progenitors, relativistic dynamics, and statistical inference. Candidates will have ample opportunities to explore other areas of gravitational-wave astronomy and will be encouraged to develop independent collaborations.

We anticipate awarding two positions. Appointments will be for a three-year term and come with generous research and travel budget. The starting date is negotiable.

The astrophysics group at Milan-Bicocca provides a vibrant environment with expertise covering all aspects of gravitational-wave astronomy, relativistic astrophysics, and numerical relativity, as well as a wider astronomical context including observational and experimental activities. The group has tight connections with the LISA Consortium, the Virgo Collaboration, and the Italian National Institute for Nuclear Physics (INFN) via the TEONGRAV national initiative. Faculty members with matching interests include Gerosa, Sesana, Colpi, Giacomazzo, and Dotti.

Milan is a beautiful, international city in the north of Italy. Mountains and lakes are just around the corner.

Successful candidates will have a PhD in Physics or related discipline, strong programming skills, and previous experience in one or more of the following topics: gravitational-wave astronomy, stellar evolution, relativistic astrophysics, general relativity, machine learning, statistical inference.

Applications should include a CV with a list of publications and a two-page statement covering research interests and plans. These should be sent to [email protected] by December 1st, 2021 for full consideration. Candidates should also arrange for at least two, but preferably three, reference letters to be sent to the same address by December 1st, 2021. We strive to build a diverse and inclusive environment and welcome expressions of interest from traditionally underrepresented groups.

For inquiries please do not hesitate to contact Davide Gerosa at [email protected].

Viola De Renzis is the latest addition to our group! Viola graduated from Rome “La Sapienza” with an MSc thesis on exotic compact objects and is now starting her PhD with me at Milan-Bicocca. Viola plays guitar, arguably better than Matt (although he runs for a million miles, and that’s when he’s tired), while Daria remains by far the best fencer in the group. Welcome, we all look forward to working with you!

It is a true honor to receive the career Prize for Young Researchers of the Italian Society for General Relativity and Gravitational Physics (SIGRAV). I was awarded the prize in the class of relativistic astrophysics. It’s amazing to be recognized in my home country; it’s great to be back! Let me thank all my mentors, advisors, collaborators, and now students who are walking with me in the adventure of science.

Here is me with the president of the society Fulvio Ricci. And here are press releases from the University of Milan-Bicocca and the INFN.

We moved! I’ve had the opportunity to relocate to Milan, in the north of Italy, very close to where I’m from. I’m now an Associate Professor at the University of Milan-Bicocca, one of the two campuses in the beautiful city of the “Madonnina“. Some of the folks in my group will be visiting Milan very often, and (spoiler alert!) we’re going to have new additions soon. I’m sad to leave the amazing group in Birmingham, but also very excited at this new tremendous opportunity.

The University of Milano-Bicocca welcomes applications for Ph.D. scholarships. The application deadline is June 16th, 2021 for positions to start later in 2021:

https://en.unimib.it/education/doctoral-research-phd-programmes/how-apply-phd-programme

In particular, I am looking for a strong, highly motivated candidate to join my newly established research group supported by the European Research Council. The candidate will work toward interpreting the phenomenology and the astrophysics of gravitational-wave sources using innovative machine-learning techniques. My activities are embedded within the wider Astrophysics group at the University of Milano-Bicocca –a world-leading research environment in strong gravity and relativistic astrophysics. Faculty members with matching interests include Colpi, Sesana, Dotti, and Giacomazzo. The candidate will have ample opportunities to work with and visit external collaborators as well.

This specific position is titled “Large catalogs of gravitational-wave events with machine learning”. Interested candidates should mention it explicitly in their application.

Milan is a beautiful, international city in the north of Italy. Mountains and lakes are just around the corner. For further information and informal inquiries please do not hesitate to contact me ([email protected]).

Spin precession in stellar-mass black hole binaries encodes information on specific formation mechanisms like tides and mass transfers. My first paper on spin precession…

Davide Gerosa , Michael Kesden, Emanuele Berti, Richard O’Shaughnessy, Ulrich Sperhake.
Physical Review D 87 (2013) 10, 104028.
arXiv:1302.4442 [gr-qc].

My very first paper was on alignment between supermassive black holes and their accretion disks. Maybe it takes a bit longer…

Giuseppe Lodato, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society Letters 429 (2013) L30-L34.
arXiv:1211.0284 [astro-ph.CO].

The quest of finding their astrophysical origin of merging black-hole binaries is now a key open problem in modern astrophysics. Stars are the natural progenitor of black holes: at the end of their lives, the core collapses and leaves behind a compact object. But once those “first-generation” black holes are around, they can potentially meet again and form “second generation” LIGO events. I first got interested in this problem in 2017 and, together with many many others researchers in the community, we explored the consequences of this “hierarchical merger” scenario in terms of both gravitational-wave physics and astrophysical environments. In this Nature Astronomy review article, Maya and I tried to condense all this body of work into a few pages. The result is (we hope) a broad and informed overview of this emerging research strand, with a whopping number of more than 270 citations! Hope you like it.

Davide Gerosa , Maya Fishbach.
Nature Astronomy 5 (2021) 749-760.
arXiv:2105.03439 [astro-ph.HE].
Review article.
Press release : Birmingham.
Other press coverage: SciTechDaily, techexplorist, sci-news, Media INAF, globalscience, futura-sciences, europapress, la Razon, astroblogs, phys.org, ScienceDaily, Mirage News Australia, World News Monitor, nanowerk, newsbeezer, SpaceDaily.

Spin precession in stellar-mass black hole binaries encodes information on specific formation mechanisms like tides and mass transfers. My first paper on spin precession…

Davide Gerosa , Michael Kesden, Emanuele Berti, Richard O’Shaughnessy, Ulrich Sperhake.
Physical Review D 87 (2013) 10, 104028.
arXiv:1302.4442 [gr-qc].

My very first paper was on alignment between supermassive black holes and their accretion disks. Maybe it takes a bit longer…

Giuseppe Lodato, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society Letters 429 (2013) L30-L34.
arXiv:1211.0284 [astro-ph.CO].

I was recently interviewed for the Italian Week of Astronomy (“Settimana dell’Astronomia”), a science festival organized by Fondazione Lombardia per l’Ambiente and supported by various Italian associations, universities, and research centers. Here is the nice clip they put together (I mean, I’m terrible at speaking to a camera, but they assembled it very well!).

I was recently interviewed for Scientific American about my recent paper on multiple-generation black holes in stellar clusters. Here is the article: “Black Hole Factories May Hide at Cores of Giant Galaxies”. Very happy to be quoted saying “I don’t think we’ve been hitting this problem hard enough”. I think it’s a nice summary of scientific research –so much to discover!

I was interviewed by our local Cambridge TV. It was a funny experience: they asked me about black holes, gravitational waves, and black hole kicks.

I wrote a post for The Birth of an Idea, which is a really beautiful blog collecting insights on how scientists start their science. Thanks Vitor for the opportunity to contribute! Here is my post:

An idea, a good one at least, is like a gift. It’s something which is not yours (indeed, you didn’t have it before!) but comes to you, it’s given to you.

I bike to work, it’s kind of ten minutes from my place to the Cambridge Maths department, but those ten minutes can be more productive than ten hours or ten days in front of my computer’s screen. It’s morning, your mind should be clear (you should pay attention to cars while biking!), but it’s actually already getting full of what you have to do today. You get to the office, sit down, turn your computer on, and start looking at your problem. You write the equations down, try putting them in a computer, it doesn’t work, just nans coming out. You ask a collaborator who hopefully knows something, write the equations down again, it doesn’t work. You check in a paper if someone else did something similar, take a break, get annoyed (and here I typically open football websites…). Oh, and you write the same equations down again, it simply doesn’t work.

At some stage, it’s time to go home, and that moment is precious to me. You know your problem so well, those equations, that crashing piece of code, but you were looking too close. When I close my laptop and get on my way home, fresh air on my face, I can look at the problem from afar. It’s like looking at those beautiful ancient mosaics. If you look very close, you only see one colored piece, but you can’t see any meaning in it. Each piece is crucial to the final piece of art, but the value of each piece is its relation to the bigger picture. You can only appreciate a mosaic if you take one step back and look to the whole picture from afar. Wow. Biking home is my step back. You’ve been looking at all pieces for days, weeks, you know the color of each piece so well that you can finally grasp the relation which puts them together.

An idea, a good one at least, is like a gift you can say thanks for.

Black holes on eccentric orbits… what does it even mean? The hard (but fun) thing is that we work in General Relativity, where coordinates don’t have a physics inside. One can always change the coordinates as they want, so they can’t be used to define observables. The eccentricity of an orbit has to do, indeed, with the shape of the orbit itself, and that can be transformed away with suitable coordinates. So, does it even sense to measure the orbital eccentricity of black-hole binaries? The one thing we are allowed to do is to find a coordinate-free estimator in General Relativity that reduces to the eccentricity we all know and love in the Newtonian limit. This is possible! The right mathematical framework for this is something called “catastrophe theory”, a funny name, but Nick likes it.

Matteo Boschini, Nicholas Loutrel, Davide Gerosa , Giulia Fumagalli.
Physical Review D 111 (2025) 024008.
arXiv:2411.00098 [gr-qc].

This is a fun IMBH story we worked out when Kostas and Luca were visiting last summer from JHU. What if (one day, who knows) we observe a highly spinning intermediate-mass black hole? If that happens, is going to be puzzling because IMBH that grow in clusters by mergers of smaller black holes tend to spin down, not up. This is a funny property of black holes, namely that extracting spins is easier than putting it in, so on average black holes slow down after they have merged many times. So if we see an IMBH with large spins, the spin must come from somewhere else. Where? Maybe gas. The argument then is that one can actually convert an IMBH spin measurement into the minimum amount of gas that must have been accreted to get that spin.

Konstantinos Kritos, Luca Reali,Davide Gerosa , Emanuele Berti.
Physical Review D 110 (2024) 123017.
arXiv:2409.15439 [astro-ph.HE].

To Stars or to gas, that is the question.
Whether ’tis nobler in the hardening to suffer
The slings and arrows of passing stars,
Or to dissipate against a sea of gas
And by disk end them. To inspiral — to merge,
No more; and by LISA to say we end
The models and the thousand PE samples
That gravity is heir to.

Alice Spadaro, Riccardo Buscicchio, David Izquierdo-Villalba, Davide Gerosa , Antoine Klein, Geraint Pratten.
Physical Review D 111 (2025) 023004.
arXiv:2409.13011 [astro-ph.HE].

The ringdown is the final bit of a gravitational-wave signal, after the two black holes have merged. It’s nice because it’s clean; GR is so powerful that all that comes out after a black hole merger has specific frequencies, the fantastic “quasi-normal modes.” While the frequencies only depend on that final BH (thanks Kerr!), the excitations of those frequencies depend on all that happened before, i.e. the merger process itself. In this summer paper by Costantino and the rest of us, we present a new accurate approximant to those amplitudes. Now go home and test GR.

Costantino Pacilio, Swetha Bhagwat, Francesco Nobili, Davide Gerosa.
Physical Review D 110 (2024) 103037.
arXiv:2408.05276 [gr-qc].

The orbits of binary black holes could be eccentric, but in practice they’re not. At least when we observe them, and that’s because of a relativistic effect that circularizes the orbit. Even if astrophysics formed black holes eccentric, relativity makes them circular when we observe them with gravitational-wave interferometers. But we’re interested in the astrophysics back then! What we find here is that the tiny residual eccentricity at detection can be crucial. Even eccentricities that are so small that we cannot tell them apart from circular can mess up the astrophysical inference. Unfortunately, this is a new systematic error that needs to be taken into account: inferring the “formation channel” of binary black holes might be even harder than we thought.

Giulia Fumagalli, Isobel Romero-Shaw, Davide Gerosa , Viola De Renzis, Konstantinos Kritos, Aleksandra Olejak.
Physical Review D 110 (2024) 063012.
arXiv:2405.14945 [astro-ph.HE].

