Davide Gerosa

January 12, 2023



Gravitational-wave populations: what’s next?

It is a pleasure to announce the workshop “Gravitational-wave populations: what’s next?” which we are currently organizing for next summer:

https://sites.google.com/unimib.it/gwpopnext

As the catalog of detected gravitational-wave events grows from O(10) to O(100) sources (but think millions in a few decades!), such increasingly detailed information is allowing us to dig deeper into the (astro)physics of compact objects. At the same time, new and more data require appropriately powerful statistical tools to be fully exploited. This highly interactive workshop (fewer talks, more working together!) will be the opportunity to share recent progress, identify what new steps are now needed, and hopefully set the stage for substantial progress in the field.

The workshop will take place on July 10-14, 2023 at the University of Milano-Bicocca, which is located near the city center of Milan, Italy. Milan is a beautiful, international city in the north of Italy and is served by three major airports with worldwide connections. The city is home to art, history, and great food; nearby excursions will take you to the Italian lakes and the stunning Alps.

While we are unable to provide travel support, the workshop will have no registration fee. The workshop will be in person without remote options.

Interested participants should register on the conference website by March 1st, 2023. Depending on the number of people registering, participants might need to be selected. We will be in touch soon after the registration deadline, so please do not make travel plans until you hear back from us. When registering please indicate which of the discussion session(s) you would like to contribute to. Early career scientists will have the opportunity to give flash talks highlighting their science.

Davide Gerosa (Milano-Bicocca), Emanuele Berti (Johns Hopkins), Salvatore Vitale (MIT)

January 1, 2023



New year, new friend

Welcome to 2023… and what better way to start the new year than welcoming a new friend! Alice Spadaro (who has recently graduated with an MSc degree here in Milan) is now officially starting her PhD in my group. Alice always smiles, likes surfing, and of course is into gravitational waves 🙂 .

November 21, 2022



Two more graduations today!

Huge congrats to two of my students who graduated today! Matteo Muriano completed a funny BSc project on black-hole merger trees. And Giovanni Cavallotto went all in for his MSc research: he basically “fixed” black-hole binary spin precession at 2PN! (which is pretty cool, stay tuned for these results!). They both defended quite brilliantly, good luck with everything now!

November 15, 2022



Eccentricity or spin precession? Distinguishing subdominant effects in gravitational-wave data

We want more! With gravitational-wave data, some quantities like the masses of the black holes are much easier to see than others. But those others are very interesting, notably spins that process and orbits that are eccentric, because they would tell us how black hole binaries came to be in the first place. So while it would be great to see those, it’s also being very hard. Some tentative claims have been made with current data, but nothing unambiguous so far. In this paper led by Isobel from Cambridge, we show that (surprise surprise…) the signals needs to be long enough before one can tell eccentricity and spin precession apart.

Isobel Romero-Shaw, Davide Gerosa, Nicholas Loutrel.
Monthly Notices of the Royal Astronomical Society 519 (2023) 5352–5357.
arXiv:2211.07528 [astro-ph.HE].

November 2, 2022



The Bardeen-Petterson effect, disk breaking, and the spin orientations of supermassive black-hole binaries

Together with my postdoc Nate, we’re proceeding our investigations on supermassive, spinning binary black holes surrounded by accretion disks (that is: a ton of gas around big monsters at the center of galaxies!). In today’s paper, we dig a bit deeper into what happens when the disk breaks. That presumably stops the interactions between the gas and the black-hole spins which could make all this funky astrophysics (spins that moves, disks that breaks, etc) actually observable with future gravitational-wave detectors. More needs to be done of course, but here we are.

Nathan Steinle, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society 519 (2023) 5031–5042.
arXiv:2211.00044 [astro-ph.HE].

October 25, 2022



Here are the new gravitational wave astronomers!

More graduations today! I had the pleasure to see three of my students defending their scientific work. Lorenzo Zanga completed his BSc project on unstable spinning black-hole binaries, Alessandro Carzaniga defended his MSc thesis on gaussianities in the LISA detector, and Alice Spadaro also presented her MSc-thesis work on the LISA mock data challenge. It’s so great to see students reaching the point of defending/arguing/explaining their science… I think it’s actually one of the best things about my job! Thank you all for sharing these months with me, I’ll see you around! (And thanks to Viola De Renzis and Riccardo Buscicchio who co-supervised Lorenzo, Alessandro, and Alice with me).

