Davide Gerosa

University of Birmingham


The Bardeen-Petterson effect in accreting supermassive black-hole binaries: a systematic approach

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.
arXiv:2004.02894 [astro-ph.GA].


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.

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


IUPAP General Relativity and Gravitation Young Scientist Prize

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 it affiliation with the  International Union of Pure and Applied Physics  (IUPAP) and “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 to all my supervisors and advisors that supported me in these past years. Here is the Birmingham press release.


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).

Matthew Mould, Davide Gerosa.
arXiv:2003.02281 [gr-qc].
Supporting material available here.


Milky Way satellites shining bright in gravitational waves

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.
arXiv:2002.10465 [astro-ph.GA].

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


Populations of double white dwarfs in Milky Way satellites and their detectability with LISA

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.
arXiv:2002.10462 [astro-ph.GA].

ps. The second half of the story is here.


Prospects for fundamental physics with LISA

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.

Enrico Barausse, et al. (322 authors incl. Davide Gerosa).
arXiv:2001.09793 [gr-qc].


Postdoc positions in our 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: j.s.cox@bham.ac.uk.
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 (d.gerosa@bham.ac.uk) and Prof. Alberto Vecchio (av@star.sr.bham.ac.uk).


“Hitting this problem hard enough” on Scientific American

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!


PhD applications now open!

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.


GrEAT PhD winter school

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.


Winter visitors

Two close collaborators will be visiting my group this winter.

Vijay Varma, postdoc at Caltech and expert of numerical relativity surrogate models, will be here on October 7-11. Get ready for his talk “Binary black hole simulations: from supercomputers to your laptop” (aka: Everything you ever wanted to know about waveform surrogates).

Giovanni Rosotti, Veni fellow in Leiden, will be here on November 4-15. He will also give a talk: The observational era of planet formation. What do planets have to do with black holes? Turns out some stages of their evolution are set by the same equations. We have a lot to learn from each other! Giovanni’s visit is supported by the GWverse COST Action (thanks EU!).


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.

Ulrich Sperhake, Roxana Rosca-Mead, Davide Gerosa, Emanuele 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.

Kaze W.K. Wong, Davide Gerosa.
Physical Review D 100 (2019) 083015.
arXiv:1909.06373 [astro-ph.HE].

ps. The nickname of this project was sigmaspops


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.

Anuradha Gupta, Davide Gerosa, K. G. Arun, Emanuele Berti, B. S. Sathyaprakash
arXiv:1909.05804 [gr-qc].


Summer research fun

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!


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?

Davide Gerosa, Emanuele Berti.
Physical Review D Rapid Communications 100 (2019) 041301R.
arXiv:1906.05295 [astro-ph.HE].
Press release: Birmingham.
Other press coverage: interestingengineeringmetro.co.ukMedia INAF (Italian), Great Lakes Ledgersciencealert.


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…

Vishal Baibhav, Emanuele Berti, Davide Gerosa, Michela Mapelli, Nicola Giacobbo, Yann Bouffanais, Ugo N. Di Carlo.
Physical Review D 100 (2019) 064060.
arXiv:1906.04197 [gr-qc].


Are stellar-mass black-hole binaries too quiet for LISA?

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.

Christopher J. Moore, Davide Gerosa, Antoine Klein.
Monthly Notices of the Royal Astronomical Society Letters 488 (2019) L94–L98.
arXiv:1905.11998 [astro-ph.HE].


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).

Yann Bouffanais, Michela Mapelli, Davide Gerosa, Ugo N. Di Carlo, Nicola Giacobbo, Emanuele Berti, Vishal Baibhav.
Astrophysical Journal, 886 (2019) 25.
arXiv:1905.11054 [astro-ph.HE].


Surrogate models for precessing binary black hole simulations with unequal masses

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.

Vijay Varma, Scott E. Field, Mark A. Scheel, Jonathan Blackman, Davide Gerosa, Leo C. Stein, Lawrence E. Kidder, Harald P. Pfeiffer.
Physical Review Research 1 (2019) 033015.
arXiv:1905.09300 [gr-qc].


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!

Davide Gerosa, Sizheng Ma, Kaze W.K. Wong, Emanuele Berti, Richard O’Shaughnessy, Yanbei Chen, Krzysztof Belczynski
Physical Review D 99 (2019) 103004.
arXiv:1902.00021 [astro-ph.HE].
Supporting material available here.


COST comes to California!