This is the first of two papers on the arxiv today: it’s fun when two long, very different projects by different people just happen to be done on the same day! This paper is by my former colleague Nate Steinle (now a postdoc in Manitoba, Canada). Here we connect the dynamics of jets in AGN disks to the spin of black holes observable by LISA. And show the latter is a diagnostic of the former! And it’s nice to see my disk-binary code being used for something I didn’t think of when I wrote it.

Nathan Steinle, Davide Gerosa , Martin G. H. Krause.
Physical Review D 110 (2024) 123034.
arXiv:2403.00066 [astro-ph.HE].

In the gravitational-wave world, we usually say a binary merger is detected if it has a sufficiently large SNR (signal-to-noise ratio). But is that true? Detection pipelines are far more complicated than that. Here we try to figure out a section threshold from what’s detected. That is: (some) people agree that these guys are GWs, so what’s your SNR threshold for detectability? It’s like reading in the minds of a GW data analyst…

Matthew Mould, Christopher J. Moore, Davide Gerosa.
Physical Review D 109 (2023) 063013.
arXiv:2311.12117 [gr-qc].

I’m obsessed with spinning black-hole binaries but, guys, spinning and eccentric black holes are even better! This is the first first-author paper by Giulia, who is not only a rising GW astronomer but also a semi-professional baker… So take two spoons of black holes, one spoon of spin dynamics, some eccentricity (but less than 0.6 ounces), and a pinch of maths. Put this in a bowl, mix it thoroughly with numerical integrations …and the result is very tasty! Spins and eccentricity shape the dynamics of black-hole binaries together , which means one can hope to measure eccentricity indirectly from the spins, but also that if you forget about eccentricity then your spin inference will be crap. Buon appetito.

Giulia Fumagalli, Davide Gerosa.
Physical Review D 108 (2023) 124055.
arXiv:2310.16893 [gr-qc].

…and we’re back to testing GR. We’ve got many gravitational-wave events and would like to use them all together to figure out if our equations for gravity are correct. And here is the issue: there’s only one set (aka catalog) of black holes that contains all the black holes we’ve observed. Now that’s obvious you’d say, and you would be right!, much like we have a single Universe to observe (I’m not a language guy but indeed “Universe” means like “the whole thing”). This effect is known in cosmology (think those low-order multiples in the usual CMB plot), so we called it “the catalog variance of testing GR”. It’s bad, but the Baron Munchauseen tells us we can bootstrap.

Costantino Pacilio, Davide Gerosa , Swetha Bhagwat.
Physical Review D Letters 109 (2024) L081302.
arXiv:2310.03811 [gr-qc].

Gravitational-wave data keep on giving us surprises. The most outstanding one IMO is an observed correlation between mass ratios and spins of the black holes, which was first found by Tom Callister and friends. That is so, so weird… to the point that virtually zero astrophysical models so far can explain it fully and consistently. Well, we can’t either (at least not fully and consistently) but we think this paper is a nice attempt. The secret seems to be the symmetry of the astrophysical environment one considers, and data tends to prefer black holes assembled in cylindrical symmetry. That’s also weird to be honest, but there’s a candidate for this setup, namely accretion disks and their migration traps. Who knows, more data will tell.

… and huge congrats to my MSc student Alessandro who managed to publish a paper even before graduating!

Alessandro Santini, Davide Gerosa , Roberto Cotesta, Emanuele Berti
arXiv:2308.12998 [astro-ph.HE].
Physical Review D 108 (2023) 083033.
Other press coverage: astrobites.

New paper from a new student! Here is Matteo Boschini’s first piece of work, where we look at predictions for the final mass and spins of black-hole remnants. That is, after two black hole merge, what’s the mass and spin of the guy they left behind? These predictions are typically done by fitting (in various ways) outputs from numerical-relativity simulations but those, unfortunately, can only handle black holes of similar masses. On the other hand, black holes with masses that are very different from each other can be handled analytically. Here we show how to put the two together with a single machine-learning fit.

Matteo Boschini, Davide Gerosa , Vijay Varma, Cristobal Armaza, Michael Boyle, Marceline S. Bonilla, Andrea Ceja, Yitian Chen, Nils Deppe, Matthew Giesler, Lawrence E. Kidder, Guillermo Lara, Oliver Long, Sizheng Ma, Keefe Mitman, Peter James Nee, Harald P. Pfeiffer, Antoni Ramos-Buades, Mark A. Scheel, Nils L. Vu, and Jooheon Yoo.
Physical Review D 108 (2023) 084015.
arXiv:2307.03435 [gr-qc].

All right, this is kind of far from my day-to-day topics but working on this paper with Alice and Riccardo was super fun. Think LISA and supermassive binary black holes. And… the detector does what it wants. That’s not true of course because the experimentalists are amazing, but there will be noise transients: unexpected blips when the gravitational-wave signal will be corrupted. Here we look at what would happen in a realistic setting when a LISA glitch happens on top of a gravitational wave from a supermassive black hole.

Alice Spadaro, Riccardo Buscicchio, Daniele Vetrugno, Antoine Klein, Davide Gerosa , Stefano Vitale, Rita Dolesi, William Joseph Weber, Monica Colpi.
Physical Review D 108 (2023) 123029.
arXiv:2306.03923 [gr-qc].

This paper is episode four in the up-down instability series. We first figured out the instability exists (episode 1), then computed when binaries go after the instability (i.e. the endpoint, episode 2), and also checked binaries are really unstable in numerical relativity (episode 3). Now we look at the inference problem with LIGO/Virgo: if unstable up-down binaries enter the sensitivity window of the detector, will we be able to tell? We phrased the problem with some fancy stats using the so-called Savage Dickey density ratio, which is the right tool to answer this question. As is too often the case, current data are not informative enough but the future is bright and loud.

Viola De Renzis, Davide Gerosa , Matthew Mould, Riccardo Buscicchio, Lorenzo Zanga.
Physical Review D 108 (2023) 024024.
arXiv:2304.13063 [gr-qc].

It’s out! The notorious (ask my students…) “ precession v2 ” paper is finally out! This took a veeeery long time; we checked and the first commit for this paper is from May 2020 (!). But the result is an exhilarating tour of spin precession at 2PN with 27 pages and 183 (!!!) numbered equations. We rewrote the entire formalism, change how we parametrize things, compute all we could in closed forms, and speed up the computational implementation. It’s cool, now performing a precession-averaged evolution is a <0.1s operation. If you’re into BH binary spin precession, this is the paper for you. All of this is now part v2 of our PRECESSION python module. So long, and thanks for all the spin.

Davide Gerosa , Giulia Fumagalli, Matthew Mould, Giovanni Cavallotto, Diego Padilla Monroy, Daria Gangardt, Viola De Renzis.
Physical Review D 108 (2023) 024042.
arXiv:2304.04801 [gr-qc].
Open-source code: homepage, repository, documentation.

Third-generation gravitational wave detectors are going to see all stellar-mass black-hole mergers in the Universe. Wooooooooo. But hang on, is this enough? Observing the sources is great, but then we need to measure them. Here we try to focus on the latter and quantify how well we will be able to measure the distance of black holes. Read the paper now, but the short answer is that 3G detectors are going to be awesome but not that awesome…

Michele Mancarella, Francesco Iacovelli, Davide Gerosa.
Physical Review D Letters 107 (2023) L101302.
arXiv:2303.16323 [gr-qc].

Lots of “firsts” today! My first -year PhD student Viola just put out her first first -author paper. This is about measuring black holes with not one, but two precessing spins. People have been trying to figure out how to tell if at least one of the two spins of a merging black-hole binary is precessing for quite some time now. And maybe we’ve even done it already for one or two of the current LIGO-Virgo events. But here I must quote that epic Italian commercial from the 90s: “two gust is megl che one” (which is a terrible Italian-English mishmash on a terrible joke to say that when you eat a Maxibon “two flavors are better than one”). In this paper we propose a strategy to identify sources that have the strongest evidence of two processing spins. Viola has been putting together simulated data for the next LIGO/Virgo data-taking period, and the result is pretty cool. If these binaries are out there in the Universe, we will be able to tell they have two spins going around!

Viola De Renzis, Davide Gerosa, Geraint Pratten, Patricia Schmidt, Matthew Mould.
Physical Review D 106 (2022) 084040.
arXiv:2207.00030 [gr-qc].

Daria’s new paper is out! (With key contributions from others in the group… This is also Viola’s first paper!).

Here we look at sub-dominant black-hole spin effects in current data from LIGO and Virgo (yeah sorry guys… our black-hole spin obsession goes on). People have looked at spin precession before, but we’re interested in even more subtle things, namely disentangling precession and nutation. This is a tricky business, which is made complicated by the fact that this piece of information is hidden behind other parameters that are easier to measure (say the masses of the two black holes). Our paper is an attempt to formulate and systematically exploit something we called “sequential prior conditioning” (which is: mix&match priors and posteriors in Bayesian stats…). Results are weak today but strong tomorrow.

Daria Gangardt, Davide Gerosa , Michael Kesden, Viola De Renzis, Nathan Steinle.
Physical Review D 106 (2022) 024019.
arXiv:2204.00026 [gr-qc].

It took a while (so many technical challenges…) but we made it! Matt‘s monster paper is finally out!

Let me introduce a fully-fledged pipeline to study populations of gravitational-wave events with deep learning. If it sounds cool, well, it is cool (just look at the flowchart in Figure 1!). We can now perform a hierarchical Bayesian analysis on GW data but, unlike current state-of-the-art applications that rely on simple functional form, we can use populations inferred from numerical simulations. This might sound like a detail but it’s not: it’s necessary to compare GW data directly against stellar physics. While we don’t do that yet here (our simulations are admittedly too simple), there’s a ton of astrophysics already in this paper. Whether you care about neural networks or hierarchical black-hole mergers (or, why not, both!), sit tight, fasten your seatbelt, and read Matt’s paper.

Matthew Mould, Davide Gerosa , Stephen R. Taylor.
Physical Review D 106 (2022) 103013.
arXiv:2203.03651 [astro-ph.HE].

Great Scott, a new paper! When analyzing gravitational-wave data, looking at one black hole at a time is not enough anymore, the fun part is looking at them all together. The issue Matt and I are tackling here is that one needs to be consistent with putting together different events when fitting the entire population. This is obvious for things that do not change (say the masses of the black holes, those are what they are), but becomes a very tricky business for varying quantities (say the spin directions, which is what we look at here). In that case, it’s dangerous to put together events taken at different stages of their evolution. And the solution to this problem is…. time travel! We show that but propagating binaries backward in time, one can put all sources on the same footing. After that, estimating the impact of the detector requires traveling forward in time, so going “back to the future”. After all, we all know that post-Newtonian black-hole binary integrations look like this:

Matthew Mould, Davide Gerosa.
Physical Review D 105 (2022) 024076.
arXiv:2110.05507 [astro-ph.HE].

No black hole is an island entire of itself.

We’ve got many gravitational wave events now. One can look at each of them individually (aka “parameter estimation”), all of them together (aka “population”), or each of them individually while they’re together. That’s what we do in this paper: we look at the properties of individual gravitational-wave events in light of the rest of the observed population. The nice thing is that all of these different ways of looking at the data are part of the same statistical tool, which is a hierarchical Bayesian scheme. Careful, heavy stats inside, don’t do this at home.

Christopher J. Moore, Davide Gerosa.
Physical Review D 104 (2021) 083008.
arXiv:2108.02462 [gr-qc].

LISA is going to be great and will detect stuff from white dwarfs to those supermassive black-hole that live at the center of galaxies. If we’re lucky (yeah, who knows how many of these we will see), LISA might also detect some smaller black holes, similar to those that LIGO now sees all the time, but at a much earlier stage of their lives. But if we’re indeed lucky, the science we would take home is outstanding. Using simulated data from the LISA Data Challenge we unleash the new amazing parameter-estimation code Balrog (don’t ask what it means, it’s just a name, not one of those surreal astronomy acronyms) at this problem. Dive into the paper for some real data-analysis fun!