Here we are, from left to right: Alessandro (sorry I cut your face in half!), me trying to be funny, Riccardo, and smiling Alice! (Lorenzo and Viola had left the room earlier…)

October 13, 2022



Late 2022 visitors: we’re alive!

My group is hosting quite a few visitors this semester. We’re alive!

  • Francesco Iacovelli is visiting us for 7 (!) months from Geneva with a grant from the Istituto Svizzero. Francesco has done some amazing work on forecasting the capabilities of Einstein Telescope.
  • Chris Moore, a longstanding collaborator from the University of Birmingham will be here at the end of October
  • Clement Bonnerot (now in Copenhagen but about to move to the UK for a faculty job, congrats!) will join us in late November.
  • Swetha Baghwat will be visiting Milan from Birmingham in November as well.
  • And Lieke van Son, Phd student at Harvard and population-synthesis mastermind, will be here in early December.
Left to right: Giulia, Viola, Michele, Lieke, Costantino, Francesco, Alice, and me

October 12, 2022



The group gets larger

So many new people are joining us this Fall!

  • Michele Mancarella is joining us as a postdoc supported by my ERC grant. He’s moving from Geneva (Switzerland) brings with him some new activities on gravitational-wave cosmology, because astronomy was not enough after all 🙂
  • Costantino Pacilio is also coming in as a postdoc on my ERC grant. Costantino is a GR tester and is providing the group with some new connections to fundamental physics.
  • Giulia Fumagalli is about to start her PhD with us, also supported by the ERC. She’s already done some amazing work with Alberto Sesana and Golam Shaifullah on pulsar timing array. Now ready for new GW adventures!
  • And spoiler alert! There’s another PhD student joining in a few months… More soon!

Welcome everybody, it’s an honor you decided to do science with us! You can read their profiles here. And if you’re also interested in my group, we have multiple openings right now. Consider applying!

September 29, 2022



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

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

Successful candidates will join the group of Prof. Davide Gerosa and will be part of the “GWmining” project funded by the European Research Council. Targeted investigations focus on the astrophysical exploitation of gravitational-wave data. We are particularly interested in candidates with expertise in population-synthesis simulations of compact binaries, gravitational-wave parameter estimation and population studies, and numerical-relativity surrogate modeling (although we are open to all candidates with a strong gravitational-wave and/or high-energy astrophysics background!). Candidates will have ample opportunities to collaborate and kickstart new projects with group members and will be strongly encouraged to develop their own independent collaborations.

We anticipate awarding up to three 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 Milano-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, the Einstein Telescope Observational Science Board, the Italian National Institute for Nuclear Physics (INFN), and the newly formed Italian Center for Supercomputing (ICSC). Faculty members with matching interests include Gerosa, Sesana, Colpi, Giacomazzo, and Dotti. For more information on Gerosa’s group see https://davidegerosa.com/group 

Milan is a beautiful, international city in the north of Italy with history, art, and outstanding food. 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 gravitational (astro)physics. Applications should include a CV with a list of publications and a two-page statement covering research interests and plans. These should be sent by November 18th, 2022 using this web form:

https://forms.gle/hnQc3N1xh53YAziH9

Candidates should also arrange for at least two, but preferably three, reference letters to be sent using the same form by November 18th, 2022. 

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 21, 2022



Andrea and Oliver are the new black-hole experts in town!

Wooo! What an amazing performance by two of my students today, who defended their BSc and MSc degrees! Oliver Rossi discussed his BSc project on black holes with large spins completed in collaboration with Viola De Renzis (PhD student in my group). Andrea Geminardi presented the results of his MSc thesis. Andrea studied the stochastic gravitational-wave background with myself, Riccardo Buscicchio (postdoc here in Milan), and Arianna Renzini (postdoc at Caltech). Hope you guys had fun working with us, we certainly did! (and I’m sorry for my pain-in-the-*** comments on your plots…). All the best for what comes next!

Here is me (center, the guy who still needs to learn how to take a selfie), Riccardo (far right), the star of the day Andrea (front right) together with Alberto (green guy) and his student Serena (left) who also brillantly defended her astrophysics MSc thesis today.

