Davide Gerosa

November 4, 2024



Orbital eccentricity in general relativity from catastrophe theory

Black holes on eccentric orbits… what does it even mean? The hard (but fun) thing is that we work in General Relativity, where coordinates don’t have a physics inside. One can always change the coordinates as they want, so they can’t be used to define observables. The eccentricity of an orbit has to do, indeed, with the shape of the orbit itself, and that can be transformed away with suitable coordinates. So, does it even sense to measure the orbital eccentricity of black-hole binaries? The one thing we are allowed to do is to find a coordinate-free estimator in General Relativity that reduces to the eccentricity we all know and love in the Newtonian limit. This is possible! The right mathematical framework for this is something called “catastrophe theory”, a funny name, but Nick likes it.

Matteo Boschini, Nicholas Loutrel, Davide Gerosa, Giulia Fumagalli.
arXiv:2411.00098 [gr-qc].

October 24, 2024



Forecasting the population properties of merging black holes

Our “popfisher” paper is finally out! (and now Viola can submit her PhD thesis). This is about next-generation (aka 3G) gravitational wave detectors. Those beasts will measure millions of black holes… and with so many of them who cares about each source individually. The important thing will be the population of objects, i.e. how those black holes are distributed as a whole. Measuring populations is an interesting but convoluted statistical problem. Here we implement a quick shortcut (the Fisher matrix) and show that yes, 3G detectors will be amazing… but more amazing for some things than for others.

Viola De Renzis, Francesco Iacovelli, Davide Gerosa, Michele  Mancarella, Costantino Pacilio.
arXiv:2410.17325 [astro-ph.HE].

October 24, 2024



4+1: October graduations

Four BSc students and one MSc student defended their research project with us this month.

  • First, huge congrats to Federica Tettoni who got her MSc degree in astrophysics. She worked with Viola De Renzis and myself on labeling black holes in gravitational wave events. Such a fun problem (and we got confused so many times!), more here.
  • Rocco Giugni (BSc) worked with Matteo Boschini on finding issues in his remnant surrogate models…
  • Giulia Foroni (BSc) worked with me on black-hole binary spin precession. The good old problem of the spin morphologies, but this time looking for two transitions at the same time.
  • Matilde Vergani (BSc) also worked with me; we looked at merger trees and their combinatorics problem (her presentation pictures of trees, I mean actual trees…).
  • Laura Tassoni (BSc) worked with Costantino Pacilio on ringdown data analysis.

Thanks all for spending some time in our research group!

October 15, 2024



A test for LISA foreground Gaussianity and stationarity. I. Galactic white-dwarf binaries

LISA will see a gazillion white dwarfs, but we won’t, or at least not individually. Those signals will actually pile up together in a mashed potato thing called foreground. But this mashed potato won’t be smooth (translate: the gravitational-wave signal won’t be stationary and Gaussian) and this structure can indeed be precious for extracting more information from LISA. But first, let’s taste this with today’s paper, i.e. characterize the foreground.

Riccardo Buscicchio, Antoine Klein, Valeriya Korol, Francesco Di Renzo, Christopher J. Moore, Davide Gerosa, Alessandro Carzaniga.
arXiv:2410.08263 [astro-ph.HE].

ps. This started as the student project of Alessandro Carzaniga, great it’s finally out!

October 8, 2024



Group accretion (close to the Eddington limit)

Our group is accreting people at the Eddington rate! There are 5 new postdocs and 2 PhD students who have just started or are about to start:

  • Zachos Roupas is joining Bicocca as a Marie Curie Fellow with an independent research program on stellar clusters.
  • Caroline Owen comes as a postdoc from Illinois, with expertise in fundamental physics and testing GR.
  • Alexandre Toubiana is joining as a postdoc from the AEI and likes gravity at all frequencies.
  • Rodrigo Tenorio is a new postdoc coming from UIB (Balearic Islands) and he’s going to be the group member who’s the closest to real GW data.
  • Tristan Bruel comes as a postdoc from Nice and will bring us back to astrophysics instead.
  • Federico De Santi graduated from the University of Pisa and will join as a PhD student.
  • Last but definitely not least, Chiara Anselmo will also join as a PhD student after an MSc degree in Rome.

Group meetings are funny and busy these days, with too many ideas going around.

September 26, 2024



Minimum gas mass accreted by spinning intermediate-mass black holes in stellar clusters

This is a fun IMBH story we worked out when Kostas and Luca were visiting last summer from JHU. What if (one day, who knows) we observe a highly spinning intermediate-mass black hole? If that happens, is going to be puzzling because IMBH that grow in clusters by mergers of smaller black holes tend to spin down, not up. This is a funny property of black holes, namely that extracting spins is easier than putting it in, so on average black holes slow down after they have merged many times. So if we see an IMBH with large spins, the spin must come from somewhere else. Where? Maybe gas. The argument then is that one can actually convert an IMBH spin measurement into the minimum amount of gas that must have been accreted to get that spin.