Riccardo Buscicchio, Antoine Klein, Elinore Roebber, Christopher J. Moore, Davide Gerosa , Eliot Finch, Alberto Vecchio.
Physical Review D 104 (2021) 044065.
arXiv:2106.05259 [astro-ph.HE].

Who are the parents of LIGO’s black holes? Stars, most likely. Things like those we see in the sky at night will eventually surrender to gravity and collapse. Some of them will form black holes. Some of them will form binary black holes. Some of them will merge. Some of them will be observed by LIGO. That’s the vanilla story at least, but it might not apply to all of the black holes that LIGO sees. For some of those, stars might be the grandparents or the great grandparents. And the parents are … just other black holes! This is today’s paper lead by Vishal Baibhav. Instead of just measuring the properties of the black holes that LIGO observes, we show we can also say something about the features of the black hole parents. Read on to explore the black-hole family tree.

Vishal Baibhav, Emanuele Berti, Davide Gerosa , Matthew Mould, Kaze W. K. Wong.
Physical Review D 104 (2021) 084002.
arXiv:2105.12140 [gr-qc].

Here is the latest in our (by now long) series of papers on black-hole binaries spin precession. This work was is championed by two outstanding PhD students, Daria (in my group) and Nate (UT Dallas). The key idea behind this paper is that, for black-hole spins, one cannot really talk about precession without talking about nutation (although we only say “precession” all the time…). The spin of, say, the Earth also does both precession (azimuthal motion) and nutation (polar motion). But, unlike in the Earth problem, for black-hole spins the two motions happen on roughly the same timescale meaning that you cannot really take them apart. Or can you? We stress the role of five parameters that characterize the combined phenomenology of precession and nutation. The hope is now to use them as building blocks for future waveforms… stay tuned!

Daria Gangardt, Nathan Steinle, Michael Kesden, Davide Gerosa , Evangelos Stoikos.
Physical Review D 103 (2021) 124026.
arXiv:2103.03894 [gr-qc].

ps. Stupid autocorrect! It’s nutation, not mutation.

Orbital eccentricity in gravitational-wave observations has been long neglected. And with good reasons! Gravitation-wave emission tends to circularize sources. By the time black holes are detectable by LIGO/Virgo/LISA/whatever, they should have had ample time to become circular. Unless something exciting goes on in their formation, things like clusters, triples, Kozai-Lidov oscillations, etc. And if that happens, we want to see it! This paper contains the first model for gravitational waveforms and black-hole remnants (final mass, spin) trained directly on eccentric numerical relativity simulations. Because eccentric is the new circular.

Tousif Islam, Vijay Varma, Jackie Lodman, Scott E. Field, Gaurav Khanna, Mark A. Scheel, Harald P. Pfeiffer, Davide Gerosa , and Lawrence E. Kidder.
Physical Review D 103 (2021) 064022.
arXiv:2101.11798 [gr-qc].

Up-down instability S01-E03.
“Previously on the up-down instability. After finding out that the instability exists (S01-E01) and calculating its analytic endpoint (S01-E02), one terrifying prospect remains. What if it’s just PN? Can all of this disappear in the strong-field regime? This challenge now needs to be faced”.

Today’s paper is the latest in our investigations of the up-down instability in binary black holes. If the primary black hole is aligned and the secondary is anti-aligned to the orbital angular momentum, the entire system is unstable to spin precession. We found this funny thing using a post-Newtonian ( read : approximate)__ treatment but we couldn’t be 100% sure that this would still be true when the black holes merge and our approximation fails. So, we got our outstanding SXS friends on board and ask them if they could see the same effect with their numerical relativity (read : the real deal!) code. And the answer is… yes! The instability is really there! And by the way, these are among the longest numerical relativity simulations ever done.

Vijay Varma, Matthew Mould, Davide Gerosa , Mark A. Scheel, Lawrence E. Kidder, Harald P. Pfeiffer.
Physical Review D 103 (2021) 064003.
arXiv:2012.07147 [gr-qc].
Supporting material available here.

Spin precession is cool, and we want to measure it. In General Relativity, the orbital plane of a binary is not fixed but moves around. This effect is related to the spin of the orbiting black holes and contains a ton of astrophysical information. The question we try to address in this paper is the following: how does one quantify “how much” precession a system has? This is typically done by condensing information into a parameter called \(\chi_{\rm p}\), which is here generalize to include two- spin effects. There are two black holes in a binary and we received numerous complaints from the secondaries: they want to join the gravitational-wave fun!

Davide Gerosa , Matthew Mould, Daria Gangardt, Patricia Schmidt, Geraint Pratten, Lucy M. Thomas.
Physical Review D 103 (2021) 064067.
arXiv:2011.11948 [gr-qc].
Open-source code: homepage, repository.

And here is my latest lockdown effort: some experiments in the wonderful and perilous world of machine learning. The idea of this paper is to teach a computer to figure out by itself if a gravitational-wave signal will be detectable or not. The problem is very similar to that of image recognition: much like classifying if an image is more likely to contain a dog or a cat, here we classify black-hole mergers based on the imprints they have in the LIGO and Virgo detectors. This is important to quantify the so-called “selection effects”: in order to figure out what the Universe does based on what we observe, we need to know very well “how” we observe and thus what we are going to miss. Our code is built using Google’s TensorFlow and it is public on Github, feel free to play with it!

Davide Gerosa , Geraint Pratten, Alberto Vecchio.
Physical Review D 102 (2020) 103020.
arXiv:2007.06585 [astro-ph.HE]
Open-source code: homepage, repository.

If General Relativity is too boring, couple it to something else. In this paper we study what happens to stellar collapse and supernova explosions if gravity is transmitted not only with the usual metric of Einstein’s theory (aka the graviton) but also an additional quantity. If this extra scalar field has a mass, it dramatically impacts the emitted gravitational waves… Which means that maybe, one day, one can use gravitational-wave data to figure out if scalar fields are coupled to gravity. Here we try to explore all the related phenomenology of stellar collapse with a large set of simulations covering the parameter space. And the overall picture is remarkably neat and simple!

Roxana Rosca-Mead, Ulrich Sperhake, Christopher J. Moore, Michalis Agathos, Davide Gerosa , Christian D. Ott.
Physical Review D 102 (2020) 044010.
arXiv:2005.09728 [gr-qc].

We’ve been knowing about the mass gap for a while, but I bet “spin gap” sounds new to you, uh? The gap in the spectrum of binary black hole masses is due to pair-instability supernovae (i.e. what happens if a giant ball of carbon and oxygen burns all at the same time). As for the spin gap, it might be that stars collapse into black holes which have a tiny tiny spin. But that’s only for black holes that come from stars: those come out of the merger of other black holes, on the other hand, are very rapidly rotating. So, there’s a gap between these two populations. Our paper today shows that, together, mass gap and spin gap are powerful tools to figure out where black holes come from. Cluster or field? Gaps will tell.

Vishal Baibhav, Davide Gerosa , Emanuele Berti, Kaze W. K. Wong, Thomas Helfer, Matthew Mould.
Physical Review D 102 (2020) 043002.
arXiv:2004.00650 [gr-qc].

Sometimes you have to look into things twice. We found the up-down instability back in 2015 and still did not really understand what was going on. Three out of four black hole binaries with spins aligned to the orbital angular momentum are stable (in the sense that the spins stay aligned), but one is not. The impostors are the “up-down” black holes –binaries where the spin of the big black holes is aligned and the spin of the small black hole is antialigned. These guys are unstable to spin precession: small perturbation will trigger large precession cycles. Matt’s paper today figures out what’s the fate of these runaways. We find that these binaries become detectable in LIGO and LISA with very specific spin configurations: the two spins are aligned with each other and equally misaligned with the orbital angular momentum. There’s a lot of interesting maths in this draft (my first paper with a proof by contradiction!) as well as some astrophysics (for you, AGN disks lover).

Matthew Mould, Davide Gerosa.
Physical Review D 101 (2020) 124037.
arXiv:2003.02281 [gr-qc].
Supporting material available here.

Today’s paper is about superkicks. These are extreme configurations of black hole binaries which receive a large recoil. Black hole recoils work much like those of, say, a cannon. As the cannonball flies, the cannon recoils backwards. Here the binary is shooting gravitational waves: as they are emitted, the system recoils in the opposite direction. In this paper we show that superkicks might be up to 25% larger if the binary is mildly eccentric. This means it’s a bit easier to kick black holes out of stellar clusters and galaxies.

Ulrich Sperhake, Roxana Rosca-Mead, Davide Gerosa , Emanuele Berti.
Physical Review D 101 (2020) 024044.
arXiv:1910.01598 [gr-qc].

Gravitational-wave astronomy is, seems obvious to say, about doing astronomy with gravitational waves. One has gravitational-wave observations (thanks LIGO and Virgo!) on hand and astrophysical models on the other hand. The more closely these two sides interact, the more we can hope to use gravitational-wave data to learn about the astrophysics of the sources. Today’s paper with JHU student Kaze Wong tries to further stimulate this dialog. And, well, one needs to throw some artificial intelligence in the game. There are three players now (astrophysics, gravitational waves, and machine learning) and things get even more interesting.

Kaze W.K. Wong, Davide Gerosa.
Physical Review D 100 (2019) 083015.
arXiv:1909.06373 [astro-ph.HE].

ps. The nickname of this project was sigmaspops

What’s in between neutron stars and black holes? It looks like neutron stars have a maximum mass of about 2 solar masses while black holes have a minimum mass of about 5. So what’s in between? That’s the popular issue of the ‘low mass gap’. Actually, now we know something must be in there. LIGO and Virgo have seen GW170817, a merger of two neutron stars, which merged in to a black hole with the right mass to populate the gap. Can this population be seen directly with (future) gravitational-wave detectors? That’s today’s paper.

Anuradha Gupta, Davide Gerosa , K. G. Arun, Emanuele Berti, Will Farr, B. S. Sathyaprakash.
Physical Review D 101 (2020) 103036.
arXiv:1909.05804 [gr-qc].

Funny things happen in supernova explosions. Funny and complicated. If the star is too massive, the explosion is unstable. The black hole it formed it not as massive as it could have been. In gravitational-wave astronomy, this means that we should not observe black holes heavier than about 50 solar masses. This does not apply, of course, to black holes that are not formed from stars, but from other black holes (yes! more black holes!). If black holes resulting from older gravitational wave events somehow stick around, they could be recycled in other generations of mergers. We point out that this can work only if their astrophysical environment is dense enough. Can we measure the escape speed of black holes “nurseries” using gravitational-wave events that should not be there because of supernova instabilities?

Davide Gerosa , Emanuele Berti.
Physical Review D Rapid Communications 100 (2019) 041301R.
arXiv:1906.05295 [astro-ph.HE].
Press release : Birmingham.
Other press coverage: Scientific American, astrobites, interestingengineering, metro.co.uk, Media INAF, Great Lakes Ledger, sciencealert, sciencetimes, mic.com.

Where do black holes come from? Sounds like a scify book title, but it’s real. These days, that’s actually the million dollar question in gravitational-wave astronomy. LIGO sees (lots of!) black holes in binaries, and those data encode information on how their stellar progenitors behave, what they like or did not like to do. This is paper is the latest attempt to understand if black holes formed alone (i.e. a single binary star forms a single binary black hole) or together (i.e. many stars exchange pairs in dense stellar environments).

Yann Bouffanais, Michela Mapelli, Davide Gerosa , Ugo N. Di Carlo, Nicola Giacobbo, Emanuele Berti, Vishal Baibhav.
Astrophysical Journal, 886 (2019) 25.
arXiv:1905.11054 [astro-ph.HE].

The prospect of multiband gravitational-wave astronomy is so so so exciting (I mean, really!). So exciting that we want to make sure once again it’s true; and this is today’s paper. Multiband means seeing the same black hole binary with both LIGO at high frequencies and LISA at low frequencies. LISA observations can serve as precursors for the LIGO mergers, and you can a whole lot of new science (astrophysics, tests of GR, smart data analysis, cosmology, etc). Here we have a new semi-analytic way to estimate the rate (i.e. how many) of multiband events, and we also explore some of the stellar physics one could constraint with them. Enjoy!

