# Davide Gerosa

May 25, 2022

## Which black hole formed first? Mass-ratio reversal in massive binary stars from gravitational-wave data

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
arXiv:2205.12329 [astro-ph.HE].

May 4, 2022

## New horizons for fundamental physics with LISA

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].

April 26, 2022

## PhD in gravitational physics!

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:

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:

• “Gravitational-wave data and black-hole binary dynamics”, supervised by Gerosa. Possible research directions include statistical inference from LIGO/Virgo and LISA data, application of machine-learning tools to gravitational-wave astronomy, and theoretical investigations of black-hole binaries.
• “Dynamics of massive black hole binaries in dense stellar systems”, supervised by Sesana and Gualandris. This is a dual-doctorate position in partnership with the University of Surrey, UK. The main focus is the understanding of binary evolution using N-body simulations and analytical modeling. The successful candidate will spend 50% of their time at Bicocca and 50% of their time at Surrey.

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].

April 22, 2022

## Long-term research appointment in computational astrophysics at Milano-Bicocca (Italy)

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]).

April 21, 2022

## Got an ISCRA-B supercomputer allocation!

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!

April 13, 2022

## “With a little help from my friends” Workshop at JHU

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/

April 8, 2022

## The last three years: multiband gravitational-wave observations of stellar-mass binary black hole

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].

April 4, 2022

## Constraining black-hole binary spin precession and nutation with sequential prior conditioning

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.
arXiv:2204.00026 [gr-qc].

March 11, 2022

## Astrophysics with the Laser Interferometer Space Antenna

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].

March 9, 2022

## Deep learning and Bayesian inference of gravitational-wave populations: hierarchical black-hole mergers

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.
arXiv:2203.03651 [astro-ph.HE].

March 2, 2022

## New class! Astrostatistics

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.

February 23, 2022

## Congrats Cecilia!

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!

February 21, 2022

## The irreducible mass of LIGO’s black holes

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.
arXiv:2202.08848 [gr-qc].

January 20, 2022

## People visiting

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.

January 7, 2022

## Cariplo Foundation Grant

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”!

December 6, 2021

## TEONGRAV

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!

November 17, 2021

## The Bardeen-Petterson effect in accreting supermassive black-hole binaries: disc breaking and critical obliquity

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 (2021) 5608–5621.
arXiv:2111.08065 [astro-ph.HE].

October 13, 2021

## Gravitational-wave population inference at past time infinity

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].

October 11, 2021

## Nate is joining us!

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.

October 4, 2021

## Postdoctoral fellowships in gravitational-wave astronomy at Milan-Bicocca (Italy)

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].

September 18, 2021

## Welcome Viola!

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!

September 8, 2021

## SIGRAV Prize for Young Researchers

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.

September 1, 2021

## Moving (back to) Milan!

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.

August 6, 2021

## Population-informed priors in gravitational-wave astronomy

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].

July 14, 2021

## Well done Max!

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.

July 11, 2021

## Settimana dell’astronomia 2021

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!).

June 24, 2021

## Modeling the outcome of supernova explosions in binary population synthesis using the stellar compactness

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].

June 10, 2021

## A new IREU friend from Missouri

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!

June 10, 2021

## Bayesian parameter estimation of stellar-mass black-hole binaries with LISA

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].

May 27, 2021

## Looking for the parents of LIGO’s black holes

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].

May 18, 2021

## Come to Milan for a PhD!

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]).

May 11, 2021

## Hierarchical mergers of stellar-mass black holes and their gravitational-wave signatures

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.

April 26, 2021

## High mass but low spin: an exclusion region to rule out hierarchical black-hole mergers as a mechanism to populate the pair-instability mass gap

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].

March 31, 2021

## Testing general relativity with gravitational-wave catalogs: the insidious nature of waveform systematics.

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: SystemAtics usiNg populatIons to Test general relativitY.

March 31, 2021

## Group study on BH binaries in AGN disks

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.

March 9, 2021

## A taxonomy of black-hole binary spin precession and nutation

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.

January 29, 2021

## Eccentric binary black hole surrogate models for the gravitational waveform and remnant properties: comparable mass, nonspinning case

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].

January 22, 2021

## HopBham!

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/

December 15, 2020

## Up-down instability of binary black holes in numerical relativity

[Intro music…Now imagine one of those voices from a TV show trailer…]

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.

November 25, 2020

## A generalized precession parameter chi_p to interpret gravitational-wave data

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: homepagerepository.

November 16, 2020

## Nicola joins the band

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!

November 10, 2020

## Inferring the properties of a population of compact binaries in presence of selection effects

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].

September 25, 2020

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.

September 17, 2020

## ERC Starting Grant

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.

September 14, 2020

## Daria’s PhD adventure starts here

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

September 11, 2020

## Congrats to MSc students

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.

July 30, 2020

## Structure of neutron stars in massive scalar-tensor gravity

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]

July 15, 2020

## Gravitational-wave selection effects using neural-network classifiers

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.

June 12, 2020

## Massive black hole binary inspiral and spin evolution in a cosmological framework

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].

May 20, 2020

## Core collapse in massive scalar-tensor gravity

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].