The COST action GWverse is an impressive network of European researchers and institutions tackling gravitational waves, black holes, etc (i.e. the things I like… sweet!). Together with conferences and outreach, they support collaborative visits between the network members, so here we come. Hey wait a minute, Caltech is kind of far from Europe isn’t it? Here’s the news: Caltech is now an international partner of GWverse, and we’re very happy to host European researchers who want to collaborate with us in sunny southern California.

We’re having our first visitors. Serguei Ossokine from the AEI, is here to work with me on a black-hole binary spin project. Yann Bouffanais from University of Padova (Italy) is coming to collaborate on formation channels.  Welcome Serguei and Yann, and thanks to COST for supporting our science!

europetocaltech.jpg


The binary black hole explorer: on-the-fly visualizations of precessing binary black holes

As you can imagine, I’m kind of obsessed with black hole binaries. So easy (let’s face it, a black hole is easy! Just mass and spin), but at the same time so terribly complicated… Happy to present our attempt to see the binary dynamics in real time. Technical blah blah: we attach a visualization tool to a numerical relativity surrogate model. Are you ready to be a binary black hole explorer?

precessing

ps. Kids can have fun with black holes too! From mikesmathpage.

Vijay Varma, Leo C. Stein, Davide Gerosa.
Classical and Quantum Gravity 36 (2019) 9, 095007.
arXiv:1811.06552 [astro-ph.HE].
Supporting material available here.


Wide nutation: binary black-hole spins repeatedly oscillating from full alignment to full anti-alignment

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.

ps. We found this wide precession during Alicia’s SURF undergraduate summer project at Caltech. Jackpot!

Davide Gerosa, Alicia Lima, Emanuele Berti, Ulrich Sperhake, Michael Kesden, Richard O’Shaughnessy.
Classical and Quantum Gravity 36 (2019) 10, 105003.
arXiv:1811.05979 [gr-qc].
Supporting material available here.


High-accuracy mass, spin, and recoil predictions of generic black-hole merger remnants

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!

ducks

Bonus note. What if you collide ducks instead of black holes? 

Vijay Varma, Davide Gerosa, François Hébert, Leo C. Stein, Hao Zhang.
Physical Review Letters 122 (2019) 011101.
arXiv:1809.09125 [gr-qc]
Press release: CaltechOle Miss.
Other press coverage: Space Dailyphys.orglongroomtasnimeuropapress (Spanish), Media INAF (video in Italian).


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.

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

Katie Chamberlain, Christopher J. Moore, Davide Gerosa, Nicolas Yunes.
Physical Review D 99 (2019) 024025.
arXiv:1809.04799 [gr-qc].


Giulio Rampa thesis prize

I was recently awarded the 2018 Giulio Rampa Thesis Prize for Outstanding Research in General Relativity. The prize is sponsored by the University of Pavia (Italy) and the Italian Society for Relativity and Gravitational Physics (SIGRAV), and was officially awarded at the 23rd SIGRAV Conference. The prize announcement reads:

Dr. Gerosa’s Ph.D. Thesis on “Source modelling at the dawn of gravitational-wave astronomy” shows an impressive ability to master a rather broad range of topics in relativistic astrophysics and gravitational wave physics. The research initiated by Dr. Gerosa in these areas has triggered follow-up work, providing new important insights and new physical scenarios. The large impact that the work of Dr. Gerosa has already had can only continue to grow.


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: this is my 25th published paper! That’s silver, right?

Davide Gerosa, Emanuele Berti, Richard O’Shaughnessy, Krzysztof Belczynski, Michael Kesden, Daniel Wysocki, Wojciech Gladysz.
Physical Review D 98 (2018) 084036.
arXiv:1808.02491 [astro-ph.HE].
Supporting material available here.


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.

Rhondale Tso, Davide Gerosa, Yanbei 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.

Stephen R. Taylor, Davide Gerosa.
Physical Review D 98 (2018) 083017.
arXiv:1806.08365 [astro-ph.HE].
Editor’s coverage in APS’s Kaleidoscope.


Numerical Relativity beyond General Relativity

Happy to report about the great success of our workshop ”Numerical Relativity beyond General Relativity”. This was organized by me, Helvi Witek and Leo Stein at the Benasque physics center (Spain), in the beautiful region of the Pyrenees, on June 3-9, 2018. Was great to see world-leading experts from so many different fields (numerical relativity, gravitational-wave data analysis, self-force, theoretical physics, cosmology, etc) interacting and reporting their progress on innovative uses of computational techniques in gravitation. Here are the conference program and (some of) the talk’s slides.

