Davide Gerosa

January 20, 2022

Lots of visitors!

Traveling is (kind of) coming back, and we’re having lots of visitors around, all supported by external research grants (congrats folks, you’re great!)

Safe travel everyone, it’s time we move our group meetings to a larger room.

December 6, 2021


My group and I are now part of TEONGRAV, which is the Italian national initiative dedicated to gravitational theory and phenomenology. TEONGRAV is run by the INFN (National Institute for Nuclear Physics) and, besides the other folks here in Milan, it counts members from Florence, Rome, Naples, Padua, Trento, and Trieste. Looking forward to new exciting collaborations, all surrounded by good Italian coffee of course!

November 17, 2021

The Bardeen-Petterson effect in accreting supermassive black-hole binaries: disc breaking and critical obliquity

Breaking things is fun! In the previous paper of this series, we looked at accretion disks around massive black-hole binaries and found things were going awry. We kept on finding configurations that our implementation could not handle… And now we know this is real! Finding disk solutions when the spin of the black hole has a large misalignment is just not possible! And that’s because the disk really breaks into different sections. We’ve now checked it with state-of-the-art hydrodynamical numerical simulations that not only confirm what we suspected but also show some funny things (like breaking being prevented by disk spirals, etc). I was serious, breaking things is real fun!

Check out Rebecca’s beautiful movies!

R. Nealon, E. Ragusa, D. Gerosa, G. Rosotti, R. Barbieri.
Monthly Notices of the Royal Astronomical Society, in press.
arXiv:2111.08065 [astro-ph.HE].

October 13, 2021

Back to the future: gravitational-wave population inference at past time infinity

Great Scott, a new paper! When analyzing gravitational-wave data, looking at one black hole at a time is not enough anymore, the fun part is looking at them all together. The issue Matt and I are tackling here is that one needs to be consistent with putting together different events when fitting the entire population. This is obvious for things that do not change (say the masses of the black holes, those are what they are), but becomes a very tricky business for varying quantities (say the spin directions, which is what we look at here). In that case, it’s dangerous to put together events taken at different stages of their evolution. And the solution to this problem is…. time travel! We show that but propagating binaries backward in time, one can put all sources on the same footing. After that, estimating the impact of the detector requires traveling forward in time, so going “back to the future”. After all, we all know that post-Newtonian black-hole binary integrations look like this:

M. Mould, D. Gerosa
arXiv:2110.05507  [astro-ph.HE].

October 11, 2021

Nate is joining us!

Nathan Steinle is officially starting his postdoc in the group today! Nate graduated with Mike Kesden at the University of Texas at Dallas and is now working with me and the rest of the Birmingham crowd. Welcome Nate! Hope you enjoy this side of the pond.

October 4, 2021

Postdoctoral fellowships in gravitational-wave astronomy at Milan-Bicocca (Italy)

The University of Milan-Bicocca (Italy) invites expressions of interest for postdoctoral positions in gravitational-wave astronomy.

Successful candidates will join Prof. Davide Gerosa and will constitute the core team of the “GWmining” project funded by the European Research Council. Targeted investigations include applications of machine-learning techniques to gravitational-wave physics, modeling of black-hole binary populations from their stellar progenitors, relativistic dynamics, and statistical inference. Candidates will have ample opportunities to explore other areas of gravitational-wave astronomy and will be encouraged to develop independent collaborations.

We anticipate awarding two positions. Appointments will be for a three-year term and come with generous research and travel budget. The starting date is negotiable.

The astrophysics group at Milan-Bicocca provides a vibrant environment with expertise covering all aspects of gravitational-wave astronomy, relativistic astrophysics, and numerical relativity, as well as a wider astronomical context including observational and experimental activities. The group has tight connections with the LISA Consortium, the Virgo Collaboration, and the Italian National Institute for Nuclear Physics (INFN) via the TEONGRAV national initiative. Faculty members with matching interests include Gerosa, Sesana, Colpi, Giacomazzo, and Dotti.

