PRL

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

D. Gerosa, V. De Renzis, F. Tettoni, M. Mould, A. Vecchio, C. Pacilio.
Physical Review Letters 134 (2025) 121402. arXiv:2409.07519 [astro-ph.HE].
PRL Editors’ Suggestion. Covered by press release.

Press release : Milano-Bicocca.
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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.

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

And what if you collide ducks instead of black holes?

V. Varma, D. Gerosa, L. C. Stein, F. H’ebert, H. Zhang.
Physical Review Letters 122 (2019) 011101. arXiv:1809.091259 [gr-qc].\

Press release: Caltech, Ole Miss.
Other press coverage: Space Daily, phys.org, longroom, tasnim, europapress (Spanish), Media INAF (video in Italian).

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

U. Sperhake, C. J. Moore, R. Rosca, M. Agathos, D. Gerosa, C. D. Ott.
Physical Review Letters 119 (2017) 201103. arXiv:1708.03651 [gr-qc].

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

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

S. Vitale, D. Gerosa, C.-J. Haster, K. Chatziioannou, A. Zimmerman.
Physical Review Letters 119 (2017) 251103. arXiv:1707.04637 [gr-qc].

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

R. O’Shaughnessy, D. Gerosa, D. Wysocki.
Physical Review Letters 119 (2017) 011101. arXiv:1704.03879 [gr-qc].
APS Editor’s choice (physics.aps.org). Covered by press release.

Press release : Rochester Institute of Technology, Caltech’s tweet.
Editor’s coverage in physics.aps.org.
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Black hole kicks are cool: powerful (up to thousands of km/s!) recoils that black holes receive following a merger. Here we show these events might leave an imprint on the emitted gravitational waves, which is potentially detectable by future interferometers.

D. Gerosa, C. J. Moore.
Physical Review Letters 117 (2016) 011101. arXiv:1606.04226 [gr-qc].
PRL Editors’ Suggestion. Covered by press release.

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

D. Gerosa, M. Kesden, R. O’Shaughnessy, A. Klein, E. Berti, U. Sperhake, D. Trifiro’.
Physical Review Letters 115 (2015) 141102. arXiv:1506.09116 [gr-qc].
PRL Editors’ Suggestion.

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

M. Kesden, D. Gerosa, R. O’Shaughnessy, E. Berti, U. Sperhake.
Physical Review Letters 114 (2015) 081103. arXiv:1411.0674 [gr-qc].
Covered by press release.

Press release : Cambridge University, Cambridge Center for Theoretical Cosmology, Ole Miss, UT Dallas.
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