Davide Gerosa , Sizheng Ma, Kaze W.K. Wong, Emanuele Berti, Richard O’Shaughnessy, Yanbei Chen, Krzysztof Belczynski
Physical Review D 99 (2019) 103004.
arXiv:1902.00021 [astro-ph.HE].
Supporting material available here.

We all know Doppler shifts, right? That’s like the biibouuubiiiiboouuuuuu of an ambulance. That happens to gravitational waves as well. Suppose you have a merging binary which is emitting gravitational waves (bibooou). If that binary is going somewhere (say it’s falling into the gravitational potential of a third body), much like the ambulance, the emitted signal will be Doppler shifted. This paper shows a very nice calculation to incorporate Doppler shifts into gravitational waves.

ps. This started out as Katie’s undergraduate summer project at Caltech. Congrats Katie!

Katie Chamberlain, Christopher J. Moore, Davide Gerosa , Nicolas Yunes.
Physical Review D 99 (2019) 024025.
arXiv:1809.04799 [gr-qc].

Today’s paper celebrates the wedding of startrack and precession (the nickname for this project was pretrack 😉 ). We use population synthesis evolution from startrack to predict the parameters of spinning black-hole binaries observed by LIGO. The spin distribution is then propagated from formation to detection using post-Newtonian evolutions from my precession code. The bottom line is that spin measurements can be used to truly reconstruct the binary formation channels, and some specific mechanisms (like mass transfers, tides, natal kicks, supernova’s instabilities etc.). Our database is publicly available (play with it!), as well as a little code to compute gravitational-wave detectabilities.

Update : this is my 25th published paper! That’s silver, right?

Davide Gerosa , Emanuele Berti, Richard O’Shaughnessy, Krzysztof Belczynski, Michael Kesden, Daniel Wysocki, Wojciech Gladysz.
Physical Review D 98 (2018) 084036.
arXiv:1808.02491 [astro-ph.HE].
Supporting material available here.

LISA is going to be amazing: supermassive black-holes, galactic white dwarfs, EMRIs… Besides all of that, LISA can help us doing LIGO’s science better. Some LIGO sources (notably, things like GW150914) will show up in LISA years in advance. LISA is going to tell us when (in time) and where (in frequency) LIGO will see these sources. In this paper, we explore the idea of adapting the LIGO noise curve if one knows that a source is coming in (because LISA told us). We apply this idea to ringdown tests of GR, and show how powerful they become.

Rhondale Tso, Davide Gerosa, Yanbei Chen.
Physical Review D 99 (2019) 124043.
arXiv:1807.00075 [gr-qc].
Other press coverage: astrobites.

Gravitational-wave astronomy is moving. Quickly. In a few years we are going to have large catalogs of many detections, and a whole lot of information to extract from them. Instead of focussing on parameters (masses, spins, redshifts) of single sources, we will want to extract hyperparameters describing physical features of the population (metallicity, natal kicks, common envelope, stellar winds, etc). Here we show how to do this in practice: read our new paper for an amazing journey through hyperlateral cubes, Gaussian process emulators, selection biases, hierarchical modeling and more.

Our tools are publicly available! Here is Steve’s Webpage and our public code.

Stephen R. Taylor, Davide Gerosa.
Physical Review D 98 (2018) 083017.
arXiv:1806.08365 [astro-ph.HE].
Editor’s coverage in APS’s Kaleidoscope.

LIGO can measure spins. Well, effective spins actually. These are special combinations of the two spins (magnitude and direction) and the binary mass ratio. There’s a ton of astrophysics that can be done just with this quantity, but one should always be careful. Today’s paper points out a few important shortcomings when dealing with effective spin measurements. Want to know more about asymmetries and selection biases?

ps. You can hardly find a better day to post a paper on the arxiv than May 4th

Ken K. Y. Ng, Salvatore Vitale, Aaron Zimmerman, Katerina Chatziioannou, Davide Gerosa , Carl-Johan Haster.
Physical Review D 98 (2018) 083007.
arXiv:1805.03046 [gr-qc].

Surrogate models are fancy interpolation schemes developed to provide accurate (well, really accurate) waveforms directly from numerical relativity simulations. The first surrogate able to model fully precessing systems came up recently (and it’s really an amazing work!). Here we exploit these advances to explore how linear momentum is emitted in generic black-hole mergers, and well as its back-reaction. Black holes get kicked!

Davide Gerosa , François Hébert, Leo C. Stein.
Physical Review D 97 (2018) 104049.
arXiv:1802.04276 [gr-qc].
Open-source code: homepage, repository.

Natal kicks imparted to neutron stars and black holes at birth can be constrained using LIGO data. Kicks cause misalignments between the spins and the orbital angular momentum. Here we compare large banks of population synthesis simulations to LIGO data using hierarchical Bayesian statistics and show that (already with 4 events!) natal kicks are constrained from both above and below. Simulated binaries are produced merging Startrack evolutions to my precession code. More on this very soon…

Update : here it is!

Daniel Wysocki, Davide Gerosa , Richard O’Shaughnessy, Krzysztof Belczynski, Wojciech Gladysz, Emanuele Berti, Michael Kesden, Daniel Holz.
Physical Review D 97 (2018) 043014.
arXiv:1709.01943 [astro-ph.HE].

Part of our series of spin precession papers, here we study nutational resonances. Those are configurations where the precession of L about J, and that of the two spins are in resonance with each other. These configurations are very generic (virtually every binary will go through resonances), but their effect on the dynamics seems to be small, unless… unless you end up in transitional precession! Transitional precession (great paper!) turns out to be a very special nutational resonance.

Xinyu Zhao, Michael Kesden, Davide Gerosa.
Physical Review D 96 (2017) 024007.
arXiv:1705.02369 [gr-qc].

What if the black holes LIGO sees are the results of a merger? I mean, we see mergers, but maybe those are second-generation ones, and the two merging black holes come from first-generation mergers. Or (more likely…) stellar mass black holes form from stars and only merge once…

Davide Gerosa , Emanuele Berti.
Physical Review D 95 (2017) 124046.
arXiv:1703.06223 [gr-qc].
Selected as PRD Editors’ Suggestion.
Other press coverage: Ars Technica.

Here we present my numerical code precession, which implements our multi-timescale way to look at spinning black-hole binaries. The paper has a detailed description of the various functions as well as lots of examples.

Update : typos in Eq. (36-37) have been fixed in v3 on the arXiv.

Davide Gerosa , Michael Kesden.
Physical Review D 93 (2016) 124066.
arXiv:1605.01067 [astro-ph.HE].
Open-source code: homepage, repository, documentation.

This is a follow up of arXiv:1403.7147, just done better. Instead of overlaps, we do real injections in LIGO parameter-estimation codes to show that spin-orbit resonances are indeed detectable.

Daniele Trifirò, Richard O’Shaughnessy, Davide Gerosa , Emanuele Berti, Michael Kesden, Tyson Littenberg, Ulrich Sperhake.
Physical Review D 93 (2016) 044071.
arXiv:1507.05587 [gr-qc].

Detailed analysis of 2PN black-hole binary spin precession using multi-timescale methods. Follow-up of the Letter arXiv:1411.0674, this paper contains the full calculation and the description of the underlying phenomenology.

Davide Gerosa , Michael Kesden, Ulrich Sperhake, Emanuele Berti, Richard O’Shaughnessy.
Physical Review D 92 (2015) 064016.
arXiv:1506.03492 [gr-qc].
Supporting material available here.

Spinning black-hole binaries might belong to two spin-orbit resonances, or families. Can you tell them apart using gravitational-wave observations? Spoiler: yes!

Bonus note: check out the title in v1 on the arxiv…

Davide Gerosa , Richard O’Shaughnessy, Michael Kesden, Emanuele Berti, Ulrich Sperhake.
Physical Review D 89 (2014) 124025.
arXiv:1403.7147 [gr-qc].

Spin precession in stellar-mass black hole binaries encodes information on specific formation mechanisms like tides and mass transfers. My first paper on spin precession…

Davide Gerosa , Michael Kesden, Emanuele Berti, Richard O’Shaughnessy, Ulrich Sperhake.
Physical Review D 87 (2013) 10, 104028.
arXiv:1302.4442 [gr-qc].

The latest news from our LIGO/Virgo friends (including some colleagues here in Birmingham) was an astrophysical surprise. The black-hole binary GW190412 is just different from every other one we have had so far. One of the two black holes is about three times larger than the other one, it’s spinning relatively fast, and that spin might even be misaligned with respect to the binary axis. That’s a lot of new things, which makes this event very challenging (but we like challenges!) to be explained with a coherent astrophysical setup. That’s what I meant by an astrophysical surprise. Today’s paper is our attempt to, first of all, quantify that GW190412 is indeed very unusual. Maybe it comes from a second-generation merger (that is, an event where one of the two black holes is the result of a previous merger). This might explain its features, but then the astrophysical host must be very unusual. So, yet another challenge.

Davide Gerosa , Salvatore Vitale, Emanuele Berti.
Physical Review Letters 125 (2020) 101103.
arXiv:2005.04243 [astro-ph.HE].
Press release : Birmingham, MIT.
Other press coverage: International Business Times, SciTechDaily, VRT, notimerica, allnewsbuzz, canaltech.

Black hole mergers are like a scattering problem. Two black holes come in, and one black hole comes out. The difference is a bunch of gravitational waves. Those are nice, of course, but the remnant black hole is important too! Here we provide accurate predictions of the mass, spin and kick of this remnant given the properties of the two merging black holes. If you need those numbers (want to build a waveform family? or test GR perhaps?) just use our python module surfinBH!

Bonus note. What if you collide ducks instead of black holes?

Vijay Varma, Davide Gerosa , François Hébert, Leo C. Stein, Hao Zhang.
Physical Review Letters 122 (2019) 011101.
arXiv:1809.09125 [gr-qc]
Press release: Caltech, Ole Miss.
Other press coverage: Space Daily, phys.org, longroom, tasnim, europapress (Spanish), Media INAF (video in Italian).

Supernova can be used to test gravity! …and if there’s a massive scalar field around, things get terribly interesting. Here we generalize arXiv:1602.06952 to study stellar collapse in massive scalar-tensor theories of gravity (that is, the graviton is coupled to a massive scalar field) with numerical simulations. The scalar-field mass introduces a dispersion relation, and different GW frequencies travel at different speeds. It might even make sense to target historic supernovae: maybe the tail of the signal is still coming to us!

Ulrich Sperhake, Christopher J. Moore, Roxana Rosca, Michalis Agathos, Davide Gerosa, Christian D. Ott.
Physical Review Letters 119 (2017) 201103.
arXiv:1708.03651 [gr-qc].

Bayesian statistics is really cool. It lets you specify clearly and openly all the assumptions that enter an analysis. What’s the effect of these prior assumptions on current inference with gravitational-wave data from black-hole binaries? Here we tackle this question head-on, and perform parameter estimation runs on LIGO data with many (astrophysically motivated!) prior assumptions. Some parameters (like chirp mass) do not suffer from prior choices but others (like the effective spin) do! Specifying the astrophysics as priors is a powerful tool to extract more science from GW data

Update : at the time of publication, this was the first independent reanalysis of any GW data! (There are many more now…). Also, use our data for your research!

Salvatore Vitale, Davide Gerosa , Carl-Johan Haster, Katerina Chatziioannou, Aaron Zimmerman.
Physical Review Letters 119 (2017) 251103.
arXiv:1707.04637 [gr-qc].
Posterior sample data release.

Black-hole data can be used to probe the lives of stars. That’s the promise of gravitational-wave astronomy, right? Here we give it a go. We present a (admittedly) very simple model showing that the misalignment of GW151226 can be easily explained with large natal kicks. I like simple things…

Richard O’Shaughnessy, Davide Gerosa , Daniel Wysocki.
Physical Review Letters 119 (2017) 011101.
arXiv:1704.03879 [astro-ph.HE].
Press release : Rochester Institute of Technology, Caltech’s tweet.
Editor’s coverage in physics.aps.org.
Other press coverage: IOP’s physicsworld.com, Science Daily, Phys.org, astronomy.com, sciencenews, iflscience.