Milan is a beautiful, international city in the north of Italy. Mountains and lakes are just around the corner.

Successful candidates will have a PhD in Physics or related discipline, strong programming skills, and previous experience in one or more of the following topics: gravitational-wave astronomy, stellar evolution, relativistic astrophysics, general relativity, machine learning, statistical inference.

Applications should include a CV with a list of publications and a two-page statement covering research interests and plans. These should be sent to [email protected] by December 1st, 2021 for full consideration. Candidates should also arrange for at least two, but preferably three, reference letters to be sent to the same address by December 1st, 2021. We strive to build a diverse and inclusive environment and welcome expressions of interest from traditionally underrepresented groups.

For inquiries please do not hesitate to contact Davide Gerosa at [email protected].

September 18, 2021

Welcome Viola!

Viola De Renzis is the latest addition to our group! Viola graduated from Rome “La Sapienza” with an MSc thesis on exotic compact objects and is now starting her PhD with me at Milan-Bicocca. Viola plays guitar, arguably better than Matt (although he runs for a million miles, and that’s when he’s tired), while Daria remains by far the best fencer in the group. Welcome, we all look forward to working with you!

September 8, 2021

SIGRAV Prize for Young Researchers

It is a true honor to receive the career Prize for Young Researchers of the Italian Society for General Relativity and Gravitational Physics (SIGRAV). I was awarded the prize in the class of relativistic astrophysics. It’s amazing to be recognized in my home country; it’s great to be back! Let me thank all my mentors, advisors, collaborators, and now students who are walking with me in the adventure of science.

Here is me with the president of the society Fulvio Ricci. And here are press releases from the University of Milan-Bicocca and the INFN.

September 1, 2021

Moving (back to) Milan!

We moved! I’ve had the opportunity to relocate to Milan, in the north of Italy, very close to where I’m from. I’m now an Associate Professor at the University of Milan-Bicocca, one of the two campuses in the beautiful city of the “Madonnina“. Some of the folks in my group will be visiting Milan very often, and (spoiler alert!) we’re going to have new additions soon. I’m sad to leave the amazing group in Birmingham, but also very excited at this new tremendous opportunity.

August 6, 2021

Population-informed priors in gravitational-wave astronomy

No black hole is an island entire of itself.

We’ve got many gravitational wave events now. One can look at each of them individually (aka “parameter estimation”), all of them together (aka “population”), or each of them individually while they’re together. That’s what we do in this paper: we look at the properties of individual gravitational-wave events in light of the rest of the observed population. The nice thing is that all of these different ways of looking at the data are part of the same statistical tool, which is a hierarchical Bayesian scheme. Careful, heavy stats inside, don’t do this at home.

Christopher J. Moore, Davide Gerosa.
arXiv:2108.02462  [gr-qc].

July 14, 2021

Well done Max!

Huge congrats to Maciej (Max) Dabrowny, who just graduated from the University of Birmingham after a very successful research project with us (Max’s project ended up in a paper!). Well done and all the best for the future.

June 24, 2021

Modeling the outcome of supernova explosions in binary population synthesis using the stellar compactness

Today we go deep into the perilous world of binary population synthesis! Using Nicola’s code MOBSE, our master student Maciej has implemented some new prescriptions for how supernovae explode and produce compact objects. In practice, we use the compactness (that’s mass over radius) of the stellar core before the explosion to decide if that specific star will form a neutron star or a black hole. This now needs to be compared carefully with gravitational-wave data, but we suggest that there are two key signatures one should look for: the lowest black hole masses and the relative merger rates between black holes and neutron stars.

Maciej Dabrowny, Nicola Giacobbo, Davide Gerosa.
arXiv:2106.12541  [astro-ph.HE].

