Davide Gerosa

July 17, 2023



gwpopnext was a blast!

Last week my group and I hosted the international workshop “Gravitational-wave populations: what’s next?.” It’s been a blast!

An unconventional conference, with almost zero talks and the vast majority of the time dedicated to discussions. I report the program here below, just to give you a feeling of what we discussed. The conference started with the question “How many of you entered the field after GW150914?” and virtually everyone raised their hand! It was so refreshing to see our field is alive.

We then went through population synthesis simulations, fancy statistical methods (I promise I’ll understand nonparametric methods one day!), intricacies of injections, catalogs, and overlap with our EM observer friends. We took a break on Wednesday for a social activity on Lake Como, with some folks diving into the lake and others hiking up to a small castle. All before dinner with a fascinating lake (and thunderstorm!) view.

Thanks all for joining and participating so actively. Huge thanks to Emanuele Berti and Salvo Vitale for co-organizing this with me, as well as the local GW group for assistance. Finally, congrats to Amanda Farah and Alex Criswell who won our SIGRAV early career prize.

And if you couldn’t make it for whatever reason no worries, we’ll do it again!

Conference program in a nutshell. These are our discussion sessions

  • Intro: the pieces of the population problem.
  • What can/should astrophysicists and pop-synthers predict?
  • What is the predictive power of pop-synth codes? Are we learning more than our assumptions?
  • Hierarchical Bayesian fits: can we keep on doing this? Technical difficulties, scaling with the number of events, selection effects.
  • Mind the outliers. Are they in or out of your fit? If you fit something well you also need to fit the rest.
  • What is a catalog? Is p_astro the way to go? (Ir)relevance of subthreshold events.
  • Mind the systematics. Are waveform/calibration impacting the population? And how about the assumed population?
  • Beyond functional forms: “non-parametric” methods. What are they and what does it even mean.
  • Beyond functional forms: “parametric but informed”. Machine learning emulators for pop-synth.
  • More populations. LISA, X-ray binaries, Gaia, you name it.
  • More than individual mergers. Stochastic backgrounds, foreground removal.
  • Adding the redshift dimension: toward 3g! Use the population to do cosmology.
  • What’s next? Summary and prospects.

July 10, 2023



Extending black-hole remnant surrogate models to extreme mass ratios

New paper from a new student! Here is Matteo Boschini’s first piece of work, where we look at predictions for the final mass and spins of black-hole remnants. That is, after two black hole merge, what’s the mass and spin of the guy they left behind? These predictions are typically done by fitting (in various ways) outputs from numerical-relativity simulations but those, unfortunately, can only handle black holes of similar masses. On the other hand, black holes with masses that are very different from each other can be handled analytically. Here we show how to put the two together with a single machine-learning fit.

Matteo Boschini, Davide Gerosa, Vijay Varma, Cristobal Armaza, Michael Boyle, Marceline S. Bonilla, Andrea Ceja, Yitian Chen, Nils Deppe, Matthew Giesler, Lawrence E. Kidder, Guillermo Lara, Oliver Long, Sizheng Ma, Keefe Mitman, Peter James Nee, Harald P. Pfeiffer, Antoni Ramos-Buades, Mark A. Scheel, Nils L. Vu, and Jooheon Yoo.
Physical Review D 108 (2023) 084015.
arXiv:2307.03435 [gr-qc].

June 21, 2023



Phd position at Milano-Bicocca under the Italian National PhD program in Space Science and Technology

The University of Milano-Bicocca (Milan, Italy) will be hiring a PhD student under the Italian National PhD program in Space Science and Technology. Candidates with interests in gravitational-wave astronomy, data analysis, and multi-messenger applications are encouraged to apply. For information please see:

The application deadline is July 6th, 2023 at 4pm CEST. For informal enquiries please contact Monica Colpi ([email protected]).

June 19, 2023



Masterclass in big data within science and industry

The advanced class “Big data within science and industry” will take place on September 22nd at the University of Milano-Bicocca (Milan, Italy).

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

Data are everywhere. Exploring scientific data is now at the heart of both scientific advances as well as industrial applications. This one-day master class provides a “learn by example” introduction to the fascinating world of big data, namely pieces of information that are so rich and structured that require targeted analysis techniques loosely referred to as machine learning or artificial intelligence.


The class is suitable for advanced MSc students, PhD students, and postdocs who wish to expand their proficiency in handling scientific data. The program features the participation of three world-leading experts from both academia and the private sector, as well as a hands-on experience for all participants.