Black hole kicks are cool: powerful (up to thousands of km/s!) recoils that black holes receive following a merger. Here we show these events might leave an imprint on the emitted gravitational waves, which is potentially detectable by future interferometers.

Davide Gerosa , Christopher J. Moore.
Physical Review Letters 117 (2016) 011101.
arXiv:1606.04226 [gr-qc].
Selected as PRL Editors’ Suggestion.
Press release : Cambridge University, Cambridge Center for Theoretical Cosmology
Other press coverage: astrobites, particlebites, Daily Mail, phys.org, Particle Bites, egno.gr, Daily Galaxy, Register, Media INAF, IneffableIsland, AstronomyNow, Accademia delle Stelle, noticiasdelaciencia, Cambridge TV.

Here we study the stability of black-hole binaries with spins (anti)aligned with the orbital angular momentum. Aligned configurations are points of equilibrium, but are they stable? If the heavier black-hole is aligned and the lighter one is anti-aligned, this turns out to be unstable! And the onset of this instability can be in the LIGO or LISA band!

Davide Gerosa , Michael Kesden, Richard O’Shaughnessy, Antoine Klein, Emanuele Berti, Ulrich Sperhake, Daniele Trifirò.
Physical Review Letters 115 (2015) 141102.
arXiv:1506.09116 [gr-qc].
Selected as PRL Editors’ Suggestion.
Supporting material available here.

2PN black-hole binary spin precession works exactly like Kepler’s two-body problem. Not kidding: just define effective potentials and divide the phase space into morphologies. The only things you need are a few timescales to play with.

Michael Kesden, Davide Gerosa , Richard O’Shaughnessy, Emanuele Berti, Ulrich Sperhake.
Physical Review Letters 114 (2015) 081103.
arXiv:1411.0674 [gr-qc].
Press release : Cambridge University, Cambridge Center for Theoretical Cosmology, Ole Miss, UT Dallas.
Other press coverage: Science Daily, phys.org, phys.org (2), Media INAF, Astroblogs, RIA, Daily News, Science World Report, Tech Times, Tech Times (2), SpaceRef, Space Daily, ECN, R&D, Daily Galaxy, scitechdaily, nanowerk.
Supporting material available here.

Surrogate models are the best of both worlds. Numerical-relativity simulations are accurate but take forever. Waveform models have larger errors but can be computed cheaply, which means they can be used in the real world and compared with data. Surrogates are as fast as the approximate waform models, but as accurate as the numerical-relativity simulations they are trained on. Don’t believe me? I don’t blame you, this does sound impossible. Check out our new paper, where we pushed this effort to binaries with spins and more unequal masses.

Vijay Varma, Scott E. Field, Mark A. Scheel, Jonathan Blackman, Davide Gerosa , Leo C. Stein, Lawrence E. Kidder, Harald P. Pfeiffer.
Physical Review Research 1 (2019) 033015.
arXiv:1905.09300 [gr-qc].

Welcome to the beautiful world of SBI, with this terrific piece of work by Pippa Cole. Here we’re looking at extreme mass-ratio inspirals (EMRIs), that is, a small black hole orbiting a big black hole, which will be (one day) detected by LISA. These signals are nasty (long and of a very complicated morphology). We’re trying something new here – a deep learning called “truncated marginal neural ratio estimation” that does not even require writing down the likelihood of the problem. Just simulate all you can. The answer, this thing is great for narrowing down the parameter space where EMRIs will be, kind of like searches do with current gravitational-wave data, but in a very different way.

Philippa S. Cole, James Alvey, Lorenzo Speri, Christoph Weniger, Uddipta Bhardwaj, Davide Gerosa , Gianfranco Bertone.
arXiv:2505.16795 [gr-qc].

This paper came out of some discussions from our “Gravitational-wave snowballs” workshop in Sexten (Italy). We were discussing the good old problem of separating black-hole binary formation channels with spin measurements. Usually one says “aligned=isolated”, “isotropic=dynamical”. But then, some binaries that formed dynamically should also be eccentric. What we then realized is that, for those eccentric binaries and only for those, spin measurements can actually tell which of the dynamical channel (because there are many…) is at play.

Jakob Stegmann, Davide Gerosa , Isobel Romero-Shaw, Giulia Fumagalli, Hiromichi Tagawa, Lorenz Zwick.
arXiv:2505.13589 [astro-ph.HE].

We’re back to predicting the excitation amplitude of black hole merger ringdowns. We already looked into the simpler case of binaries with aligned spins, and now tried to study the full problem of binaries with misaligned (i.e. processing) spins. Well, this is a hard problem! It’s not even clear which mode is the stronger one anymore, and finding suitable coordinates is not at all trivial. While this is just a first exploration, there’s so much interesting phenomenology here! Do it yourself with the postmerger package.

Francesco Nobili, Swetha Bhagwat, Costantino Pacilio, Davide Gerosa.
arXiv:2504.17021 [gr-qc].

Great paper led by our former MSc student Alessandro today! This is about combining the distributions of gravitational waves and galaxies to do cosmology. These two probes measure different things (distance and redshift, respectively), so their distributions will “match” only if the cosmological model is right. You can actually use this to measure the cosmological model itself. Short answer: putting together 3G detectors and Euclid is a great idea.

Alessandro Pedrotti, Michele Mancarella, Julien Bel, Davide Gerosa.
arXiv:2504.10482 [astro-ph.CO].

Long gravitational-wave signals are, well, long. And long often means painful, as more data need to be stored and processed. Kind of intuitively, the solution might be that of cutting things into chunks, so that long becomes short. Here we apply this idea to the popular inner product entering all gravitational-wave pipelines; this is a key building block of everything we do. The answer is that using SFTs, “Short-time Fourier Transforms”, can make things faster by more than 3 orders of magnitudes, sometimes 5. We think this is the solution to future gravitational-wave data analysis problems (think LISA and 3G…).

Rodrigo Tenorio, Davide Gerosa.
arXiv:2502.11823 [gr-qc].

When inferring black holes from gravitational-wave data, we tend to do two things, one after the other. First, we consider each event individually and measure its parameters (masses, spins, etc). Then we consider all the events together and measure the population properties. This is what we do all the time, but, actually, if objects are now part of a population, those parameters should be looked at again in light of all the others. This full problem (all parameters of all the events plus the population parameters) is daunting, and in the past we used an indirect and somewhat convoluted approach. We got back to it now, and this time, we managed to do it head-on. Let me introduce this giant 500-dimensional sampling of the full population problem!

Michele Mancarella, Davide Gerosa.
arXiv:2502.12156 [gr-qc].

General relativity has this beautiful property that coordinates are meaningless. You can change them at will, which means they don’t contain any physics. And, believe it or not, some of the popular formulations we use to write down the dynamics of eccentric binary black holes still have coordinates in them. They go away if you take an average of an orbit (Peters, the man!) but that’s killing some information. In this paper we go back to those old results and show how those gauges can actually be absorbed into the formulation itself. The paper is on the maths-heavy side of things, but the results are great. Peters, you were basically right, but not quite.

Giulia Fumagalli, Nicholas Loutrel, Davide Gerosa , Matteo Boschini.
arXiv:2502.06952 [gr-qc].

Quasar 3C 186 strikes back! Matteo and I got interested in this funny quasar last year (see this one). When our paper hit the arxiv, we got contacted by the real astronomers who take actual data, who told us they had even more beautiful data. We ended up contributing with our relativistic model and… well… everything seems to work. 3C 186 is indeed a recoiling black hole (it might be a rare one, but we’ve observed it nonetheless). The abstract says “decisive,” and this is indeed the right word.

Marco Chiaberge, Takahiro Morishita, Matteo Boschini, Stefano Bianchi, Alessandro Capetti, Gianluca Castignani, Davide Gerosa , Masahiro Konishi, Shuhei Koyama, Kosuke Kushibiki, Erini Lambrides, Eileen T. Meyer, Kentaro Motohara, Massimo Stiavelli, Hidenori Takahashi, Grant R. Tremblay, Colin Norman.
arXiv:2501.18730 [astro-ph.GA].

Gravitational-wave population people talk all the time about parametric vs non-parametric methods. Parametric methods mean imposing our astrophysical knowledge on how we look at GW data. This is great, we do want to extract astrophysical knowledge, but what if we don’t know what to look for? The statisticians tell us to go non-parametric, which means using a flexible model that can fit whatever you want. That’s great, but what do we learn then? In other words, where’s the boundary between flexibility and interpretability? Today’s paper shows that one can conceptually separate these two processes and extract parametric results from non-parametric fits. I’m very proud of this piece of work, which was Cecilia Fabbri‘s MSc thesis project and was actually kickstarted by one of my previous students, Alessandro Santini. We even wrote a poem about this!

Cecilia Maria Fabbri, Davide Gerosa , Alessandro Santini, Matthew Mould, Alexandre Toubiana, Jonathan Gair.
arXiv:2501.17233 [astro-ph.HE].

Black holes on eccentric orbits… what does it even mean? The hard (but fun) thing is that we work in General Relativity, where coordinates don’t have a physics inside. One can always change the coordinates as they want, so they can’t be used to define observables. The eccentricity of an orbit has to do, indeed, with the shape of the orbit itself, and that can be transformed away with suitable coordinates. So, does it even sense to measure the orbital eccentricity of black-hole binaries? The one thing we are allowed to do is to find a coordinate-free estimator in General Relativity that reduces to the eccentricity we all know and love in the Newtonian limit. This is possible! The right mathematical framework for this is something called “catastrophe theory”, a funny name, but Nick likes it.

Matteo Boschini, Nicholas Loutrel, Davide Gerosa , Giulia Fumagalli.
Physical Review D 111 (2025) 024008.
arXiv:2411.00098 [gr-qc].

Our “popfisher” paper is finally out! (and now Viola can submit her PhD thesis). This is about next-generation (aka 3G) gravitational wave detectors. Those beasts will measure millions of black holes… and with so many of them who cares about each source individually. The important thing will be the population of objects, i.e. how those black holes are distributed as a whole. Measuring populations is an interesting but convoluted statistical problem. Here we implement a quick shortcut (the Fisher matrix) and show that yes, 3G detectors will be amazing… but more amazing for some things than for others.

Viola De Renzis, Francesco Iacovelli, Davide Gerosa , Michele Mancarella, Costantino Pacilio.
Physical Review D 111 (2025) 044048.
arXiv:2410.17325 [astro-ph.HE].

LISA will see a gazillion white dwarfs, but we won’t, or at least not individually. Those signals will actually pile up together in a mashed potato thing called foreground. But this mashed potato won’t be smooth (translate: the gravitational-wave signal won’t be stationary and Gaussian) and this structure can indeed be precious for extracting more information from LISA. But first, let’s taste this with today’s paper, i.e. characterize the foreground.

Riccardo Buscicchio, Antoine Klein, Valeriya Korol, Francesco Di Renzo, Christopher J. Moore, Davide Gerosa , Alessandro Carzaniga.
arXiv:2410.08263 [astro-ph.HE].

ps. This started as the student project of Alessandro Carzaniga, great it’s finally out!

This is a fun IMBH story we worked out when Kostas and Luca were visiting last summer from JHU. What if (one day, who knows) we observe a highly spinning intermediate-mass black hole? If that happens, is going to be puzzling because IMBH that grow in clusters by mergers of smaller black holes tend to spin down, not up. This is a funny property of black holes, namely that extracting spins is easier than putting it in, so on average black holes slow down after they have merged many times. So if we see an IMBH with large spins, the spin must come from somewhere else. Where? Maybe gas. The argument then is that one can actually convert an IMBH spin measurement into the minimum amount of gas that must have been accreted to get that spin.