June 10, 2021

A new IREU friend from Missouri

We have a new friend in the group! Meredith Vogel is joining us for her undergraduate summer research project. Meredith is e-visiting us from Missouri State University (but will soon start her grad school at the University of Florida*) and will be working with Matt on numerical-relativity surrogate models. Meredith’s project is part of the IREU (International Summer Research) program, which is a great opportunity for US students to visit groups abroad, including us! Welcome Meredith, looking forward to seeing your great science.

* That’s the place were I saw a real alligator. On campus!

June 10, 2021

Bayesian parameter estimation of stellar-mass black-hole binaries with LISA

LISA is going to be great and will detect stuff from white dwarfs to those supermassive black-hole that live at the center of galaxies. If we’re lucky (yeah, who knows how many of these we will see), LISA might also detect some smaller black holes, similar to those that LIGO now sees all the time, but at a much earlier stage of their lives. But if we’re indeed lucky, the science we would take home is outstanding. Using simulated data from the LISA Data Challenge we unleash the new amazing parameter-estimation code Balrog (don’t ask what it means, it’s just a name, not one of those surreal astronomy acronyms) at this problem. Dive into the paper for some real data-analysis fun!

Riccardo Buscicchio, Antoine Klein, Elinore Roebber, Christopher J. Moore, Davide Gerosa, Eliot Finch, Alberto Vecchio
arXiv:2106.05259  [astro-ph.HE].

May 27, 2021

Looking for the parents of LIGO’s black holes

Who are the parents of LIGO’s black holes? Stars, most likely. Things like those we see in the sky at night will eventually surrender to gravity and collapse. Some of them will form black holes. Some of them will form binary black holes. Some of them will merge. Some of them will be observed by LIGO. That’s the vanilla story at least, but it might not apply to all of the black holes that LIGO sees. For some of those, stars might be the grandparents or the great grandparents. And the parents are … just other black holes! This is today’s paper lead by Vishal Baibhav. Instead of just measuring the properties of the black holes that LIGO observes, we show we can also say something about the features of the black hole parents. Read on to explore the black-hole family tree.

Vishal Baibhav, Emanuele Berti, Davide Gerosa, Matthew Mould, Kaze W. K. Wong.
arXiv:2105.12140 [gr-qc]

May 18, 2021

Come to Milan for a PhD!

The University of Milano-Bicocca welcomes applications for Ph.D. scholarships. The application deadline is June 16th, 2021 for positions to start later in 2021:


In particular, I am looking for a strong, highly motivated candidate to join my newly established research group supported by the European Research Council. The candidate will work toward interpreting the phenomenology and the astrophysics of gravitational-wave sources using innovative machine-learning techniques. My activities are embedded within the wider Astrophysics group at the University of Milano-Bicocca –a world-leading research environment in strong gravity and relativistic astrophysics. Faculty members with matching interests include Colpi, Sesana, Dotti, and Giacomazzo. The candidate will have ample opportunities to work with and visit external collaborators as well.

This specific position is titled “Large catalogs of gravitational-wave events with machine learning”. Interested candidates should mention it explicitly in their application.

Milan is a beautiful, international city in the north of Italy. Mountains and lakes are just around the corner. For further information and informal inquiries please do not hesitate to contact me ([email protected]). 

May 11, 2021

Hierarchical mergers of stellar-mass black holes and their gravitational-wave signatures

The quest of finding their astrophysical origin of merging black-hole binaries is now a key open problem in modern astrophysics. Stars are the natural progenitor of black holes: at the end of their lives, the core collapses and leaves behind a compact object. But once those “first-generation” black holes are around, they can potentially meet again and form “second generation” LIGO events. I first got interested in this problem in 2017 and, together with many many others researchers in the community, we explored the consequences of this “hierarchical merger” scenario in terms of both gravitational-wave physics and astrophysical environments. In this Nature Astronomy review article, Maya and I tried to condense all this body of work into a few pages. The result is (we hope) a broad and informed overview of this emerging research strand, with a whopping number of more than 270 citations! Hope you like it.

Davide Gerosa, Maya Fishbach.
Nature Astronomy, in press.
arXiv:2105.03439 [astro-ph.HE].

