Davide Gerosa

May 4, 2018



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].

March 24, 2018



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.

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February 12, 2018



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: homepage, repository.

December 18, 2017



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].

November 27, 2017



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].

September 6, 2017



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].

August 11, 2017



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].

July 14, 2017



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.

July 13, 2017



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 :

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June 21, 2017



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].

May 31, 2017



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.

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May 5, 2017



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].

April 12, 2017



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.

March 30, 2017



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: homepage, repository.

March 18, 2017



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.

December 16, 2016



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].

June 14, 2016



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 University, Cambridge Center for Theoretical Cosmology
Other press coverage: Daily Mail, phys.org, Particle Bites, egno.gr (Greek), Daily Galaxy, Register, Media INAF (Italian), IneffableIsland, AstronomyNow, Accademia delle Stelle (Italian), noticiasdelaciencia (Portuguese). Blog posts on astrobites and particlebites. TV interview, aired on Cambridge TV.

May 3, 2016



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.

March 25, 2016



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!

February 22, 2016



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.

July 20, 2015



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].

July 9, 2015



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.

June 30, 2015



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.

June 10, 2015



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.

May 27, 2015



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.

March 23, 2015



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].

November 3, 2014



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.

May 27, 2014



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.

March 27, 2014



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].

February 18, 2013



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].