I only wish the rain would have stopped for more than a few hours over the entire week. This is us with Einstein; we’re all beyond!

nulrelconf.jpg


Black holes, gravitational waves and fundamental physics: a roadmap

This is a massive review born out of the European COST Action CA16104 Gravitational waves, black holes and fundamental physics (GWverse). We summarize the status of the field of gravitational-wave astronomy and lie down a roadmap for the immediate future.

Leor Barack, et al. (199 authors incl. Davide Gerosa).
Classical and Quantum Gravity 36 (2019) 14, 143001.
arXiv:1806.05195 [gr-qc].
Editor’s coverage in physicsworld.com.


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

Ken K. Y. Ng, Salvatore Vitale, Aaron Zimmerman, Katerina Chatziioannou, Davide Gerosa, Carl-Johan Haster.
Physical Review D 98 (2018) 083007.
arXiv:1805.03046 [gr-qc].


34th Pacific Coast Gravity Meeting

The 34th edition of the Pacific Coast Gravity Meeting, sponsored by the APS, was held at Caltech on March 16-17, 2018. This’ year edition was organized by me, Leo Stein and a few others, and was dedicated to Jim Isenberg who first started the Pacific Gravity meetings 34 years ago. We had a beautiful blend of people (including some very talented undergrads!) and topics (from numerical relativity, to quantum gravity, high-energy physics and gravitational-wave astronomy). I hope everybody had fun. I surely did!

Here is the conference program, and this below is the logo that I designed (It’s supposed to be Newton’s apple with some gravitational waves in Caltech’s orange color; I know, I’m a scientist, not an artist…). And congrats to Maria Okounkova who won the best student talk award of the APS.

apple-transparent.png


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 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!

Davide Gerosa, François Hébert, Leo C. Stein.
Physical Review D 97 (2018) 104049.
arXiv:1802.04276 [gr-qc].
Open-source code: homepagerepository.


Reanalysis of LIGO black-hole coalescences with alternative prior assumptions

These are proceedings of the IAU Symposium 338 “Gravitational Wave Astrophysics”, held in Baton Rouge LA on October 16-19, 2017. My contribution is based on arXiv:1707.04637, where we look at the first binary black hole data using different Bayesian priors. During that conference, we had the announcement of the first neutron start event, GW170817, and I was presenting black-hole science: so obsolete…

Davide Gerosa, Salvatore Vitale, Carl-Johan Haster, Katerina Chatziioannou, Aaron Zimmerman.
Proceedings of the International Astronomical Union, 13 (S338), 22-28.
arXiv:1712.06635 [astro-ph.HE].


Surprises from the spins: astrophysics and relativity with detections of spinning black-hole mergers

These are my proceedings for the 12th Edoardo Amaldi Conference on Gravitational Waves (July 9-14, 2017, Pasadena CA). I summarize how to use black-hole spin dynamics to learn about the lives of stars using gravitational-wave data. There are surprises…

Before the talk, I was awarded the 2016 Stefano Braccini Thesis prize (here is Salvo’s tweet about it).

Davide Gerosa.
Journal of Physics: Conference Series 957 (2018) 1, 012014.
arXiv:1711.10038 [astro-ph.HE].


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!

Daniel Wysocki, Davide Gerosa, Richard O’Shaughnessy, Krzysztof Belczynski, Wojciech Gladysz, Emanuele Berti, Michael Kesden, Daniel Holz.
Physical Review D 97 (2018) 043014.
arXiv:1709.01943 [astro-ph.HE].


Long-lived inverse chirp signals from core collapse in massive scalar-tensor gravity

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!

Ulrich Sperhake, Christopher J. Moore, Roxana Rosca, Michalis Agathos, Davide Gerosa, Christian D. Ott.
Physical Review Letters 119 (2017) 201103.
arXiv:1708.03651 [gr-qc].


Impact of Bayesian priors on the characterization of binary black hole coalescences

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!

Salvatore Vitale, Davide Gerosa, Carl-Johan Haster, Katerina Chatziioannou, Aaron Zimmerman.
Physical Review Letters 119 (2017) 251103.
arXiv:1707.04637 [gr-qc].
Posterior sample data release.


Stefano Braccini thesis prize

I was awarded the 2016 Stefano Braccini PhD Thesis Prize by the Gravitational Wave International Committee (GWIC).  The prize announcement reads:

Dr. Gerosa received his Ph.D. from the University of Cambridge and was nominated by his adviser, Prof. Ulrich Sperhake. Dr. Gerosa’s thesis includes a wide variety of topics relevant to gravitational waves, as well as other topics in astrophysics: astrophysical explorations of accretion disks, analytically challenging work in mathematical relativity and post-Newtonian theory, and numerical relativity coding of supernova core-collapse in relativity and modified gravity.