Konstantinos Kritos, Luca Reali, Davide Gerosa, Emanuele Berti.
arXiv:2409.13011 [astro-ph.HE].

September 23, 2024



Stars or gas? Constraining the hardening processes of massive black-hole binaries with LISA

To Stars or to gas, that is the question.
Whether ’tis nobler in the hardening to suffer
The slings and arrows of passing stars,
Or to dissipate against a sea of gas
And by disk end them. To inspiral — to merge,
No more; and by LISA to say we end
The models and the thousand PE samples
That gravity is heir to.

Alice Spadaro, Riccardo Buscicchio, David Izquierdo-Villalba, Davide Gerosa, Antoine Klein, Geraint Pratten.
arXiv:2409.13011 [astro-ph.HE].

September 20, 2024



Cecilia et al.

Four students just graduated with projects in our group…

First, huge congrats to Cecilia Fabbri who got her MSc in Astrophysics. Cecilia (you might remember her) worked on an exciting applied statistics problem (which has already ended up in a poem, but soon in a paper). Her problem got like 10 more people hooked beside us, so we really have to finish it now! From my side, it’s always amazing to see scientists like her growing so much. Cecilia be moving on with a PhD in Nottingham (UK) with Steve Green (and when you come back to visit you’ll tell me everything I don’t understand about simulation-based inference!). Good luck!

We also supervised three BSc students who defended their short projects:

  • Matteo Pagani worked with Ssohrab Borhanian on testing Fisher Matrix codes (spoiler, it’s tricky).
  • William Toscani worked with Giulia Fumagalli on eccentric binary black holes, revisiting the old “isotropic stays isotropic” problem in PN dynamics.
  • Francesca Rattegni worked with Matteo “Bormio” Bonetti on how the Galaxy can cause Kozai-Lidov oscillations on wide binaries.

Congrats all, Spritz time now.

Here’s Cecilia (wearing Laurel crowns is a very Italian thing to do when graduating!)

September 13, 2024



Which is which? Identification of the two compact objects in gravitational-wave binaries.

All right I think this is great (but it took me a long time to convince myself and the others that’s the case!) In gravitational-wave astronomy we measure binaries, that is, pairs of two objects. Our signals have information about the pair as a whole. At the same time, we care very much about separating those two objects and measuring the properties of individual black holes and neutron stars. We always do that operation without thinking twice, just say that for each posterior sample object “1” is that with the larger mass and object “2” is that with the lower mass. But is that ok? Surely it’s a choice, but is it the best one? What does it even mean to pick the “best” labels? I think machine learning can help us here and that this problem can be framed using the language of semi-supervised clustering. The results? Well, they seem very significant. Measurements of the black-hole spins are more accurate, you can tell more easily if that’s a black hole or a neutron star, and overall the posterior distributions just look nicer (go away nasty multimodalities and non-Gaussianities!).

Davide Gerosa, Viola De Renzis, Federica Tettoni, Matthew Mould, Alberto Vecchio, Costantino Pacilio
arXiv:2409.07519 [gr-qc].

August 9, 2024



postmerger: A flexible mapping of ringdown amplitudes for non-precessing binary black holes

The ringdown is the final bit of a gravitational-wave signal, after the two black holes have merged. It’s nice because it’s clean; GR is so powerful that all that comes out after a black hole merger has specific frequencies, the fantastic “quasi-normal modes.” While the frequencies only depend on that final BH (thanks Kerr!), the excitations of those frequencies depend on all that happened before, i.e. the merger process itself. In this summer paper by Costantino and the rest of us, we present a new accurate approximant to those amplitudes. Now go home and test GR.

Costantino Pacilio, Swetha Bhagwat, Francesco Nobili, Davide Gerosa
arXiv:2408.05276 [gr-qc].

July 22, 2024



Computer-science graduation

Usually my students graduate in Physics, but not this time… Together with Matteo Boschini, I had the pleasure of supervising a student majoring in Computer Science. Alessandro Crespi got his BSc degree with a project on Simulation Design, which is really a computing thing but has lots of physics applications. That was so much fun! It is truly true that putting different expertise/approaches/ideas makes things better.

July 16, 2024



Many visitors

We’re having a few visitors this summer, with lots of science going around. Welcome Jam Sadiq from SISSA (Italy), Rossella Gamba from Berkeley (USA), Abhishek Chowdhuri from IIT Gandhinagar (India), Luca Reali from JHU (USA), and Kostas Kritos also from JHU (USA), thanks for joining us for a bit.

