PRD

Orbital eccentricity in general relativity from catastrophe theory

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

M. Boschini, N. Loutrel, D. Gerosa, G. Fumagalli.
Physical Review D 111 (2025) 024008. arXiv:2411.00098 [gr-qc].


Minimum gas mass accreted by spinning intermediate-mass black holes in stellar clusters

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

K. Kritos, L. Reali, D. Gerosa, E. Berti.
Physical Review D 110 (2024) 123017. arXiv:2409.15439 [astro-ph.HE].


Stars or gas? Constraining the hardening processes of massive black-hole binaries with LISA

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

A. Spadaro, R. Buscicchio, D. Izquierdo-Villalba, D. Gerosa, A. Klein, G. Pratten.
Physical Review D 111 (2025) 023004 . arXiv:2409.13011 [astro-ph.HE].


Flexible mapping of ringdown amplitudes for nonprecessing binary black holes

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

C. Pacilio, S. Bhagwat, F. Nobili, D. Gerosa.
Physical Review D 110 (2024) 103037. arXiv:2408.05276 [gr-qc].


Residual eccentricity as a systematic uncertainty on the formation channels of binary black holes

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

G. Fumagalli, I. Romero-Shaw, D. Gerosa, V. De Renzis, K. Kritos, A. Olejak.
Physical Review D 110 (2024) 063012. arXiv:2405.14945 [astro-ph.HE].


Probing AGN jet precession with LISA

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

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


Calibrating signal-to-noise ratio detection thresholds using gravitational-wave catalogs

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

M. Mould, C. J. Moore, D. Gerosa.
Physical Review D 109 (2024) 063013. arXiv:2311.12117 [gr-qc].


Spin-eccentricity interplay in merging binary black holes

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

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


Catalog variance of testing general relativity with gravitational-wave data

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

C. Pacilio, D. Gerosa, S. Bhagwat.
Physical Review D 109 (2024) L081302. arXiv:2310.03811 [gr-qc].


Black-hole mergers in disk-like environments could explain the observed \(q-\chi_{\rm eff}\) correlation

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

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

A. Santini, D. Gerosa, R. Cotesta, E. Berti.
Physical Review D 108 (2023) 083033. arXiv:2308.12998 [astro-ph.HE].

Other press coverage: astrobites.


Extending black-hole remnant surrogate models to extreme mass ratios

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

M. Boschini, D. Gerosa, V. Varma, C. Armaza, M. Boyle, M. S. Bonilla, A. Ceja, Y. Chen, N. Deppe, M. Giesler, L. E. Kidder, G. Lara, O. Long, S. Ma, K. Mitman, P. J. Nee, H. P. Pfeiffer, A. Ramos-Buades, M. A. Scheel, N. L. Vu, J. Yoo.
Physical Review D 108 (2023) 084015. arXiv:2307.03435 [gr-qc].


Glitch systematics on the observation of massive black-hole binaries with LISA

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

A. Spadaro, R. Buscicchio, D. Vetrugno, A. Klein, D. Gerosa, S. Vitale, R. Dolesi, W. J. Weber, M. Colpi.
Physical Review D 108 (2023) 123029. arXiv:2306.03923 [gr-qc].


Parameter estimation of binary black holes in the endpoint of the up-down instability

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

V. De Renzis, D. Gerosa, M. Mould, R. Buscicchio, L. Zanga.
Physical Review D 108 (2023) 024024. arXiv:2304.13063 [gr-qc].


Efficient multi-timescale dynamics of precessing black-hole binaries

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

D. Gerosa, G. Fumagalli, M. Mould, G. Cavallotto, D. Padilla Monroy, D. Gangardt, V. De Renzis.
Physical Review D 108 (2023) 024042. arXiv:2304.04801 [gr-qc].
Open source code.


Inferring, not just detecting: metrics for high-redshift sources observed with third-generation gravitational-wave detectors

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

M. Mancarella, F. Iacovelli, D. Gerosa.
Physical Review D 107 (2023) L101302. arXiv:2303.16323 [gr-qc].


Characterization of merging black holes with two precessing spins

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

V. De Renzis, D. Gerosa, G. Pratten, P. Schmidt, M. Mould.
Physical Review D 106 (2022) 084040. arXiv:2207.00030 [gr-qc].