The prize was officially awarded at the 12th Edoardo Amaldi Conference on Gravitational Waves. Here is a picture tweeted by Salvo :

DEor_9FUAAES7kH.jpg


The evolutionary roads leading to low effective spins, high black hole masses, and O1/O2 rates of LIGO/Virgo binary black holes.

Looks like some of the LIGO black holes have low spins (better, low effective spins). In this paper we show these values can be accommodated with standard “field binaries”, i.e. formation channels where binary black holes form from binary stars.

Krzysztof Belczynski, Jakub Klencki, Carl E. Fields, Aleksandra Olejak, Emanuele Berti, Georges Meynet, Christopher L. Fryer, Daniel E. Holz, Richard O’Shaughnessy, Duncan A. Brown, Tomasz Bulik, Sching C. Leung, Ken’ichi Nomoto, Piero Madau, Raphael Hirschi, Etienne Kaiser, Samuel Jones, Samaresh Mondal, Martyna Chruslinska, Paweł Drozda, Davide Gerosa, Zoheyr Doctor, Mirek Giersz, Sylvia Ekström, Cyril Georgy, Abbas Askar, Vishal Baibhav, Daniel Wysocki, T. Natan, Will M. Farr, Grzegorz Wiktorowicz, M. Coleman Miller, Ben Farr, Jean-Pierre Lasota.
Astronomy & Astrophysics, in press.
arXiv:1706.07053 [astro-ph.HE].


The disc migration issue: from protoplanets to supermassive black holes

Our workshop “The disc migration issue: from protoplanets to supermassive black holes” took place in May (2017) at the Cambridge Institute of Astronomy. Chaired by Cathie Clarke and co-organized by me, Giovanni Rosotti and a few other people, we tried to bring together people working on both planetary and black-hole physics, to understand what we have in common… Much like planets migrate in protoplanetary discs, supermassive black holes are also brought together by gas interactions. Same physics, different scales, right?

Here is the conference program (with some of the talk’s slides) and below is our beautiful logo (there are discs, waves, inspirals, and King’s College!). Thanks to the KAVLI and Templeton foundations for making this possible.

DM_web-banner.jpg


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.

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


Inferences about supernova physics from gravitational-wave measurements: GW151226 spin misalignment as an indicator of strong black-hole natal kicks

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…

Richard O’Shaughnessy, Davide Gerosa, Daniel Wysocki.
Physical Review Letters 119 (2017) 011101.
arXiv:1704.03879 [astro-ph.HE].
Press release: Rochester Institute of Technology, Caltech’s tweet.
Editor’s coverage in physics.aps.org.
Other press coverage: IOP’s physicsworld.com, Science Daily, Phys.org, astronomy.com, sciencenews, iflscience.


filltex: Automatic queries to ADS and INSPIRE databases to fill LaTex bibliography

My little latex project to compile bibliographies in a smart way was published by JOSS. I really liked JOSS: it’s an innovative way to get recognition for your carefully crafted software, encouraging open science and good code practice. It’s really about publishing your code, not a paper that describes the code: they peer-review the repository, openly with pull requests.

Davide Gerosa, Michele Vallisneri.
The Journal of Open Source Software 2 (2017) 13.
Open-source code: homepagerepository.


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…

Davide Gerosa, Emanuele Berti.
Physical Review D 95 (2017) 124046.
arXiv:1703.06223 [gr-qc].
Selected as PRD Editors’ Suggestion.
Other press coverage: Ars Technica.


On the equal-mass limit of precessing black-hole binaries

Equal-mass binaries correspond to a discontinuous limit in the spin precession equations. A new constant of motion pops up, which can be exploited to study the dynamics. This is a really neat calculation done with Jakub, a Cambridge undergraduate student. Also, my first paper at Caltech!

Davide Gerosa, Ulrich Sperhake, Jakub Vošmera.
Classical and Quantum Gravity 34 (2017) 6 ,064004.
arXiv:1612.05263 [gr-qc].


Black-hole kicks as new gravitational-wave observables

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.