July 8, 2024



Go Daria go!

Daria Gangardt has just defended her PhD thesis at the University of Birmingham. The thesis is called “Black-hole dynamics and their environments” and jumps from black-hole spins all the way to AGN discs. Daria, it has been a true pleasure working with you, all the way since your very first summer project and through your supervisor changing countries. I’m both honored and proud that you completed your PhD with me, all the best with everything. Time for drinks now! Go Dr. Daria!

July 1, 2024



Challenges and future perspectives in gravitational-wave astronomy: O4 and beyond

The workshop “Challenges and future perspectives in gravitational-wave astronomy: O4 and beyond” will take place at the Lorentz Center (Leiden, Netherlands) from October 14th to October 18th, 2024.

Our goal is to foster an interdisciplinary discussion (with astrophysicists, data analysts, and machine learners) about how current and future observations of gravitational and electromagnetic waves can be used to shed light on the physics of compact-object formation and evolution.

We encourage interested participants to apply by July 21st, 2024 at:
https://www.lorentzcenter.nl/challenges-and-future-perspectives-in-gravitational-wave-astronomy-o4-and-beyond.html

Lorentz Workshops@Oort are scientific meetings for small groups of up to 55 participants, including both senior and junior scientists. We will dedicate a considerable amount of time to discussion sessions, thus stimulating an interactive atmosphere and encouraging collaboration between participants. The venue Lorentz Center@Oort is located at the Faculty of Science campus of Leiden University, the Netherlands. The Lorentz Center provides each participant with office space as well as various practical services such as arranging accommodations at the nearby hotel Van der Valk Hotel Leiden/Tulip Inn Leiden at a special rate, visa assistance, and bike rental. For more information see: www.lorentzcenter.nl

SOC: Fabio Antonini (chair), Maya Fishbach, Davide Gerosa, Laura Nuttall, Rosalba Perna, Simon Portegies Zwart.

June 23, 2024



One population fit to rule them all

Three fits for the non-parametric under data sky,
Seven for the astrophysicists in their clusters of stars,
Nine for powerlaw+peaks doomed to die,
One for the sampler on his python throne
In the land of LIGO where the data lie.
One population fit to rule them all
One population fit to find them features
One population fit to Bayes them all, and in the stats bind them. 
In the land of LIGO where the data lie.

May 27, 2024



Residual eccentricity as a systematic uncertainty on the formation channels of binary black holes

The orbits of binary black holes could be eccentric, but in practice they’re not. At least when we observe them, and that’s because of a relativistic effect that circularizes the orbit. Even if astrophysics formed black holes eccentric, relativity makes them circular when we observe them with gravitational-wave interferometers. But we’re interested in the astrophysics back then! What we find here is that the tiny residual eccentricity at detection can be crucial. Even eccentricities that are so small that we cannot tell them apart from circular can mess up the astrophysical inference. Unfortunately, this is a new systematic error that needs to be taken into account: inferring the “formation channel” of binary black holes might be even harder than we thought.

Giulia Fumagalli, Isobel Romero-Shaw, Davide Gerosa, Viola De Renzis, Konstantinos Kritos, Aleksandra Olejak.
Physical Review D 110 (2024) 063012.
arXiv:2405.14945 [astro-ph.HE].

May 23, 2024



Gravitational-wave snowballs, populations, and models

We are organizing “Gravitational-wave snowballs, populations, and models” — a workshop to be held in Sexten, in the Dolomites region of Italy, January 20-24, 2025:
https://sites.google.com/unimib.it/gwsnowballs

Our goal is to bring together researchers at the forefront of both forward astrophysical modeling of compact object binary formation and gravitational-wave data analysis in preparation for the upcoming O4 data release of LIGO/Virgo, for discussions focused on population-level modeling and inference.

The meeting will be held at Bad Moos Hotel right next to the ski slopes and the conference program will have appropriate breaks for snow activities; more details are available at
https://sites.google.com/unimib.it/gwsnowballs/logistics

We hope you will consider applying to participate. Space is limited to 40 people. Please apply online at
https://sites.google.com/unimib.it/gwsnowballs/registration
by July 15, 2024.  We plan to notify accepted participants by the end of July.