April 26, 2021

High mass but low spin: an exclusion region to rule out hierarchical black-hole mergers as a mechanism to populate the pair-instability mass gap

Hierarchical mergers are the new black. LIGO is seeing black holes that are just too big to be there. The reason is that stars, which collapse and produce black holes, do some funny things when they get too massive. Notably, they start to spontaneously produce positrons and electrons instead of keeping their own photons. Long story short: those missing photons make the temperature go up, ignite an explosion that disrupts the core and prevents black-hole formation. This “mass gap” is a solid prediction from our astrophysics friends. In some previous papers, we and other groups pointed out that one can bypass stars and form black holes from previous black holes (and goodbye my dear maximum mass limit!). But now our astrophysics friends are telling us they can also evade the limit with some more elaborate astro-magic (winds, rotation, dredge-up, reaction rates, accretion). Today’s paper is about telling the two apart, with a key prediction: a black hole with large mass but low spin would raise a glass to the astro-wizards.

Davide Gerosa, Nicola Giacobbo, Alberto Vecchio.
Astrophysical Journal, 915 (2021) 56.
arXiv:2104.11247 [astro-ph.HE].

March 31, 2021

Testing general relativity with gravitational-wave catalogs: the insidious nature of waveform systematics.

General Relativity works well. But we still want to test it, and I guess that’s because it actually works too well (you know, all those quantum things that don’t really fit, etc). And we want to test it with gravitational-wave data, and not just because it’s the new cool thing to do (though it is!) but also because they gravitational waves give us insight into the strong-field regime of gravity where new things, if they are there at all, should show up. Now, all of this sounds great but, in practice, one has to deal with the actual model used to analyze the data. Errors in these signal models (aka waveforms), which are somewhat inevitable, can trick us into thinking we have seen a deviation from General Relativity. So, before you go out on the street and shout that Einstein was wrong, keep calm and mind your waveform.

Christopher J. Moore, Eliot Finch, Riccardo Buscicchio, Davide Gerosa.
iScience 24 (2021) 102577.
arXiv:2103.16486 [gr-qc].
Other press coverage: indiescience, sciencedaily, phys.org, astronomy.com, physicsworld.

ps. The codename for this paper was SANITY: SystemAtics usiNg populatIons to Test general relativitY.

March 31, 2021

Group study on BH binaries in AGN disks

This is a quick update some of our group activities… In the past few months we’ve been busy learning about the formation of stellar-mass black-hole binaries in the disks of active galactic nuclei. We organized a journal club and studied one paper each week on this “new” formation channel for LIGO sources. We discussed a ton of topics, going from disk accretion to migration traps, LIGO rates, AGN variability, GW counterparts, hierarchical mergers, all the way to EMRIs.

Here is a log of all the sessions: davidegerosa.com/bhbin-agndisks

Let me thanks all those who took part and presented papers including Daria, Matt (1), Chris, Eliot, Matt (2), Alberto, Evan, Riccardo, and Sean.

March 9, 2021

A taxonomy of black-hole binary spin precession and nutation

Here is the latest in our (by now long) series of papers on black-hole binaries spin precession. This work was is championed by two outstanding PhD students, Daria (in my group) and Nate (UT Dallas). The key idea behind this paper is that, for black-hole spins, one cannot really talk about precession without talking about nutation (although we only say “precession” all the time…). The spin of, say, the Earth also does both precession (azimuthal motion) and nutation (polar motion). But, unlike in the Earth problem, for black-hole spins the two motions happen on roughly the same timescale meaning that you cannot really take them apart. Or can you? We stress the role of five parameters that characterize the combined phenomenology of precession and nutation. The hope is now to use them as building blocks for future waveforms… stay tuned!

Daria Gangardt, Nathan Steinle, Michael Kesden, Davide Gerosa, Evangelos Stoikos.
Physical Review D 103 (2021) 124026.
arXiv:2103.03894 [gr-qc].

ps. Stupid autocorrect! It’s nutation, not mutation.

