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.
D. Gangardt, A. A. Trani, C. Bonnerot, D. Gerosa.
Monthly Notices of the Royal Astronomical Society 530 (2024) 3986–3997. arXiv:2403.00060 [astro-ph.HE].
Open source code.
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.
First thing you do after a 4h 15m viva? Eat a cookie baked by Giulia!
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.
M. Mould, D. Gerosa, F. S. Broekgaarden, N. 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!
A. Klein, G. Pratten, R. Buscicchio, P. Schmidt, C. J. Moore, E. Finch, A. Bonino, L. M. Thomas, N. Williams, D. Gerosa, S. McGee, M. Nicholl, A. 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.
M. Mould, D. Gerosa, S. 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:
ps. The v1 title on the arxiv was more explicit… too bad they took it away.
M. Mould, D. 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.
C. J. Moore, D. 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.
M. Dabrowny, N. Giacobbo, D. Gerosa.
Rendiconti Lincei 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!
R. Buscicchio, A. Klein, E. Roebber, C. J. Moore, D. Gerosa, E. Finch, A. 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.
V. Baibhav, E. Berti, D. Gerosa, M. Mould, K. 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.
D. Gerosa, M. Fishbach.
Nature Astronomy 5 (2021) 749-760. arXiv:2105.03439 [gr-qc].
Review article. Covered by press release.
The next few years are expected to be a golden age for pulsar timing array (PTA) science. The recent tentative claim of a detection of an astrophysical signal in the NANOGrav 12.5-year data set is expected to be confirmed, thereby opening a new observational window on supermassive black holes. In order to better follow these developments, Chris Moore and I will run a spring journal club in which we aim to review some key papers in the field. More info: [davidegerosa.com/ptaprimer/][/ptaprimer].
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.
D. Gerosa, N. Giacobbo, A. 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.
ps. The codename for this paper was SANITY: S ystemA tics usiN g populatI ons to T est general relativitY.
C. J. Moore, E. Finch, R. Buscicchio, D. Gerosa.
iScience 24 (2021) 102577. arXiv:2103.16486 [gr-qc].
Other press coverage: indiescience, sciencedaily, phys.org, astronomy.com, physicsworld.
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!
ps. Stupid autocorrect! It’s nutation, not mutation.
D. Gangardt, N. Steinle, M. Kesden, D. Gerosa, E. Stoikos.
Physical Review D 103 (2021) 124026. arXiv:2103.03894 [gr-qc].
We just received our new computing servers (thanks Royal Society). These are two machines of 96 cores each and a ton of RAM, and will support our activities in computational astrophysiscs. Their nicknames are xwing and tiefighter. Huge thanks David Stops for helping with the setup.
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.
T. Islam, V. Varma, J. Lodman, S. E. Field, G. Khanna, M. A. Scheel, H. P. Pfeiffer, D. Gerosa, L. 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
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.
V. Varma, M. Mould, D. Gerosa, M. A. Scheel, L. E. Kidder, H. P. Pfeiffer.
Physical Review D 103 (2021) 064003. arXiv:2012.07147 [gr-qc].
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!
D. Gerosa, M. Mould, D. Gangardt, P. Schmidt, G. Pratten, L. M. Thomas.
Physical Review D 103 (2021) 064067. arXiv:2011.11948 [gr-qc].
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).
S. Vitale, D. Gerosa, W. M. Farr, S. R. Taylor.
Chapter in: Handbook of Gravitational Wave Astronomy, Springer, Singapore. 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 (click on “Premio Ministro della Cultura”).
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!
R. Rosca-Mead, C. J. Moore, U. Sperhake, M. Agathos, D. 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!
D. Gerosa, G. Pratten, A. Vecchio.
Physical Review D 102 (2020) 103020. arXiv:2007.06585 [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.
M. Sayeb, L. Blecha, L. Z. Kelley, D. Gerosa, M. Kesden, J. Thomas.
Monthly Notices of the Royal Astronomical Society 501 (2021) 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!