Konstantinos Kritos, Luca Reali,Davide Gerosa , Emanuele Berti.
Physical Review D 110 (2024) 123017.
arXiv:2409.15439 [astro-ph.HE].

To Stars or to gas, that is the question.
Whether ’tis nobler in the hardening to suffer
The slings and arrows of passing stars,
Or to dissipate against a sea of gas
And by disk end them. To inspiral — to merge,
No more; and by LISA to say we end
The models and the thousand PE samples
That gravity is heir to.

Alice Spadaro, Riccardo Buscicchio, David Izquierdo-Villalba, Davide Gerosa , Antoine Klein, Geraint Pratten.
Physical Review D 111 (2025) 023004.
arXiv:2409.13011 [astro-ph.HE].

All right I think this is great (but it took me a long time to convince myself and the others that’s the case!) In gravitational-wave astronomy we measure binaries, that is, pairs of two objects. Our signals have information about the pair as a whole. At the same time, we care very much about separating those two objects and measuring the properties of individual black holes and neutron stars. We always do that operation without thinking twice, just say that for each posterior sample object “1” is that with the larger mass and object “2” is that with the lower mass. But is that ok? Surely it’s a choice, but is it the best one? What does it even mean to pick the “best” labels? I think machine learning can help us here and that this problem can be framed using the language of semi-supervised clustering. The results? Well, they seem very significant. Measurements of the black-hole spins are more accurate, you can tell more easily if that’s a black hole or a neutron star, and overall the posterior distributions just look nicer (go away nasty multimodalities and non-Gaussianities!).

Davide Gerosa , Viola De Renzis, Federica Tettoni, Matthew Mould, Alberto Vecchio, Costantino Pacilio.
Physical Review Letters 134 (2025) 121402.
arXiv:2409.07519 [gr-qc].
Selected as PRL Editors’ Suggestion.
Press release : Milano-Bicocca.
Other press coverage: ilgiorno, lescienze, ansa.it, adnkronos (1), adnkronos (2), 30science, agenparl.eu, cagliarilivemagazine, ilcentrotirreno, ilgiornaleditalia, laragione, lospecialegiornale, meteoweb, msn.com, occhioche, padovanews, prpchannel, sardegnalive, smartphonology, tgabruzzo24, vetrinatv, unicaradio, altoadige, ecodibergamo, roboreporter, saluteh24, salutedomani.

The ringdown is the final bit of a gravitational-wave signal, after the two black holes have merged. It’s nice because it’s clean; GR is so powerful that all that comes out after a black hole merger has specific frequencies, the fantastic “quasi-normal modes.” While the frequencies only depend on that final BH (thanks Kerr!), the excitations of those frequencies depend on all that happened before, i.e. the merger process itself. In this summer paper by Costantino and the rest of us, we present a new accurate approximant to those amplitudes. Now go home and test GR.

Costantino Pacilio, Swetha Bhagwat, Francesco Nobili, Davide Gerosa.
Physical Review D 110 (2024) 103037.
arXiv:2408.05276 [gr-qc].

The orbits of binary black holes could be eccentric, but in practice they’re not. At least when we observe them, and that’s because of a relativistic effect that circularizes the orbit. Even if astrophysics formed black holes eccentric, relativity makes them circular when we observe them with gravitational-wave interferometers. But we’re interested in the astrophysics back then! What we find here is that the tiny residual eccentricity at detection can be crucial. Even eccentricities that are so small that we cannot tell them apart from circular can mess up the astrophysical inference. Unfortunately, this is a new systematic error that needs to be taken into account: inferring the “formation channel” of binary black holes might be even harder than we thought.

Giulia Fumagalli, Isobel Romero-Shaw, Davide Gerosa , Viola De Renzis, Konstantinos Kritos, Aleksandra Olejak.
Physical Review D 110 (2024) 063012.
arXiv:2405.14945 [astro-ph.HE].

… and we’re back to selection effects. That means modeling what you cannot see. The black holes that gravitational-wave detectors observe are not representative of those that are out there in the Universe. Some are easier to see, some are harder. Quantifying how much easier and harder is crucial to properly understand the underlying astrophysics. In this paper (which came out of a BSc student project!), we go back to the basics and work out gravitational-wave selection effects one step after the other, using and refining the most common approximation. Two things to remember: including noise fluctuations is easy, and a signal-to-noise ratio threshold of 11 is probably ok.

Davide Gerosa , Malvina Bellotti.
Classical and Quantum Gravity 41 (2024) 125002.
arXiv:2404.16930 [astro-ph.HE].

Population 3 stars are like “the original” stars. Those formed with material that comes straight from the Big Bang. It would be very (like, a lot!) cool to see them with gravitational-wave detectors. But can we tell them apart? Or do they look like all the other stars? Here is an attempt with a fancy machine-learning classifier.

Filippo Santoliquido, Ulyana Dupletsa, Jacopo Tissino, Marica Branchesi, Francesco Iacovelli, Giuliano Iorio, Michela Mapelli, Davide Gerosa , Jan Harms, Mario Pasquato.
Astronomy & Astrophysics 690 (2024) A362.
arXiv:2404.10048 [astro-ph.HE].

This is the first of two papers on the arxiv today: it’s fun when two long, very different projects by different people just happen to be done on the same day! This paper is by my former colleague Nate Steinle (now a postdoc in Manitoba, Canada). Here we connect the dynamics of jets in AGN disks to the spin of black holes observable by LISA. And show the latter is a diagnostic of the former! And it’s nice to see my disk-binary code being used for something I didn’t think of when I wrote it.

Nathan Steinle, Davide Gerosa , Martin G. H. Krause.
Physical Review D 110 (2024) 123034.
arXiv:2403.00066 [astro-ph.HE].

And the second paper on the arxiv today is Daria’s masterpiece! pAGN (which Daria says you should read “pagan”) is a brand new, super cool code that implements the hydrodynamics of AGN disks, at least in their most popular one-dimensional fashion. Those solutions have been around for a long time but their details were, well, let’s say unclear. Daria went through everything from beginning to end, coming up with the “one-stop solution for your AGN disc needs” (that was actually the working title of the paper…). So pip install pAGN and have fun.

Daria Gangardt, Alessandro Alberto Trani, Clément Bonnerot, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society, 530 (2024) 3986–3997.
arXiv:2403.00060 [astro-ph.HE].

Not sure what happened here, how the hell did I end up writing a paper with actual radio data that needed to be reduced … Call me an ambulance.

The guy here is 3C186 which is not a postcode but a quasar. A funny one because it’s not centered on the galaxy (it’s a bit off) and it’s also going at another velocity (ciao ciao). One of the leading explanations is that 3C186 is a recoiling black hole, the remnant of black-hole merger is being kicked away (yeah these things can happen). 3C186 also has a radio jet, and that should point in the direction of the black-hole spin. The funny thing is that spin and the kick appear perpendicular to each other, and this is fun because theory says they should actually be parallel. We looked into this a bit carefully and discovered it’s all a lie! The spin and the kick both point along the line of sight and appear perpendicular only because of a super strong projection effect. If this is true, the radio jet should also point straight to us! We then tried to test this with whatever ratio data we could grab (where is that ambulance) and found that… mmh, well, it’s a maybe.

Matteo Boschini, Davide Gerosa , Om Sharan Salafia, Massimo Dotti.
Astronomy & Astrophysics 686 (2024) A245.
arXiv:2402.08740 [astro-ph.GA].

In the gravitational-wave world, we usually say a binary merger is detected if it has a sufficiently large SNR (signal-to-noise ratio). But is that true? Detection pipelines are far more complicated than that. Here we try to figure out a section threshold from what’s detected. That is: (some) people agree that these guys are GWs, so what’s your SNR threshold for detectability? It’s like reading in the minds of a GW data analyst…

Matthew Mould, Christopher J. Moore, Davide Gerosa.
Physical Review D 109 (2023) 063013.
arXiv:2311.12117 [gr-qc].

I’m obsessed with spinning black-hole binaries but, guys, spinning and eccentric black holes are even better! This is the first first-author paper by Giulia, who is not only a rising GW astronomer but also a semi-professional baker… So take two spoons of black holes, one spoon of spin dynamics, some eccentricity (but less than 0.6 ounces), and a pinch of maths. Put this in a bowl, mix it thoroughly with numerical integrations …and the result is very tasty! Spins and eccentricity shape the dynamics of black-hole binaries together , which means one can hope to measure eccentricity indirectly from the spins, but also that if you forget about eccentricity then your spin inference will be crap. Buon appetito.

Giulia Fumagalli, Davide Gerosa.
Physical Review D 108 (2023) 124055.
arXiv:2310.16893 [gr-qc].

…and we’re back to testing GR. We’ve got many gravitational-wave events and would like to use them all together to figure out if our equations for gravity are correct. And here is the issue: there’s only one set (aka catalog) of black holes that contains all the black holes we’ve observed. Now that’s obvious you’d say, and you would be right!, much like we have a single Universe to observe (I’m not a language guy but indeed “Universe” means like “the whole thing”). This effect is known in cosmology (think those low-order multiples in the usual CMB plot), so we called it “the catalog variance of testing GR”. It’s bad, but the Baron Munchauseen tells us we can bootstrap.

Costantino Pacilio, Davide Gerosa , Swetha Bhagwat.
Physical Review D Letters 109 (2024) L081302.
arXiv:2310.03811 [gr-qc].

Gravitational-wave data keep on giving us surprises. The most outstanding one IMO is an observed correlation between mass ratios and spins of the black holes, which was first found by Tom Callister and friends. That is so, so weird… to the point that virtually zero astrophysical models so far can explain it fully and consistently. Well, we can’t either (at least not fully and consistently) but we think this paper is a nice attempt. The secret seems to be the symmetry of the astrophysical environment one considers, and data tends to prefer black holes assembled in cylindrical symmetry. That’s also weird to be honest, but there’s a candidate for this setup, namely accretion disks and their migration traps. Who knows, more data will tell.

… and huge congrats to my MSc student Alessandro who managed to publish a paper even before graduating!

Alessandro Santini, Davide Gerosa , Roberto Cotesta, Emanuele Berti
arXiv:2308.12998 [astro-ph.HE].
Physical Review D 108 (2023) 083033.
Other press coverage: astrobites.

New paper from a new student! Here is Matteo Boschini’s first piece of work, where we look at predictions for the final mass and spins of black-hole remnants. That is, after two black hole merge, what’s the mass and spin of the guy they left behind? These predictions are typically done by fitting (in various ways) outputs from numerical-relativity simulations but those, unfortunately, can only handle black holes of similar masses. On the other hand, black holes with masses that are very different from each other can be handled analytically. Here we show how to put the two together with a single machine-learning fit.

Matteo Boschini, Davide Gerosa , Vijay Varma, Cristobal Armaza, Michael Boyle, Marceline S. Bonilla, Andrea Ceja, Yitian Chen, Nils Deppe, Matthew Giesler, Lawrence E. Kidder, Guillermo Lara, Oliver Long, Sizheng Ma, Keefe Mitman, Peter James Nee, Harald P. Pfeiffer, Antoni Ramos-Buades, Mark A. Scheel, Nils L. Vu, and Jooheon Yoo.
Physical Review D 108 (2023) 084015.
arXiv:2307.03435 [gr-qc].

All right, this is kind of far from my day-to-day topics but working on this paper with Alice and Riccardo was super fun. Think LISA and supermassive binary black holes. And… the detector does what it wants. That’s not true of course because the experimentalists are amazing, but there will be noise transients: unexpected blips when the gravitational-wave signal will be corrupted. Here we look at what would happen in a realistic setting when a LISA glitch happens on top of a gravitational wave from a supermassive black hole.

Alice Spadaro, Riccardo Buscicchio, Daniele Vetrugno, Antoine Klein, Davide Gerosa , Stefano Vitale, Rita Dolesi, William Joseph Weber, Monica Colpi.
Physical Review D 108 (2023) 123029.
arXiv:2306.03923 [gr-qc].