September 19, 2022



Job opportunities for Marie Curie past holders and applicants

The Italian government has pushed a hiring program dedicated to holders and applicants of Marie Curie Fellowships from the EU. The call targets those that have either (i) completed a successful Marie Curie Fellowship in the past 4 years or (ii) applied unsuccessfully in the past 4 years but were awarded the so-called “Seal of Excellence”. 

For both categories, successful candidates will be awarded a 3yr senior researcher position (at the so-called RTDA level in the Italian system). RTDAs are hired as full employees with related benefits and have limited teaching duties. On top of this, candidates in the Marie Curie winners strand (i) will also be offered a substantial startup grant to hire their own PhD students and postdocs. 

All Italian institutions can act as hosts, so I encourage you to contact one of us in the country for more information.

In particular, the gravitational-wave group at the University of Milano-Bicocca provides a vibrant environment with activities ranging from relativistic astrophysics. gravitational-wave data analysis, numerical relativity, and gravity theory. The group counts faculty members Gerosa, Sesana, Colpi, Giacomazzo, and Dotti as well as tens of students and postdocs. The city of Milan is a jewel in the north of Italy with a charming international vibe (as well as mountains, history, art, and outstanding food). 

The internal application deadline is October 18th. If you’re eligible and/or interested in applying with us, please get in touch asap ([email protected]) and we’ll go from there.

Here are the relevant webpages (scroll down for the English text):

(i) Marie Curie past winners

https://www.unimib.it/ricerca/opportunita/finanziamenti-alla-ricerca/finanziamenti-nazionali/bando-giovani-ricercatori-vincitori-msca-young-researchers-msca-grants-winners

(ii) Seal of Excellence holders:

https://www.unimib.it/ricerca/opportunita/finanziamenti-alla-ricerca/finanziamenti-nazionali/bando-giovani-ricercatori-seal-excellence-msca-call-young-researchers-seal-excellence-msca

August 30, 2022



Italy has a brand new Center for Supercomputing (ICSC)… and we’re on it!

The Italian government is pushing a major inverstment program in High-Performance Computing, and we’re part of it! The new ICSC (Italian Center for Supercomputing) will manage >300M Euros going towards early-career researchers, PhD scholarships, and computing infrastructure. The University of Milano-Bicocca is part of the founding member of ICSC, with our research group providing some core activities for the Bicocca contribution. If you’re interested in computational (astro)physics, stay tuned for several upcoming opportunities!

July 4, 2022



Characterization of merging black holes with two precessing spins

Lots of “firsts” today! My first-year PhD student Viola just put out her first first-author paper. This is about measuring black holes with not one, but two precessing spins. People have been trying to figure out how to tell if at least one of the two spins of a merging black-hole binary is precessing for quite some time now. And maybe we’ve even done it already for one or two of the current LIGO-Virgo events. But here I must quote that epic Italian commercial from the 90s: “two gust is megl che one” (which is a terrible Italian-English mishmash on a terrible joke to say that when you eat a Maxibon “two flavors are better than one”). In this paper we propose a strategy to identify sources that have the strongest evidence of two processing spins. Viola has been putting together simulated data for the next LIGO/Virgo data-taking period, and the result is pretty cool. If these binaries are out there in the Universe, we will be able to tell they have two spins going around!

Viola De Renzis, Davide Gerosa, Geraint Pratten, Patricia Schmidt, Matthew Mould.
Physical Review D 106 (2022) 084040.
arXiv:2207.00030 [gr-qc].

June 1, 2022



New summer means new summer projects

We’re having four (!) summer students joining the group this year!

  • Diego Padilla Monroy from Florida International University (Miami) will be working with me in Milan supported by the IREU program.
  • Derin Sivrioglu from Grinnell College (Iowa) will be working with Daria Gangartd in Milan.
  • Sayan Neogi from the Indian Institute of Science, Education and Research (Pune, India) will be working with Matt Mould in Birmingham.
  • Sarah Al Humaikani from Paris (France) will be working with Nathan Steinle in Birmingham.

Welcome all! We look forward to seeing your summer discoveries!