January 29, 2021

Eccentric binary black hole surrogate models for the gravitational waveform and remnant properties: comparable mass, nonspinning case

Orbital eccentricity in gravitational-wave observations has been long neglected. And with good reasons! Gravitation-wave emission tends to circularize sources. By the time black holes are detectable by LIGO/Virgo/LISA/whatever, they should have had ample time to become circular. Unless something exciting goes on in their formation, things like clusters, triples, Kozai-Lidov oscillations, etc. And if that happens, we want to see it! This paper contains the first model for gravitational waveforms and black-hole remnants (final mass, spin) trained directly on eccentric numerical relativity simulations. Because eccentric is the new circular.

Tousif Islam, Vijay Varma, Jackie Lodman, Scott E. Field, Gaurav Khanna, Mark A. Scheel, Harald P. Pfeiffer, Davide Gerosa, and Lawrence E. Kidder.
Physical Review D 103 (2021) 064022.
arXiv:2101.11798 [gr-qc].

January 22, 2021


We are running a virtual workshop with my group (Bham) and Emanuele Berti’s group at Johns Hopkins University (Hop). It’s an attempt to feel a bit less lonely during the COVID pandemic. Hope this is the opportunity to start new projects! And we’re a funny crowd…

For more: davidegerosa.com/hopbham-workshop/

December 15, 2020

Up-down instability of binary black holes in numerical relativity

[Intro music…Now imagine one of those voices from a TV show trailer…]

Up-down instability S01-E03.
“Previously on the up-down instability. After finding out that the instability exists (S01-E01) and calculating its analytic endpoint (S01-E02), one terrifying prospect remains. What if it’s just PN? Can all of this disappear in the strong-field regime? This challenge now needs to be faced”.

Today’s paper is the latest in our investigations of the up-down instability in binary black holes. If the primary black hole is aligned and the secondary is anti-aligned to the orbital angular momentum, the entire system is unstable to spin precession. We found this funny thing using a post-Newtonian (read: approximate) treatment but we couldn’t be 100% sure that this would still be true when the black holes merge and our approximation fails. So, we got our outstanding SXS friends on board and ask them if they could see the same effect with their numerical relativity (read: the real deal!) code. And the answer is… yes! The instability is really there! And by the way, these are among the longest numerical relativity simulations ever done.

Vijay Varma, Matthew Mould, Davide Gerosa, Mark A. Scheel, Lawrence E. Kidder, Harald P. Pfeiffer.
Physical Review D 103 (2021) 064003.
arXiv:2012.07147 [gr-qc].
Supporting material available here.

November 25, 2020

A generalized precession parameter chi_p to interpret gravitational-wave data

Spin precession is cool, and we want to measure it. In General Relativity, the orbital plane of a binary is not fixed but moves around. This effect is related to the spin of the orbiting black holes and contains a ton of astrophysical information. The question we try to address in this paper is the following: how does one quantify “how much” precession a system has? This is typically done by condensing information into a parameter called \(\chi_{\rm p}\), which is here generalize to include two- spin effects. There are two black holes in a binary and we received numerous complaints from the secondaries: they want to join the gravitational-wave fun!

Davide Gerosa, Matthew Mould, Daria Gangardt, Patricia Schmidt, Geraint Pratten, Lucy M. Thomas.
Physical Review D 103 (2021) 064067.
arXiv:2011.11948 [gr-qc].
Open-source code: homepagerepository.

November 16, 2020

Nicola joins the band

It’s a great pleasure to welcome Nicola Giacobbo, who starts his postdoc with us today. Nicola completed his PhD and first postdoc year in Padova, and is an expert in population-synthesis simulations, compact binary progenitors, stellar physics, and all those funny things. Welcome Nicola!