For students enrolled in the Physics and Astronomy PhD program here at Milano-Bicocca, this 8-hour program will be recognized with 1 CFU. In any case, we are happy to provide attendance certificates.

Interested students should register by September 8th, 2023. Participation is free of charge. We hope to accommodate everyone, but depending on the number of people registering, participants might need to be selected.

Davide Gerosa, Michele Fumagalli (Milano-Bicocca)

June 8, 2023



Dr. Matt!

Please let me introduce Dr Matthew Mould… After N papers (where N is a lot) and a 4h+15min viva discussion, Matt has completed his PhD in gravitational-wave astronomy at the University of Birmingham. WooooO! The examiners were Annelies Mortier from Birmingham and Uli Sperhake from Cambridge, who went through a thesis with more than 600 references…. Matt will be continuing his already successful career with a postdoc at MIT, LIGO lab. From my side, Matt is (actually, was!) my first PhD student and spending 3+ years working with him has been amazing. Thanks, Matt for teaching me Bayesian stats and never letting go when I was saying crap.

First thing you do after a 4h 15m viva? Eat a carefully baked cookie by Giulia!

June 7, 2023



Glitch systematics on the observation of massive black-hole binaries with LISA

All right, this is kind of far from my day-to-day topics but working on this paper with Alice and Riccardo was super fun. Think LISA and supermassive binary black holes. And… the detector does what it wants. That’s not true of course because the experimentalists are amazing, but there will be noise transients: unexpected blips when the gravitational-wave signal will be corrupted. Here we look at what would happen in a realistic setting when a LISA glitch happens on top of a gravitational wave from a supermassive black hole.

Alice Spadaro, Riccardo Buscicchio, Daniele Vetrugno, Antoine Klein, Davide Gerosa, Stefano Vitale, Rita Dolesi, William Joseph Weber, Monica Colpi.
Physical Review D 108 (2023) 123029.
arXiv:2306.03923 [gr-qc].

June 6, 2023



Let’s PRIN!

Happy to report we got a grant from the Italian PRIN program! This is in collaboration with Andrea Maselli from GSSI in L’Aquila. The title is “Gravitational-wave astronomy as a mature field: characterizing selection biases and environmental effects”. Stay tuned for more research (and more positions to join our group!).

June 3, 2023



IREU summer time

Welcome Harrison Blake! My group is hosting a student from the IREU program in Gravitational Physics, which is administered by the University of Florida. Harrison is visiting from Ohio State University and will be working with Michele Mancarella on forecasting the science with can do with gravitational waves from the Moon…

May 30, 2023



One to many: comparing single gravitational-wave events to astrophysical populations

We do population analysis in gravitational waves all the time now. That is: we compare many observations from GW experiments against many simulated datapoints from simulations. But what if you only have one observation? That could be a LIGO guy that is kind of an outlier (think GW190521) or maybe a datapoint from a future detector (think LISA) that feels lonely in his parameter space. Don’t look further, this is stats for you (and Matt’s last paper as a grad student…)

Matthew Mould, Davide Gerosa, Marco Dall’Amico, Michela Mapelli.
Monthly Notices of the Royal Astronomical Society, 525 (2023) 3986–3997
arXiv:2305.18539 [astro-ph.HE].

May 17, 2023



QLUSTER: quick clusters of merging binary black holes

We’ve got the best name ever for a numerical code! Let me introduce QLUSTER which, guess what, simulates clusters. We finally put out a piece of code that was originally developed in 2019 and later used in several papers. It’s a very very simple treatment of black-hole binary formation in dense stellar environments, with the goal of predicting gravitational waves from repeated mergers. The code is available at github.com/mdmould/qluster and a short description is provided in the proceedings of the 2023 edition of the amazing Moriond conference.

Davide Gerosa, Matthew Mould.
Proceedings of the 57th Rencontres de Moriond.
arXiv:2305.04987 [astro-ph.HE].

April 27, 2023



Parameter estimation of binary black holes in the endpoint of the up-down instability

This paper is episode four in the up-down instability series. We first figured out the instability exists (episode 1), then computed when binaries go after the instability (i.e. the endpoint, episode 2), and also checked binaries are really unstable in numerical relativity (episode 3). Now we look at the inference problem with LIGO/Virgo: if unstable up-down binaries enter the sensitivity window of the detector, will we be able to tell? We phrased the problem with some fancy stats using the so-called Savage Dickey density ratio, which is the right tool to answer this question. As is too often the case, current data are not informative enough but the future is bright and loud.