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.

D. Gangardt, D. Gerosa, M. Kesden, V. De Renzis, N. Steinle.
Physical Review D 106 (2022) 024019. Erratum: 107 (2023) 109901. arXiv:2204.00026 [gr-qc].


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.

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


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:

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].


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.

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


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!

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].


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.

V. Baibhav, E. Berti, D. Gerosa, M. Mould, K. W. K. Wong.
Physical Review D 104 (2021) 084002. arXiv:2105.12140 [gr-qc].


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!

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].


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.

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].


Up-down instability of binary black holes in numerical relativity

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].


A generalized precession parameter \(\chi_{\rm p}\) 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!

D. Gerosa, M. Mould, D. Gangardt, P. Schmidt, G. Pratten, L. M. Thomas.
Physical Review D 103 (2021) 064067. arXiv:2011.11948 [gr-qc].


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!

D. Gerosa, G. Pratten, A. Vecchio.
Physical Review D 102 (2020) 103020. arXiv:2007.06585 [astro-ph.HE].


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!

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 mass gap, the spin gap, and the origin of merging binary black holes

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].


Endpoint of the up-down instability in precessing binary black holes

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].


Amplification of superkicks in black-hole binaries through orbital eccentricity

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].


Machine-learning interpolation of population-synthesis simulations to interpret gravitational-wave observations: a case study

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].


Black holes in the low mass gap: Implications for gravitational wave observations

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].


Escape speed of stellar clusters from multiple-generation black-hole mergers in the upper mass gap

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.


Gravitational-wave detection rates for compact binaries formed in isolation: LIGO/Virgo O3 and beyond

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].


Constraining the fraction of binary black holes formed in isolation and young star clusters with gravitational-wave data

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].


Multiband gravitational-wave event rates and stellar physics

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].


Frequency-domain waveform approximants capturing Doppler shifts

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

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

K. Chamberlain, C. J. Moore, D. Gerosa, N. Yunes.
Physical Review D 99 (2019) 024025. arXiv:1809.04799 [gr-qc].


Spin orientations of merging black holes formed from the evolution of stellar binaries

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

Update : I think this is my 25th published paper!

D. Gerosa, E. Berti, R. O’Shaughnessy, K. Belczynski, M. Kesden, D. Wysocki, W. Gladysz.
Physical Review D 98 (2018) 084036. arXiv:1808.02491 [astro-ph.HE].


Optimizing LIGO with LISA forewarnings to improve black-hole spectroscopy

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.


Mining gravitational-wave catalogs to understand binary stellar evolution: a new hierarchical bayesian framework

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

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

S. R. Taylor, D. Gerosa.
Physical Review D 98 (2018) 083017. arXiv:1806.08365 [astro-ph.HE].

Editor’s coverage in APS’s Kaleidoscope.


Gravitational-wave astrophysics with effective-spin measurements: asymmetries and selection biases

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

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

K. K. Y. Ng, S. Vitale, A. Zimmerman, K. Chatziioannou, D. Gerosa, C.-J. Haster.
Physical Review D 98 (2018) 083007. arXiv:1805.03046 [gr-qc].


Black-hole kicks from numerical-relativity surrogate models

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

D. Gerosa, F. H’ebert, L. C. Stein.
Physical Review D 97 (2018) 104049. arXiv:1802.04276 [gr-qc].


Explaining LIGO’s observations via isolated binary evolution with natal kicks

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

Update : here it is!

D. Wysocki, D. Gerosa, R. O’Shaughnessy, K. Belczynski, W. Gladysz, E. Berti, M. Kesden, D. Holz.
Physical Review D 97 (2018) 043014. arXiv:1709.01943 [astro-ph.HE].


Nutational resonances, transitional precession, and precession-averaged evolution in binary black-hole systems

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

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


Are merging black holes born from stellar collapse or previous mergers?

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

D. Gerosa, E. Berti.
Physical Review D 95 (2017) 124046. arXiv:1703.06223 [gr-qc].
PRD Editors’ Suggestion.

Other press coverage: Ars Technica.