Davide Gerosa, Christopher J. Moore.
Physical Review Letters 117 (2016) 011101.
arXiv:1606.04226 [gr-qc].
Selected as PRL Editors’ Suggestion.
Press releases: Cambridge UniversityCambridge Center for Theoretical Cosmology
Other press coverage: Daily Mailphys.org, Particle Bitesegno.gr (Greek), Daily Galaxy, RegisterMedia INAF (Italian), IneffableIsland, AstronomyNow, Accademia delle Stelle (Italian), noticiasdelaciencia (Portuguese). Blog posts on astrobites and particlebites. TV interview, aired on Cambridge TV.


PRECESSION: Dynamics of spinning black-hole binaries with python

Here we present my numerical code precession, which implements our multi-timescale way to look at spinning black-hole binaries. The paper has a detailed description of the various functions as well as lots of examples.

Update: typos in Eq. (36-37) have been fixed in v3 on the arXiv.

Davide Gerosa, Michael Kesden.
Physical Review D 93 (2016) 124066.
arXiv:1605.01067 [astro-ph.HE].
Open-source code: homepage, repository, documentation.


NASA Einstein Fellowhip

I was awarded a NASA Einstein Fellowship to conduct three years of postdoctoral research at Caltech. My proposal is titled “Strong gravity to the realm of observational astronomy”. Here is a passage from NASA’s press release:

“We are very pleased to welcome this talented group of young scientists as the incoming Einstein Fellows,” said Belinda Wilkes, Director of the Chandra X-ray Center at the Smithsonian Astrophysical Observatory that manages the Einstein Fellows program for NASA. “Their research will advance the quest to better understand the physics of the cosmos in a variety of directions.”

Sunny California, here I come!


Numerical simulations of stellar collapse in scalar-tensor theories of gravity

Here we present 1+1 numerical-relativity simulation of stellar collapse in scalar-tensor theories, where gravity is mediated by the usual metric coupled to an additional scalar field. Bottom line: you can test General Relativity with supernovae explosions!

Davide Gerosa, Ulrich Sperhake, Christian D. Ott.
Classical and Quantum Gravity 33 (2016) 13 , 135002.
arXiv:1602.06952 [gr-qc].
Supporting material available here.


Distinguishing black-hole spin-orbit resonances by their gravitational wave signatures. II: Full parameter estimation

This is a follow up of arXiv:1403.7147, just done better. Instead of overlaps, we do real injections in LIGO parameter-estimation codes to show that spin-orbit resonances are indeed detectable.

Daniele Trifirò, Richard O’Shaughnessy, Davide Gerosa, Emanuele Berti, Michael Kesden, Tyson Littenberg, Ulrich Sperhake.
Physical Review D 93 (2016) 044071.
arXiv:1507.05587 [gr-qc].


The birth of an idea

I wrote a post for The Birth of an Idea, which is a really beautiful blog collecting insights on how scientists start their science. Thanks Vitor for the opportunity to contribute! Here is my post:

Bikes and Colorful Pieces

An idea, a good one at least, is like a gift. It’s something which is not yours (indeed, you didn’t have it before!) but comes to you, it’s given to you.

I bike to work, it’s kind of ten minutes from my place to the Cambridge Maths department, but those ten minutes can be more productive than ten hours or ten days in front of my computer’s screen. It’s morning, your mind should be clear (you should pay attention to cars while biking!), but it’s actually already getting full of what you have to do today. You get to the office, sit down, turn your computer on, and start looking at your problem. You write the equations down, try putting them in a computer, it doesn’t work, just nans coming out. You ask a collaborator who hopefully knows something, write the equations down again, it doesn’t work. You check in a paper if someone else did something similar, take a break, get annoyed (and here I typically open football websites…). Oh, and you write the same equations down again, it simply doesn’t work.

At some stage, it’s time to go home, and that moment is precious to me. You know your problem so well, those equations, that crashing piece of code, but you were looking too close. When I close my laptop and get on my way home, fresh air on my face, I can look at the problem from afar. It’s like looking at those beautiful ancient mosaics. If you look very close, you only see one colored piece, but you can’t see any meaning in it. Each piece is crucial to the final piece of art, but the value of each piece is its relation to the bigger picture. You can only appreciate a mosaic if you take one step back and look to the whole picture from afar. Wow. Biking home is my step back. You’ve been looking at all pieces for days, weeks, you know the color of each piece so well that you can finally grasp the relation which puts them together.

An idea, a good one at least, is like a gift you can say thanks for.


Precessional instability in binary black holes with aligned spins

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!

