Davide Gerosa

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

Roxana Rosca-Mead, Christopher J. Moore, Ulrich Sperhake, Michalis Agathos, Davide Gerosa.
arXiv:2007.14429 [gr-qc]

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

Davide Gerosa, Geraint Pratten, Alberto Vecchio.
arXiv:2007.06585 [astro-ph.HE]
Open-source code: homepage, repository.

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

Mohammad Sayeb, Laura Blecha, Luke Zoltan Kelley, Davide Gerosa, Michael Kesden, July Thomas.
arXiv:2006.06647 [astro-ph.GA]

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

Roxana Rosca-Mead, Ulrich Sperhake, Christopher J. Moore, Michalis Agathos, Davide Gerosa, Christian D. Ott
arXiv:2005.09728 [gr-qc].

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

Davide Gerosa, Salvatore Vitale, Emanuele Berti.
arXiv:2005.04243 [astro-ph.HE].

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

Luca Reali, Matthew Mould, Davide Gerosa, Vijay Varma.
arXiv:2005.01747 [gr-qc].

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

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

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

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

Vishal Baibhav, Davide Gerosa, Emanuele Berti, Kaze W. K. Wong, Thomas Helfer, Matthew Mould.
arXiv:2004.00650 [gr-qc].

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

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

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

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

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

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

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

ps. The second half of the story is here.

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

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