May 25, 2022



Which black hole formed first? Mass-ratio reversal in massive binary stars from gravitational-wave data

Big stars burn everything they have, die fast, and produce big black holes. So when you see two black holes together, it’s likely that the big black hole comes from the big star. Or maybe not? Before dying, the big star can drop some mass onto the other guy, making it bigger! So now, the initially big star still produces the first black hole, but, at the end of the day, that might not be the more massive black hole anymore! This scenario is called “mass-ratio reversal” and our astrophysics friends have put together many models out there showing this is indeed possible for a good fraction of the black holes that produce gravitational-wave events. So here we ask the data: given the events LIGO and Virgo have seen so far, what’s the evidence for mass-ratio reversal in binary stars? Read Matt’s paper to find out.

Matthew Mould, Davide Gerosa, Floor S. Broekgaarden, Nathan Steinle.
Monthly Notices of the Royal Astronomical Society 517 (2022) 2738–2745.
arXiv:2205.12329 [astro-ph.HE].

April 26, 2022



PhD in gravitational physics!

The University of Milano-Bicocca welcomes applications for Ph.D. scholarships. The application deadline is May 20th, 2022 for positions starting in the Fall of 2022:

https://en.unimib.it/education/postgraduates/doctoral-research-phd-programmes/applying-doctorate/calls-application

In particular, the theoretical astrophysics group is looking for strong, highly motivated candidates to join our activities in black-hole binary dynamics, gravitational-wave data exploitation, and numerical relativity. Faculty members with matching interests include Gerosa, Sesana, Colpi, Dotti, and Giacomazzo. The candidates will have ample opportunities to work with and visit external collaborators as well.  

Our PhD admission program includes a number of “open” scholarships, covering all research activities in the department (including ours!). All candidates are considered for those by default.  In addition, our group sponsors two specific positions:

  • “Gravitational-wave data and black-hole binary dynamics”, supervised by Gerosa. Possible research directions include statistical inference from LIGO/Virgo and LISA data, application of machine-learning tools to gravitational-wave astronomy, and theoretical investigations of black-hole binaries.
  • “Dynamics of massive black hole binaries in dense stellar systems”, supervised by Sesana and Gualandris. This is a dual-doctorate position in partnership with the University of Surrey, UK. The main focus is the understanding of binary evolution using N-body simulations and analytical modeling. The successful candidate will spend 50% of their time at Bicocca and 50% of their time at Surrey.

Candidates wishing to be considered for these additional positions should mention it explicitly in their application.

More information on the astrophysics group at Bicocca can be found at astro.fisica.unimib.it. For informal inquiries please do not hesitate to contact [email protected] or [email protected].

April 22, 2022



Long-term research appointment in computational astrophysics at Milano-Bicocca (Italy)

The University of Milano-Bicocca (Italy) invites expressions of interest for a 3+2 year research position in HPC applications to astrophysics.

The astrophysics group at Milano-Bicocca provides a vibrant environment with expertise covering all aspects of gravitational-wave astronomy, relativistic astrophysics, galactic dynamics, and numerical relativity. This is embedded in a wider astronomical context including both observational and experimental activities. Our group has tight connections with the LISA Consortium, the Virgo Collaboration, the Einstein Telescope Science Board, the European Pulsar Timing Array, and the Italian National Institute for Nuclear Physics (INFN) via the TEONGRAV national initiative. Staff members with matching interests include Colpi, Dotti, Gerosa, Giacomazzo, Lupi, and Sesana.

Milan is a beautiful, international city in the north of Italy. Mountains and lakes are just around the corner. Art, culture, and food are outstanding. The city hosts three international airports with worldwide connections.

This recruitment campaign is part of a wider national initiative supporting HPC-related computational activities throughout the country. This is a major investment program directly supported by the European Union. It will provide the most ideal context for ambitious candidates wishing to develop and apply state-of-the-art computational and machine-learning tools to current astrophysical and gravitational-wave modeling issues.

The researcher will be appointed at the so-called “RTDA” level for 3 years. The contract can also be extended for 2 more years depending on funding availability.  The starting date is negotiable, with the earliest and latest dates on January 1st, 2023 and May 1st, 2023, respectively. RTDA researchers are full-time university employees (with full benefits, such as health insurance and pension plan), have limited teaching duties, and are eligible to fully supervise research MSc student projects. This is an ideal setup for early-career researchers wishing to transition toward research independence and start developing their own group. 