We do population analysis in gravitational waves all the time now. That is: we compare many observations from GW experiments against many simulated datapoints from simulations. But what if you only have one observation? That could be a LIGO guy that is kind of an outlier (think GW190521) or maybe a datapoint from a future detector (think LISA) that feels lonely in his parameter space. Don’t look further, this is stats for you (and Matt’s last paper as a grad student…)

Matthew Mould, Davide Gerosa , Marco Dall’Amico, Michela Mapelli.
Monthly Notices of the Royal Astronomical Society, 525 (2023) 3986–3997
arXiv:2305.18539 [astro-ph.HE].

This paper is episode four in the up-down instability series. We first figured out the instability exists (episode 1), then computed when binaries go after the instability (i.e. the endpoint, episode 2), and also checked binaries are really unstable in numerical relativity (episode 3). Now we look at the inference problem with LIGO/Virgo: if unstable up-down binaries enter the sensitivity window of the detector, will we be able to tell? We phrased the problem with some fancy stats using the so-called Savage Dickey density ratio, which is the right tool to answer this question. As is too often the case, current data are not informative enough but the future is bright and loud.

Viola De Renzis, Davide Gerosa , Matthew Mould, Riccardo Buscicchio, Lorenzo Zanga.
Physical Review D 108 (2023) 024024.
arXiv:2304.13063 [gr-qc].

It’s out! The notorious (ask my students…) “ precession v2 ” paper is finally out! This took a veeeery long time; we checked and the first commit for this paper is from May 2020 (!). But the result is an exhilarating tour of spin precession at 2PN with 27 pages and 183 (!!!) numbered equations. We rewrote the entire formalism, change how we parametrize things, compute all we could in closed forms, and speed up the computational implementation. It’s cool, now performing a precession-averaged evolution is a <0.1s operation. If you’re into BH binary spin precession, this is the paper for you. All of this is now part v2 of our PRECESSION python module. So long, and thanks for all the spin.

Davide Gerosa , Giulia Fumagalli, Matthew Mould, Giovanni Cavallotto, Diego Padilla Monroy, Daria Gangardt, Viola De Renzis.
Physical Review D 108 (2023) 024042.
arXiv:2304.04801 [gr-qc].
Open-source code: homepage, repository, documentation.

Third-generation gravitational wave detectors are going to see all stellar-mass black-hole mergers in the Universe. Wooooooooo. But hang on, is this enough? Observing the sources is great, but then we need to measure them. Here we try to focus on the latter and quantify how well we will be able to measure the distance of black holes. Read the paper now, but the short answer is that 3G detectors are going to be awesome but not that awesome…

Michele Mancarella, Francesco Iacovelli, Davide Gerosa.
Physical Review D Letters 107 (2023) L101302.
arXiv:2303.16323 [gr-qc].

We want more! With gravitational-wave data, some quantities like the masses of the black holes are much easier to see than others. But those others are very interesting, notably spins that process and orbits that are eccentric, because they would tell us how black hole binaries came to be in the first place. So while it would be great to see those, it’s also being very hard. Some tentative claims have been made with current data, but nothing unambiguous so far. In this paper led by Isobel from Cambridge, we show that (surprise surprise…) the signals needs to be long enough before one can tell eccentricity and spin precession apart.

Isobel Romero-Shaw, Davide Gerosa , Nicholas Loutrel.
Monthly Notices of the Royal Astronomical Society 519 (2023) 5352–5357.
arXiv:2211.07528 [astro-ph.HE].

Together with my postdoc Nate, we’re proceeding our investigations on supermassive, spinning binary black holes surrounded by accretion disks (that is: a ton of gas around big monsters at the center of galaxies!). In today’s paper, we dig a bit deeper into what happens when the disk breaks. That presumably stops the interactions between the gas and the black-hole spins which could make all this funky astrophysics (spins that moves, disks that breaks, etc) actually observable with future gravitational-wave detectors. More needs to be done of course, but here we are.

Nathan Steinle, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society 519 (2023) 5031–5042.
arXiv:2211.00044 [astro-ph.HE].

Lots of “firsts” today! My first -year PhD student Viola just put out her first first -author paper. This is about measuring black holes with not one, but two precessing spins. People have been trying to figure out how to tell if at least one of the two spins of a merging black-hole binary is precessing for quite some time now. And maybe we’ve even done it already for one or two of the current LIGO-Virgo events. But here I must quote that epic Italian commercial from the 90s: “two gust is megl che one” (which is a terrible Italian-English mishmash on a terrible joke to say that when you eat a Maxibon “two flavors are better than one”). In this paper we propose a strategy to identify sources that have the strongest evidence of two processing spins. Viola has been putting together simulated data for the next LIGO/Virgo data-taking period, and the result is pretty cool. If these binaries are out there in the Universe, we will be able to tell they have two spins going around!

Viola De Renzis, Davide Gerosa, Geraint Pratten, Patricia Schmidt, Matthew Mould.
Physical Review D 106 (2022) 084040.
arXiv:2207.00030 [gr-qc].

Big stars burn everything they have, die fast, and produce big black holes. So when you see two black holes together, it’s likely that the big black hole comes from the big star. Or maybe not? Before dying, the big star can drop some mass onto the other guy, making it bigger! So now, the initially big star still produces the first black hole, but, at the end of the day, that might not be the more massive black hole anymore! This scenario is called “mass-ratio reversal” and our astrophysics friends have put together many models out there showing this is indeed possible for a good fraction of the black holes that produce gravitational-wave events. So here we ask the data: given the events LIGO and Virgo have seen so far, what’s the evidence for mass-ratio reversal in binary stars? Read Matt’s paper to find out.

Matthew Mould, Davide Gerosa , Floor S. Broekgaarden, Nathan Steinle.
Monthly Notices of the Royal Astronomical Society 517 (2022) 2738–2745.
arXiv:2205.12329 [astro-ph.HE].

Observing gravitational waves from the ground (i.e. LIGO, Virgo, etc) give us a unique view on “the last three minutes” of the life of compact objects before they merge with each other. Going to space (I’m talking to you, LISA!) will instead give us “the last three years”. Completed together with the rest of the Birmingham crowd, this paper provides a realistic view of this truly amazing landscape. LISA observations at low frequencies in the 2030s will be paired with high-frequency data from LIGO’s successors (the so-called 3rd generation detectors). Together (and that’s crucial, together!) LISA and 3g detectors will tell us the full story of the life of merging black holes. LIGO alone is like catching up with a movie because you were late at the theatre, LISA alone is like a huge cliffhanger before the series finale… multiband observations are a bingewatching experience!

Antoine Klein, Geraint Pratten, Riccardo Buscicchio, Patricia Schmidt, Christopher J. Moore, Eliot Finch, Alice Bonino, Lucy M. Thomas, Natalie Williams, Davide Gerosa , Sean McGee, Matt Nicholl and Alberto Vecchio.
arXiv:2204.03423 [gr-qc].

Daria’s new paper is out! (With key contributions from others in the group… This is also Viola’s first paper!).

Here we look at sub-dominant black-hole spin effects in current data from LIGO and Virgo (yeah sorry guys… our black-hole spin obsession goes on). People have looked at spin precession before, but we’re interested in even more subtle things, namely disentangling precession and nutation. This is a tricky business, which is made complicated by the fact that this piece of information is hidden behind other parameters that are easier to measure (say the masses of the two black holes). Our paper is an attempt to formulate and systematically exploit something we called “sequential prior conditioning” (which is: mix&match priors and posteriors in Bayesian stats…). Results are weak today but strong tomorrow.

Daria Gangardt, Davide Gerosa , Michael Kesden, Viola De Renzis, Nathan Steinle.
Physical Review D 106 (2022) 024019.
arXiv:2204.00026 [gr-qc].

It took a while (so many technical challenges…) but we made it! Matt‘s monster paper is finally out!

Let me introduce a fully-fledged pipeline to study populations of gravitational-wave events with deep learning. If it sounds cool, well, it is cool (just look at the flowchart in Figure 1!). We can now perform a hierarchical Bayesian analysis on GW data but, unlike current state-of-the-art applications that rely on simple functional form, we can use populations inferred from numerical simulations. This might sound like a detail but it’s not: it’s necessary to compare GW data directly against stellar physics. While we don’t do that yet here (our simulations are admittedly too simple), there’s a ton of astrophysics already in this paper. Whether you care about neural networks or hierarchical black-hole mergers (or, why not, both!), sit tight, fasten your seatbelt, and read Matt’s paper.

Matthew Mould, Davide Gerosa , Stephen R. Taylor.
Physical Review D 106 (2022) 103013.
arXiv:2203.03651 [astro-ph.HE].

Spinning black holes are weird (well, all black holes are weird but those that spin are the worse!). They have a funny thing called ergoregion where orbiting particles can have negative energy. Penrose was the first to realize that this can be exploited to extract energy from the black hole itself. The thing is, even if you figure out how to do it, you’re inevitably going to spin the black hole down. At the end of the day, you’re left with a fossil black hole that does not have any spin. The mass of that leftover black hole (“ What’s for lunch dear? Fancy some sushi or prefer a black hole?”) is called irreducible mass. Hawking (another giant!) figured out this has to do with thermodynamics.

Long story short, in this paper we compute the irreducible mass of the black holes detected in gravitational waves by LIGO. It was funny to re-discover that gravitational wave detection was indeed the motivation behind Hawking original proof of the area theorem (he had Weber‘s claimed detection in mind at the time). The story behind our paper starts as a toy calculation with my undergraduate student Cecilia and ended up in a neat, hopefully informative exploitation of LIGO data. We reparametrized LIGO’s black-hole properties using the rotational and rotational contributions to their total energy, we ranked current gravitational-wave events according to their “irreversibility”, and we compute a sort of population version of the area law. Enjoy!

Davide Gerosa , Cecilia Maria Fabbri, Ulrich Sperhake.
Classical and Quantum Gravity 39 (2020) 175008.
arXiv:2202.08848 [gr-qc].

Breaking things is fun! In the previous paper of this series, we looked at accretion disks around massive black-hole binaries and found things were going awry. We kept on finding configurations that our implementation could not handle… And now we know this is real! Finding disk solutions when the spin of the black hole has a large misalignment is just not possible! And that’s because the disk really breaks into different sections. We’ve now checked it with state-of-the-art hydrodynamical numerical simulations that not only confirm what we suspected but also show some funny things (like breaking being prevented by disk spirals, etc). I was serious, breaking things is real fun!

Check out Rebecca’s beautiful movies!

Rebecca Nealon, Enrico Ragusa, Davide Gerosa , Giovanni Rosotti, Riccardo Barbieri.
Monthly Notices of the Royal Astronomical Society 509 (2022) 5608–5621.
arXiv:2111.08065 [astro-ph.HE].

Great Scott, a new paper! When analyzing gravitational-wave data, looking at one black hole at a time is not enough anymore, the fun part is looking at them all together. The issue Matt and I are tackling here is that one needs to be consistent with putting together different events when fitting the entire population. This is obvious for things that do not change (say the masses of the black holes, those are what they are), but becomes a very tricky business for varying quantities (say the spin directions, which is what we look at here). In that case, it’s dangerous to put together events taken at different stages of their evolution. And the solution to this problem is…. time travel! We show that but propagating binaries backward in time, one can put all sources on the same footing. After that, estimating the impact of the detector requires traveling forward in time, so going “back to the future”. After all, we all know that post-Newtonian black-hole binary integrations look like this:

Matthew Mould, Davide Gerosa.
Physical Review D 105 (2022) 024076.
arXiv:2110.05507 [astro-ph.HE].

No black hole is an island entire of itself.

We’ve got many gravitational wave events now. One can look at each of them individually (aka “parameter estimation”), all of them together (aka “population”), or each of them individually while they’re together. That’s what we do in this paper: we look at the properties of individual gravitational-wave events in light of the rest of the observed population. The nice thing is that all of these different ways of looking at the data are part of the same statistical tool, which is a hierarchical Bayesian scheme. Careful, heavy stats inside, don’t do this at home.

Christopher J. Moore, Davide Gerosa.
Physical Review D 104 (2021) 083008.
arXiv:2108.02462 [gr-qc].