Davide Gerosa, Michael Kesden, Richard O’Shaughnessy, Antoine Klein, Emanuele Berti, Ulrich Sperhake, Daniele Trifirò.
Physical Review Letters 115 (2015) 141102.
arXiv:1506.09116 [gr-qc].
Selected as PRL Editors’ Suggestion.
Supporting material available here.


Multi-timescale analysis of phase transitions in precessing black-hole binaries

Detailed analysis of 2PN black-hole binary spin precession using multi-timescale methods. Follow-up of the Letter arXiv:1411.0674, this paper contains the full calculation and the description of the underlying phenomenology.

Davide Gerosa, Michael Kesden, Ulrich Sperhake, Emanuele Berti, Richard O’Shaughnessy.
Physical Review D 92 (2015) 064016.
arXiv:1506.03492 [gr-qc].
Supporting material available here.


Tensor-multi-scalar theories: relativistic stars and 3+1 decomposition

What happens if you throw a scalar field into General Relativity? And if you throw more than one? Here is a paper on the phenomenology of neutron stars in theories with more than one scalar field coupled to gravity.

Michael Horbatsch, Hector O. Silva, Davide Gerosa, Paolo Pani, Emanuele Berti, Leonardo Gualtieri, Ulrich Sperhake.
Classical and Quantum Gravity 32 (2015) 20, 204001.
arXiv:1505.07462 [gr-qc].
Featured in CQG+. Selected as IOPselect.


Spin alignment and differential accretion in merging black hole binaries

Supermassive black holes in binaries and their accretion discs… Spins align on some timescale, but migration also takes place. Do gas discs have enough time to align the spins? Well, the secret is the mass ratio: light secondaries might prevent primaries from aligning. A great collaboration between gravitational-wave and planet researchers!

Davide Gerosa, Benedetta Veronesi, Giuseppe Lodato, Giovanni Rosotti.
Monthly Notices of the Royal Astronomical Society 451 (2015) 3941-3954.
arXiv:1503.06807 [astro-ph.GA].


Effective potentials and morphological transitions for binary black-hole spin precession

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.

Michael Kesden, Davide Gerosa, Richard O’Shaughnessy, Emanuele Berti, Ulrich Sperhake.
Physical Review Letters 114 (2015) 081103.
arXiv:1411.0674 [gr-qc].
Press releases: Cambridge University, Cambridge Center for Theoretical Cosmology, Ole Miss, UT Dallas.
Other press coverage: Science Daily, phys.org, phys.org (2), Media INAF (Italian), Astroblogs (Dutch), RIA (Russian), Daily News, Science World Report, Tech Times, Tech Times (2)SpaceRef, Space Daily, ECN, R&D, Daily Galaxy, scitechdaily, nanowerk
Supporting material available here.


Rival families: waveforms from resonant black-hole binaries as probes of their astrophysical formation history

These are my proceedings of the 3rd Session of the Sant Cugat Forum on Astrophysics, held in beautiful Sant Cugat, near Barcelona on April 22-25, 2014. My contribution is this paper: have a look if you want to know more about rival families in spinning black-hole binaries.

Davide Gerosa. 
Astrophysics and Space Science Proceedings 40 (2015) 137-145.


Missing black holes in brightest cluster galaxies as evidence for the occurrence of superkicks in nature

Black-hole kicks are powerful. I mean, really powerful. They can even eject supermassive black holes from the heavier galaxies in our Universe. And then these galaxies are left “empty”…

Davide Gerosa, Alberto Sesana.
Monthly Notices of the Royal Astronomical Society 446 (2015) 38-55.
arXiv:1405.2072 [astro-ph.GA].


Distinguishing black-hole spin-orbit resonances by their gravitational-wave signatures

Spinning black-hole binaries might belong to two spin-orbit resonances, or families. Can you tell them apart using gravitational-wave observations? Spoiler: yes!

Bonus note: check out the title in v1 on the arxiv…

Davide Gerosa, Richard O’Shaughnessy, Michael Kesden, Emanuele Berti, Ulrich Sperhake.
Physical Review D 89 (2014) 124025.
arXiv:1403.7147 [gr-qc].


Resonant-plane locking and spin alignment in stellar-mass black-hole binaries: a diagnostic of compact-binary formation

Spin precession in stellar-mass black hole binaries encodes information on specific formation mechanisms like tides and mass transfers. My first paper on spin precession…

Davide Gerosa, Michael Kesden, Emanuele Berti, Richard O’Shaughnessy, Ulrich Sperhake.
Physical Review D 87 (2013) 10, 104028.
arXiv:1302.4442 [gr-qc].