Viola De Renzis, Davide Gerosa, Matthew Mould, Riccardo Buscicchio, Lorenzo Zanga.
Physical Review D 108 (2023) 024024.
arXiv:2304.13063 [gr-qc].

April 12, 2023



Efficient multi-timescale dynamics of precessing black-hole binaries 

It’s out! The notorious (ask my students…) “precession v2” paper is finally out! This took a veeeery long time; we checked and the first commit for this paper is from May 2020 (!). But the result is an exhilarating tour of spin precession at 2PN with 27 pages and 183 (!!!) numbered equations. We rewrote the entire formalism, change how we parametrize things, compute all we could in closed forms, and speed up the computational implementation. It’s cool, now performing a precession-averaged evolution is a <0.1s operation. If you’re into BH binary spin precession, this is the paper for you. All of this is now part v2 of our PRECESSION python module. So long, and thanks for all the spin.

Davide Gerosa, Giulia Fumagalli, Matthew Mould, Giovanni Cavallotto, Diego Padilla Monroy, Daria Gangardt, Viola De Renzis.
Physical Review D 108 (2023) 024042.
arXiv:2304.04801 [gr-qc].
Open-source code: homepage, repository, documentation.

March 29, 2023



Inferring, not just detecting: metrics for high-redshift sources observed with third-generation gravitational-wave detectors

Third-generation gravitational wave detectors are going to see all stellar-mass black-hole mergers in the Universe. Wooooooooo. But hang on, is this enough? Observing the sources is great, but then we need to measure them. Here we try to focus on the latter and quantify how well we will be able to measure the distance of black holes. Read the paper now, but the short answer is that 3G detectors are going to be awesome but not that awesome…

Michele Mancarella, Francesco Iacovelli, Davide Gerosa.
Physical Review D Letters 107 (2023) L101302.
arXiv:2303.16323 [gr-qc].

March 23, 2023



PhD positions in gravitational-wave astronomy at Milano-Bicocca

The University of Milano-Bicocca welcomes applications for PhD scholarships. The application deadline is April 19th, 2023 for positions starting in the Fall of 2023:

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

In particular, the theoretical astrophysics group is looking for 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. 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 this year is advertising an additional “project” scholarship titled “Gravitational-wave source modeling” and supervised by Gerosa. Candidates wishing to be considered for this additional opportunity should indicate it explicitly when applying (the number of this position FIS.3).

We strive to build an inclusive group and welcome applications from all interested candidates. More information on the astrophysics group at Bicocca can be found at astro.fisica.unimib.it. For informal inquiries and expressions of interest please do not hesitate to contact [email protected]

March 22, 2023



Spring graduations!

It’s student time! Massive congratulations to two of my students who just graduated.

The star of the day is Matteo Boschini, who completed his MSc project with me after a long visit at the AEI (Postdam, Germany) to collaborate with Vijay Varma. Matteo worked out an amazing extension of current numerical-relativity surrogate models… stay tuned for a paper because this is going to be cool!

Daniele Chirico completed his BSc studies with a sweet research project on supernova explosions, orbits, and kicks. He’s staying in Milan for his MSc degree now, so wait a bit for his successes!

That’s Matteo discussing black-hole remnants…

February 27, 2023



We should learn from our students: LISA and beyond

The student reps of our department (codename: redshift) have organized a stellar event today. Curiosity and interest in the LISA space mission brought them to design a full day of talks from leading experts in the field. They put Stefano Vitale, Alessandra Buonanno, and Bernard Schutz in the same room with the (astro)physics students and, well, a few of us who tagged along. The result was an amazing rollercoaster called “LISA and beyond” across the wonders of the experimental design by Stefano (is this truly going to work?!?), some amazing order-of-magnitude calculations that Bernard pulled off (wish I could do that!), and a broad vision by Alessandra across the discoveries we had and those we will soon be seeing (can’t wait, can’t wait!). Our students engaged with the speakers, asked questions, and organized a round table touching topics like the carbon footprint of space missions, gender equality, and how to manage a research group. Such ingenuity and enthusiasm are what keeps science alive! We should learn from our students and do science like that.

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 3, 2022



Super Arianna!

Very happy to report that Arianna Renzini (currently a postdoc at Caltech) was awarded a prestigious Marie Skłodowska-Curie Fellowship from the European Union, to be hosted here with my group. Arianna will bring expertise in modeling the gravitational-wave stochastic background, which is a key target for both current and future experiments. Arianna’s proposal is titled “Stochastic rewind and fast-forward: calibrating LISA with LIGO’s black holes and stochastic background.” Huge congrats, can’t wait to welcome you here.

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].