Today we go deep into the perilous world of binary population synthesis! Using Nicola’s code MOBSE, our master student Maciej has implemented some new prescriptions for how supernovae explode and produce compact objects. In practice, we use the compactness (that’s mass over radius) of the stellar core before the explosion to decide if that specific star will form a neutron star or a black hole. This now needs to be compared carefully with gravitational-wave data, but we suggest that there are two key signatures one should look for: the lowest black hole masses and the relative merger rates between black holes and neutron stars.

Maciej Dabrowny, Nicola Giacobbo, Davide Gerosa.
Rendiconti Lincei. Scienze Fisiche e Naturali 32 (2021) 665–673.
arXiv:2106.12541 [astro-ph.HE].

LISA is going to be great and will detect stuff from white dwarfs to those supermassive black-hole that live at the center of galaxies. If we’re lucky (yeah, who knows how many of these we will see), LISA might also detect some smaller black holes, similar to those that LIGO now sees all the time, but at a much earlier stage of their lives. But if we’re indeed lucky, the science we would take home is outstanding. Using simulated data from the LISA Data Challenge we unleash the new amazing parameter-estimation code Balrog (don’t ask what it means, it’s just a name, not one of those surreal astronomy acronyms) at this problem. Dive into the paper for some real data-analysis fun!

Riccardo Buscicchio, Antoine Klein, Elinore Roebber, Christopher J. Moore, Davide Gerosa , Eliot Finch, Alberto Vecchio.
Physical Review D 104 (2021) 044065.
arXiv:2106.05259 [astro-ph.HE].

Who are the parents of LIGO’s black holes? Stars, most likely. Things like those we see in the sky at night will eventually surrender to gravity and collapse. Some of them will form black holes. Some of them will form binary black holes. Some of them will merge. Some of them will be observed by LIGO. That’s the vanilla story at least, but it might not apply to all of the black holes that LIGO sees. For some of those, stars might be the grandparents or the great grandparents. And the parents are … just other black holes! This is today’s paper lead by Vishal Baibhav. Instead of just measuring the properties of the black holes that LIGO observes, we show we can also say something about the features of the black hole parents. Read on to explore the black-hole family tree.

Vishal Baibhav, Emanuele Berti, Davide Gerosa , Matthew Mould, Kaze W. K. Wong.
Physical Review D 104 (2021) 084002.
arXiv:2105.12140 [gr-qc].

The quest of finding their astrophysical origin of merging black-hole binaries is now a key open problem in modern astrophysics. Stars are the natural progenitor of black holes: at the end of their lives, the core collapses and leaves behind a compact object. But once those “first-generation” black holes are around, they can potentially meet again and form “second generation” LIGO events. I first got interested in this problem in 2017 and, together with many many others researchers in the community, we explored the consequences of this “hierarchical merger” scenario in terms of both gravitational-wave physics and astrophysical environments. In this Nature Astronomy review article, Maya and I tried to condense all this body of work into a few pages. The result is (we hope) a broad and informed overview of this emerging research strand, with a whopping number of more than 270 citations! Hope you like it.

Davide Gerosa , Maya Fishbach.
Nature Astronomy 5 (2021) 749-760.
arXiv:2105.03439 [astro-ph.HE].
Review article.
Press release : Birmingham.
Other press coverage: SciTechDaily, techexplorist, sci-news, Media INAF, globalscience, futura-sciences, europapress, la Razon, astroblogs, phys.org, ScienceDaily, Mirage News Australia, World News Monitor, nanowerk, newsbeezer, SpaceDaily.

Hierarchical mergers are the new black. LIGO is seeing black holes that are just too big to be there. The reason is that stars, which collapse and produce black holes, do some funny things when they get too massive. Notably, they start to spontaneously produce positrons and electrons instead of keeping their own photons. Long story short: those missing photons make the temperature go up, ignite an explosion that disrupts the core and prevents black-hole formation. This “mass gap” is a solid prediction from our astrophysics friends. In some previous papers, we and other groups pointed out that one can bypass stars and form black holes from previous black holes (and goodbye my dear maximum mass limit!). But now our astrophysics friends are telling us they can also evade the limit with some more elaborate astro-magic (winds, rotation, dredge-up, reaction rates, accretion). Today’s paper is about telling the two apart, with a key prediction: a black hole with large mass but low spin would raise a glass to the astro-wizards.

Davide Gerosa , Nicola Giacobbo, Alberto Vecchio.
Astrophysical Journal, 915 (2021) 56.
arXiv:2104.11247 [astro-ph.HE].

General Relativity works well. But we still want to test it, and I guess that’s because it actually works too well (you know, all those quantum things that don’t really fit, etc). And we want to test it with gravitational-wave data, and not just because it’s the new cool thing to do (though it is!) but also because they gravitational waves give us insight into the strong-field regime of gravity where new things, if they are there at all, should show up. Now, all of this sounds great but, in practice, one has to deal with the actual model used to analyze the data. Errors in these signal models (aka waveforms), which are somewhat inevitable, can trick us into thinking we have seen a deviation from General Relativity. So, before you go out on the street and shout that Einstein was wrong, keep calm and mind your waveform.

Christopher J. Moore, Eliot Finch, Riccardo Buscicchio, Davide Gerosa.
iScience 24 (2021) 102577.
arXiv:2103.16486 [gr-qc].
Other press coverage: indiescience, sciencedaily, phys.org, astronomy.com, physicsworld.

ps. The codename for this paper was SANITY: S ystemA tics usiN g populatI ons to T est general relativitY.

Here is the latest in our (by now long) series of papers on black-hole binaries spin precession. This work was is championed by two outstanding PhD students, Daria (in my group) and Nate (UT Dallas). The key idea behind this paper is that, for black-hole spins, one cannot really talk about precession without talking about nutation (although we only say “precession” all the time…). The spin of, say, the Earth also does both precession (azimuthal motion) and nutation (polar motion). But, unlike in the Earth problem, for black-hole spins the two motions happen on roughly the same timescale meaning that you cannot really take them apart. Or can you? We stress the role of five parameters that characterize the combined phenomenology of precession and nutation. The hope is now to use them as building blocks for future waveforms… stay tuned!

Daria Gangardt, Nathan Steinle, Michael Kesden, Davide Gerosa , Evangelos Stoikos.
Physical Review D 103 (2021) 124026.
arXiv:2103.03894 [gr-qc].

ps. Stupid autocorrect! It’s nutation, not mutation.

Orbital eccentricity in gravitational-wave observations has been long neglected. And with good reasons! Gravitation-wave emission tends to circularize sources. By the time black holes are detectable by LIGO/Virgo/LISA/whatever, they should have had ample time to become circular. Unless something exciting goes on in their formation, things like clusters, triples, Kozai-Lidov oscillations, etc. And if that happens, we want to see it! This paper contains the first model for gravitational waveforms and black-hole remnants (final mass, spin) trained directly on eccentric numerical relativity simulations. Because eccentric is the new circular.

Tousif Islam, Vijay Varma, Jackie Lodman, Scott E. Field, Gaurav Khanna, Mark A. Scheel, Harald P. Pfeiffer, Davide Gerosa , and Lawrence E. Kidder.
Physical Review D 103 (2021) 064022.
arXiv:2101.11798 [gr-qc].

Up-down instability S01-E03.
“Previously on the up-down instability. After finding out that the instability exists (S01-E01) and calculating its analytic endpoint (S01-E02), one terrifying prospect remains. What if it’s just PN? Can all of this disappear in the strong-field regime? This challenge now needs to be faced”.

Today’s paper is the latest in our investigations of the up-down instability in binary black holes. If the primary black hole is aligned and the secondary is anti-aligned to the orbital angular momentum, the entire system is unstable to spin precession. We found this funny thing using a post-Newtonian ( read : approximate)__ treatment but we couldn’t be 100% sure that this would still be true when the black holes merge and our approximation fails. So, we got our outstanding SXS friends on board and ask them if they could see the same effect with their numerical relativity (read : the real deal!) code. And the answer is… yes! The instability is really there! And by the way, these are among the longest numerical relativity simulations ever done.

Vijay Varma, Matthew Mould, Davide Gerosa , Mark A. Scheel, Lawrence E. Kidder, Harald P. Pfeiffer.
Physical Review D 103 (2021) 064003.
arXiv:2012.07147 [gr-qc].
Supporting material available here.

Spin precession is cool, and we want to measure it. In General Relativity, the orbital plane of a binary is not fixed but moves around. This effect is related to the spin of the orbiting black holes and contains a ton of astrophysical information. The question we try to address in this paper is the following: how does one quantify “how much” precession a system has? This is typically done by condensing information into a parameter called \(\chi_{\rm p}\), which is here generalize to include two- spin effects. There are two black holes in a binary and we received numerous complaints from the secondaries: they want to join the gravitational-wave fun!

Davide Gerosa , Matthew Mould, Daria Gangardt, Patricia Schmidt, Geraint Pratten, Lucy M. Thomas.
Physical Review D 103 (2021) 064067.
arXiv:2011.11948 [gr-qc].
Open-source code: homepage, repository.

And here is the latest episode in the series of our massive scalar-tensor gravity papers… After stellar collapse, we now look at how neutron stars look like in this strange theory of gravity (recap: “massive scalar-tensor” means that gravity is mediated by the usual metric plus a scalar field which as a mass). Result: not only the theory is strange, stars are strange too! If you want to get a neutron star of 40 solar masses, look no further, massive scalar-tensor is the theory for you. More seriously, we explore all the different families of static solutions, highlighting a remarkable phenomenology. This is the kind of predictions we need to test gravity with astrophysical sources!

Roxana Rosca-Mead, Christopher J. Moore, Ulrich Sperhake, Michalis Agathos, Davide Gerosa.
Symmetry 12 (2020) 1384.
arXiv:2007.14429 [gr-qc]

And here is my latest lockdown effort: some experiments in the wonderful and perilous world of machine learning. The idea of this paper is to teach a computer to figure out by itself if a gravitational-wave signal will be detectable or not. The problem is very similar to that of image recognition: much like classifying if an image is more likely to contain a dog or a cat, here we classify black-hole mergers based on the imprints they have in the LIGO and Virgo detectors. This is important to quantify the so-called “selection effects”: in order to figure out what the Universe does based on what we observe, we need to know very well “how” we observe and thus what we are going to miss. Our code is built using Google’s TensorFlow and it is public on Github, feel free to play with it!

Davide Gerosa , Geraint Pratten, Alberto Vecchio.
Physical Review D 102 (2020) 103020.
arXiv:2007.06585 [astro-ph.HE]
Open-source code: homepage, repository.

Supermassive black hole inspiral and spin evolution are deeply connected. In the early stages when black holes are brought together by star scattering and accretion, spin orientations can change because of interactions with the environment. Later on, when gravitational waves are driving the mergers, spins change because of relativistic couplings. In this paper we try to follow this complicated evolution in a full cosmological framework, using products of the Illustris simulation suite, a new sub-resolution model, and post-Newtonian integrations.

Mohammad Sayeb, Laura Blecha, Luke Zoltan Kelley, Davide Gerosa , Michael Kesden, July Thomas.
Monthly Notices of the Royal Astronomical Society 501 (2020) 2531–2546.
arXiv:2006.06647 [astro-ph.GA].

If General Relativity is too boring, couple it to something else. In this paper we study what happens to stellar collapse and supernova explosions if gravity is transmitted not only with the usual metric of Einstein’s theory (aka the graviton) but also an additional quantity. If this extra scalar field has a mass, it dramatically impacts the emitted gravitational waves… Which means that maybe, one day, one can use gravitational-wave data to figure out if scalar fields are coupled to gravity. Here we try to explore all the related phenomenology of stellar collapse with a large set of simulations covering the parameter space. And the overall picture is remarkably neat and simple!

Roxana Rosca-Mead, Ulrich Sperhake, Christopher J. Moore, Michalis Agathos, Davide Gerosa , Christian D. Ott.
Physical Review D 102 (2020) 044010.
arXiv:2005.09728 [gr-qc].