Caltech

The 2017 paper “Are merging black holes born from stellar collapse or previous mergers? ” that I wrote with Emanuele Berti was selected 2025 Frontiers of Science Award. These prizes are awarded by the International Congress of Basic Science (ICBS), sponsored by the City of Beijing and the Yanqi Lake Beijing Institute of Mathematical Sciences and Application (BIMSA). Every year, they select influential recent papers in Physics, Maths, and Computer Science.

The complete list of Physics papers selected for awards is available here. Ours is one of only three papers that were selected in the category Astrophysics and Cosmology – Theory. The award citation reads:

This investigations combines gravitational-wave observations with population synthesis models to distinguish between binary black holes formed through isolated stellar evolutions versus those created through hierarchical mergers in dense stellar environments.

I’m so happy to see how a seemingly simple idea we had (“What if LIGO’s black holes merge multiple times?”) went so far! Our paper was published in Physical Review D in 2017, selected as an Editor’s Suggestion back then… and now got an award!

And that’s me collecting the prize in Beijing…

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].

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!

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? Here!

ps. Folks are having fun with this! From mikesmathpage.

V. Varma, L. C. Stein, D. Gerosa.
Classical and Quantum Gravity 36 (2019) 095007. arXiv:1811.06552 [astro-ph.HE].

Latest in the series of our spin-precession papers, here we found a thing that was worthy of a new name: wide nutation(we had wide precession before, but this is better). These are black-hole binary configurations where the angle between any of the two spins and the orbital angular momentum changes a lot. Can’t change more actually: spins goes from full alignment to full anti-alignment. And they do it many times.

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

D. Gerosa, A. Lima, E. Berti, U. Sperhake, M. Kesden, R. O’Shaughnessy.
Classical and Quantum Gravity 36 (2019) 105003. arXiv:1811.05979 [gr-qc].

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).

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].

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].

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.

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.

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.

L. Barack, et al. (199 authors incl. D. Gerosa).
Classical and Quantum Gravity 36 (2019) 143001. arXiv:1806.05195 [gr-qc].

Editor’s coverage in physicsworld.com.

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].

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].

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…

D. Gerosa, S. Vitale, C.-J. Haster, K. Chatziioannou, A. Zimmerman.
IAU Proceedigs 338 (2018) 22-28. arXiv:1712.06635 [astro-ph.HE].

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.

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

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].

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].

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.

K. Belczynski, J. Klencki, C. E. Fields, A. Olejak, E. Berti, G. Meynet, C. L. Fryer, D. E. Holz, R. O’Shaughnessy, D. A. Brown, T. Bulik, S. C. Leung, K. Nomoto, P. Madau, R, Hirschi, E. Kaiser, S. Jones, S. Mondal, M. Chruslinska, P. Drozda, D. Gerosa, Z. Doctor, M. Giersz, S. Ekstr:om, C. Georgy, A. Askar, V. Baibhav, D. Wysocki, T. Natan, W. M. Farr, G. Wiktorowicz, M. C. Miller, B. Farr, J.-P. Lasota.
Astronomy & Astrophysics 636 (2020) A104. arXiv:1706.07053 [astro-ph.HE].

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].

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.
Other press coverage: IOP’s physicsworld.com, Science Daily, Phys.org, astronomy.com, sciencenews, iflscience.

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.

D. Gerosa, M. Vallisneri.
Journal of Open Source Software 2 (2017) 13.
Open source code.

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.

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!

D. Gerosa, U. Sperhake, J. Vosmera.
Classical and Quantum Gravity 34 (2017) 064004. arXiv:1612.05263 [gr-qc].

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!

In summer 2013 I completed a summer internship at Caltech in the USA. Happy to share that my work there has been selected for publication in their Caltech Undergraduate Research Journal (CURJ).

D. Gerosa.
Caltech Undergraduate Research Journal (CURJ) 15:1 (2014) 17-26.