The successful candidate will have a PhD in Physics, Astronomy, Computer Science, or related discipline, strong programming skills, and previous experience in one or more of the following topics: HPC workflows, GPU software development, computational astrophysics, gravitational-wave astronomy, numerical relativity, statistical data analysis, machine learning.

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 June 15th, 2022 for full consideration. Candidates should also arrange for two reference letters to be sent to [email protected] by June 15th, 2022. 

We strive to build a diverse and inclusive environment and welcome expressions of interest from traditionally underrepresented groups. Women are especially encouraged to apply. For inquiries please do not hesitate to contact Bruno Giacomazzo ([email protected]) or Davide Gerosa ([email protected]).

April 21, 2022



Got an ISCRA-B supercomputer allocation!

I was just awarded a large allocation on the Italian national supercomputer at CINECA. My PhD student Viola De Renzis (our parameter-estimation expert!) is the co-I on our proposal. Our award is part of the so-called ISCRA Class B program (which is their medium-size allocation scheme) and amounts to 1.2M CPUh on the Galileo cluster (that is: we’re going to have to crunch a ton of numbers now!). Viola and I will study the extraction of spin-spin couplings from black-hole binaries using gravitational-wave data and stochastic sampling techniques. Stay tuned!

April 13, 2022



“With a little help from my friends” Workshop at JHU

We’re at Johns Hopkins University (Baltimore) today, for a brainstorming workshop we organized together with the gravity groups at JHU and Penn State. A ton of interesting people, cool science, fun numerics, big black holes, future detectors, and many new exciting projects we all want to start. The idea is to get “a little help from my (gravity) friends”. Have a look at what we’re up to: davidegerosa.com/with-a-little-help-from-my-friends-workshop/

April 8, 2022



The last three years: multiband gravitational-wave observations of stellar-mass binary black hole

Observing gravitational waves from the ground (i.e. LIGO, Virgo, etc) give us a unique view on “the last three minutes” of the life of compact objects before they merge with each other. Going to space (I’m talking to you, LISA!) will instead give us “the last three years”. Completed together with the rest of the Birmingham crowd, this paper provides a realistic view of this truly amazing landscape. LISA observations at low frequencies in the 2030s will be paired with high-frequency data from LIGO’s successors (the so-called 3rd generation detectors). Together (and that’s crucial, together!) LISA and 3g detectors will tell us the full story of the life of merging black holes. LIGO alone is like catching up with a movie because you were late at the theatre, LISA alone is like a huge cliffhanger before the series finale… multiband observations are a bingewatching experience!

Antoine Klein, Geraint Pratten, Riccardo Buscicchio, Patricia Schmidt, Christopher J. Moore, Eliot Finch, Alice Bonino, Lucy M. Thomas, Natalie Williams, Davide Gerosa, Sean McGee, Matt Nicholl and Alberto Vecchio.
arXiv:2204.03423 [gr-qc].

April 4, 2022



Constraining black-hole binary spin precession and nutation with sequential prior conditioning

Daria’s new paper is out! (With key contributions from others in the group… This is also Viola’s first paper!).

Here we look at sub-dominant black-hole spin effects in current data from LIGO and Virgo (yeah sorry guys… our black-hole spin obsession goes on). People have looked at spin precession before, but we’re interested in even more subtle things, namely disentangling precession and nutation. This is a tricky business, which is made complicated by the fact that this piece of information is hidden behind other parameters that are easier to measure (say the masses of the two black holes). Our paper is an attempt to formulate and systematically exploit something we called “sequential prior conditioning” (which is: mix&match priors and posteriors in Bayesian stats…). Results are weak today but strong tomorrow.

Daria Gangardt, Davide Gerosa, Michael Kesden, Viola De Renzis, Nathan Steinle.
Physical Review D 106 (2022) 024019.
arXiv:2204.00026 [gr-qc].

March 11, 2022



Astrophysics with the Laser Interferometer Space Antenna

LISA astrophysics is awesome and everything you might ever want to know is written this paper. [Sorry for the short blog post, but there isn’t much else to say really…] A huge thanks to all the captains that put this massive community-wide effort together.

Pau Amaro-Seoane, et al. (155 authors incl. Davide Gerosa).
arXiv:2203.06016 [gr-qc].

March 9, 2022



Deep learning and Bayesian inference of gravitational-wave populations: hierarchical black-hole mergers

It took a while (so many technical challenges…) but we made it! Matt‘s monster paper is finally out!