November 10, 2020

Inferring the properties of a population of compact binaries in presence of selection effects

If you want to know what’s out there, you need to figure out what’s missing. And gravitational-wave astronomy is no exception. We are trying to infer how things like black holes and neutron stars behave in the Universe given a limited number of observations, which are somehow selected by our detectors. This is a very general problem which is common to a variety of fields of science. We provide a hopefully pedagogical introduction to population inference, deriving all the necessary statistics from the ground up. In other terms, here is what you always wanted to know about this population business everyone is talking about but never dared to ask.

This document is going to be part of a truly massive “Handbook of Gravitational Wave Astronomy” soon to be published by Springer (not really a handbook I would say, you probably need a truck to carry it around).

Salvatore Vitale, Davide Gerosa, Will M. Farr, Stephen R. Taylor.
Chapter of “Handbook of Gravitational Wave Astronomy”; Springer Singapore (2021).
arXiv:2007.05579 [astro-ph.IM].

September 17, 2020

ERC Starting Grant

I was awarded a Starting Grant from the European Research Council for my program titled “Gravitational-wave data mining”. My team and I will look into gravitational-wave data, machine-learning tools, black-hole binary dynamics, stellar-evolution simulations, etc. The total awarded amount is 1.5M EUR. Here is the press release from the Birmingham news office.

Thank you Europe, you’re great.

September 11, 2020

Congrats to MSc students

Congratulations to my Master’s students that graduate this year:  Abdullah Aziz and Julian Chan from the University of Birmingham, and Beatrice Basset from the University of Lyon. Well done all, and good luck with your future adventures.

July 30, 2020

Structure of neutron stars in massive scalar-tensor gravity

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.
Symmetry 12 (2020) 1384.
arXiv:2007.14429 [gr-qc]

July 15, 2020

Gravitational-wave selection effects using neural-network classifiers

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.
Physical Review D 102 (2020) 103020.
arXiv:2007.06585 [astro-ph.HE]
Open-source code: homepage, repository.

June 12, 2020

Massive black hole binary inspiral and spin evolution in a cosmological framework

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.
Monthly Notices of the Royal Astronomical Society 501 (2020) 2531–2546.
arXiv:2006.06647 [astro-ph.GA].

May 20, 2020

Core collapse in massive scalar-tensor gravity

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.
Physical Review D 102 (2020) 044010.
arXiv:2005.09728 [gr-qc].

May 12, 2020

Astrophysical implications of GW190412 as a remnant of a previous black-hole merger

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.
Physical Review Letters 125 (2020) 101103.
arXiv:2005.04243 [astro-ph.HE].
Press release: Birmingham, MIT.
Other press coverage: International Business Times, SciTechDaily, VRT (Dutch), notimerica (Spanish), allnewsbuzz, canaltech (Portuguese).

May 6, 2020

Mapping the asymptotic inspiral of precessing binary black holes to their merger remnants

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].

April 8, 2020

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.
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.

April 3, 2020

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.
Physical Review D 102 (2020) 043002.
arXiv:2004.00650 [gr-qc].

March 16, 2020

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. For more see the Birmingham press release, the Springer press release, and the IUPAP newsletter.

March 5, 2020

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

February 26, 2020

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.
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?

February 26, 2020

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.
Astronomy & Astrophysics 638 (2020) A153.
arXiv:2002.10462 [astro-ph.GA].

ps. The second half of the story is here.

January 28, 2020

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).
General Relativity and Gravitation 52 (2020) 8, 81.
arXiv:2001.09793 [gr-qc].

December 13, 2019

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: [email protected]
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 ([email protected]) and Prof. Alberto Vecchio ([email protected]).

December 4, 2019

“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!

November 20, 2019

ESA Voyage 2050

I was selected by the European Space Agency to join the Voyage 2050 Topical Teams. Voyage 2050 is ESA’s long-term programmatic plan to select scientific missions to be launched between 2035 and 2050. I am part of the review panel tasked to evaluate mission proposals focussed on “The Extreme Universe, including gravitational waves, black holes, and compact objects“.