R. Rosca-Mead, U. Sperhake, C. J. Moore, M. Agathos, D. Gerosa, C. 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.
D. Gerosa, S. Vitale, E. Berti.
Physical Review Letters 125 (2020) 101103. arXiv:2005.04243 [astro-ph.HE].
Covered by press release.
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!
L. Reali, M. Mould, D. Gerosa, V. 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.
ps. Here is a Twitter thread by P. Armitage.
D. Gerosa, G. Rosotti, R. Barbieri.
Monthly Notices of the Royal Astronomical Society 496 (2020) 3060-3075. arXiv:2004.02894 [astro-ph.GA].
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.
V. Baibhav, D. Gerosa, E. Berti, K. W. K. Wong, T. Helfer, M. 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).
M. Mould, D. Gerosa.
Physical Review D 101 (2020) 124037. arXiv:2003.02281 [gr-qc].
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!
ps. The second half of the story is here.
V. Korol, S. Toonen, A. Klein, V. Belokurov, F. Vincenzo, R. Buscicchio, D. Gerosa, C. J. Moore, E. Roebber, E. M. Rossi, A. Vecchio.
Astronomy & Astrophysics 638 (2020) A153. arXiv:2002.10462 [astro-ph.GA].
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…
ps. Here is the first half of the story.
ps2. The code still needs a name. Suggestions?
E. Roebber, R. Buscicchio, A. Vecchio, C. J. Moore, A. Klein, V. Korol, S. Toonen, D. Gerosa, J. Goldstein, S. M. Gaebel, T. E. Woods.
Astrophysical Journal 894 (2020) L15. arXiv:2002.10465 [astro-ph.GA].
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.
E. Barausse, et al. (320 authors incl. D. Gerosa).
General Relativity and Gravitation 52 (2020) 8, 81. arXiv:2001.09793 [gr-qc].
I was recently awarded a research grant from the Royal Society (woooo!). My research proposal is titled “The supermassive black-hole binary puzzle: putting the pieces together.” This was in response of a solicitation for early career scientists who are establishing their research group.
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.
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.
U. Sperhake, R. Rosca-Mead, D. Gerosa, E. 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.
ps. The nickname of this project was sigmaspops
K. W. K. Wong, D. Gerosa.
Physical Review D 100 (2019) 083015. arXiv:1909.06373 [astro-ph.HE].
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.
A. Gupta, D. Gerosa, K. G. Arun, E. Berti, W. 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?
D. Gerosa, E. Berti.
Physical Review D 100 (2019) 041301R. arXiv:1906.05295 [astro-ph.HE].
Covered by press release.
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…
V. Baibhav, E. Berti, D. Gerosa, M. Mapelli, N. Giacobbo, Y. Bouffanais, U. 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.
C. J. Moore, D. Gerosa, A. Klein.
Monthly Notices of the Royal Astronomical Society 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).
Y. Bouffanais, M. Mapelli, D. Gerosa, U. N. Di Carlo, N. Giacobbo, E. Berti, V. 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.
V. Varma, S. E. Field, M. A. Scheel, J. Blackman, D. Gerosa, L. C. Stein, L. E. Kidder, H. 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!
D. Gerosa, S. Ma, K. W. K. Wong, E. Berti, R. O’Shaughnessy, Y. Chen, K. Belczynski.
Physical Review D 99 (2019) 103004. arXiv:1902.00021 [astro-ph.HE].
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.
We found this wide precession during Alicia’s SURF undergraduate summer project at Caltech!
D. Gerosa, A. Lima, E. Berti, U. Sperhake, M. Kesden, R. O’Shaughnessy.
Classical and Quantum Gravity 36 (2019) 105003. arXiv:1811.05979 [gr-qc].
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.
R. Tso, D. Gerosa, Y. Chen.
Physical Review D 99 (2019) 124043. arXiv:1807.00075 [gr-qc].
Other press coverage: astrobites.