Let me introduce a fully-fledged pipeline to study populations of gravitational-wave events with deep learning. If it sounds cool, well, it is cool (just look at the flowchart in Figure 1!). We can now perform a hierarchical Bayesian analysis on GW data but, unlike current state-of-the-art applications that rely on simple functional form, we can use populations inferred from numerical simulations. This might sound like a detail but it’s not: it’s necessary to compare GW data directly against stellar physics. While we don’t do that yet here (our simulations are admittedly too simple), there’s a ton of astrophysics already in this paper. Whether you care about neural networks or hierarchical black-hole mergers (or, why not, both!), sit tight, fasten your seatbelt, and read Matt’s paper.

Matthew Mould, Davide Gerosa, Stephen R. Taylor.
Physical Review D 106 (2022) 103013.
arXiv:2203.03651 [astro-ph.HE].

March 2, 2022



New class! Astrostatistics

I just had the first lectures of a class I’m teaching for the first time: Astrostatistics and Machine Learning (sounds exciting? Well, it is!). This is an advanced course for the MSc degree in Astrophysics and Space Science at the University of Milano-Bicocca. My students and I will travel across data inference, Bayesian wonders, sampling, regression, classification, and become best friends with deep learning. All of this is applied to astrophysical datasets.

The entire class is available under the form of jupyter notebooks at github.com/dgerosa/astrostatistics_bicocca_2022. The repository is hooked up with the mybinder service.

February 23, 2022



Congrats Cecilia!

Huge congrats to my student Cecilia Fabbri who got her Bachelor’s degree today. Cecilia defended (quite brilliantly!) her project titled “Constraining the black-hole irreducible mass with current gravitational-wave data”. Her work ended up in our recent draft (arxiv:2202.08848). Cecilia is continuing with a Master’s degree in astrophysics at Milano-Bicocca, stay tuned for her future successes!

February 21, 2022



The irreducible mass of LIGO’s black holes

Spinning black holes are weird (well, all black holes are weird but those that spin are the worse!). They have a funny thing called ergoregion where orbiting particles can have negative energy. Penrose was the first to realize that this can be exploited to extract energy from the black hole itself. The thing is, even if you figure out how to do it, you’re inevitably going to spin the black hole down. At the end of the day, you’re left with a fossil black hole that does not have any spin. The mass of that leftover black hole (“What’s for lunch dear? Fancy some sushi or prefer a black hole?”) is called irreducible mass. Hawking (another giant!) figured out this has to do with thermodynamics.

Long story short, in this paper we compute the irreducible mass of the black holes detected in gravitational waves by LIGO. It was funny to re-discover that gravitational wave detection was indeed the motivation behind Hawking original proof of the area theorem (he had Weber‘s claimed detection in mind at the time). The story behind our paper starts as a toy calculation with my undergraduate student Cecilia and ended up in a neat, hopefully informative exploitation of LIGO data. We reparametrized LIGO’s black-hole properties using the rotational and rotational contributions to their total energy, we ranked current gravitational-wave events according to their “irreversibility”, and we compute a sort of population version of the area law. Enjoy!

Davide Gerosa, Cecilia Maria Fabbri, Ulrich Sperhake.
Classical and Quantum Gravity 39 (2020) 175008.
arXiv:2202.08848 [gr-qc].

January 20, 2022



People visiting

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



TEONGRAV

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!

Rebecca Nealon, Enrico Ragusa, Davide Gerosa, Giovanni Rosotti, Riccardo Barbieri.
Monthly Notices of the Royal Astronomical Society 509 (2022) 5608–5621.
arXiv:2111.08065 [astro-ph.HE].

October 13, 2021



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:

Matthew Mould, Davide Gerosa.
Physical Review D 105 (2022) 024076.
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.
Physical Review D 104 (2021) 083008.
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.
Rendiconti Lincei. Scienze Fisiche e Naturali 32 (2021) 665–673.
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.
Physical Review D 104 (2021) 044065.
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.
Physical Review D 104 (2021) 084002.
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:

https://en.unimib.it/education/doctoral-research-phd-programmes/how-apply-phd-programme

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 5 (2021) 749-760.
arXiv:2105.03439 [astro-ph.HE].
Review article.

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.