Ilya Mandel
Davide Gerosa
Salvatore Vitale

April 29, 2024



Quick recipes for gravitational-wave selection effects

… and we’re back to selection effects. That means modeling what you cannot see. The black holes that gravitational-wave detectors observe are not representative of those that are out there in the Universe. Some are easier to see, some are harder. Quantifying how much easier and harder is crucial to properly understand the underlying astrophysics. In this paper (which came out of a BSc student project!), we go back to the basics and work out gravitational-wave selection effects one step after the other, using and refining the most common approximation. Two things to remember: including noise fluctuations is easy, and a signal-to-noise ratio threshold of 11 is probably ok.

Davide Gerosa, Malvina Bellotti.
Classical and Quantum Gravity 41 (2024) 125002.
arXiv:2404.16930 [astro-ph.HE].

April 18, 2024



Linking Advances in our Understanding of Theoretical Astrophysics and Relativity to Observations (LAUTARO)

This week we’re hosting researchers from the Gran Sasso Science Institute (GSSI) for a joint mini-conference / workshop / group meeting. More here:

davidegerosa.com/lautaro

This is part of a PRIN grant we have together (thanks Italy) with support from other grants as well (thanks Europe). The meeting has the best title ever (that was actually my idea…), the best logo ever (that was Giulia’s idea), and the best organization ever (huge thanks Costantino and Sara!).

April 16, 2024



Classifying binary black holes from Population III stars with the Einstein Telescope: a machine-learning approach

Population 3 stars are like “the original” stars. Those formed with material that comes straight from the Big Bang. It would be very (like, a lot!) cool to see them with gravitational-wave detectors. But can we tell them apart? Or do they look like all the other stars? Here is an attempt with a fancy machine-learning classifier.

Filippo Santoliquido, Ulyana Dupletsa, Jacopo Tissino, Marica Branchesi, Francesco Iacovelli, Giuliano Iorio, Michela Mapelli, Davide Gerosa, Jan Harms, Mario Pasquato.
arXiv:2404.10048 [astro-ph.HE].

April 12, 2024



PhDs with us! 2024 admissions

The University of Milano-Bicocca welcomes applications for PhD scholarships. This year’s application deadline is May 14th, 2024 (noon CEST) for positions starting in the Fall of 2024:

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

In particular, we are looking for highly motivated candidates to join our activities in black-hole binary dynamics and gravitational-wave data exploitation. Milano-Bicocca hosts a large group in gravitational-wave physics, covering activities ranging from astrophysical/numerical modeling to data analysis. The group counts 7 faculty members (Bortolas, Colpi, Dotti, Gerosa, Giacomazzo, Sesana, and an upcoming new hire) together with several postdocs (of which two prize fellows) and PhD students. Candidates will also have ample opportunities to work with and visit external collaborators.

Our PhD admission program includes several “open” scholarships, covering all research activities in the department (including ours!). All candidates are considered for those by default. In addition, we are advertising an additional “project” scholarship titled “Gravitational-wave source modeling” which will be supervised by Prof. Davide Gerosa. Candidates wishing to be considered for this opportunity should indicate it explicitly when applying (the number of this position FIS.8). For more information on Gerosa’s group see www.davidegerosa.com/group

We strive to build an inclusive group and welcome applications from all interested candidates. For informal inquiries, expressions of interest, and application tips please do not hesitate to contact [email protected]

March 23, 2024



Three more

Three more students graduated in March with research projects completed in our group!

  • Alessandro Pedrotti defended his MSc thesis working with Michele Mancarella on gravitational-wave cosmology going from crazy calculations to fun correlations and all the way to Einstein Telescope! Alessandro is now moving on with his career with a research placement at the University of Aix-Marseille. Congrats!
  • Annalisa Amigoni completed a BSc project with Ssohrab: more fun with 3g detectors…
  • Alice Palladino also completed a BSc project; she worked with Viola and me on a strange and mind-twisting “ordering” problem using the LIGO posterior (how many times did we get confused on this!)

March 6, 2024



Teaching for the new AI degree

On top of “astrostats” for the MSc degree in Astrophysics, this semester I’m excited to start teaching for the new BSc degree in Artificial Intelligence. This course is delivered jointly by the University of Milano-Bicocca (my place), the University of Milano-Statale (“the other” uni in town), and the University of Pavia (south of here…). My class is actually a lab, the full (too long) title is “Laboratory of Machine Learning Applied to Physical Systems.” The class material is available here:

github.com/dgerosa/machinelearning4physics_bicocca_2024

Can’t wait to see what these AI students can do! Hope to learn from you as much as you learn from me.

March 4, 2024



pAGN: the one-stop solution for AGN disc modeling

And the second paper on the arxiv today is Daria’s masterpiece! pAGN (which Daria says you should read “pagan”) is a brand new, super cool code that implements the hydrodynamics of AGN disks, at least in their most popular one-dimensional fashion. Those solutions have been around for a long time but their details were, well, let’s say unclear. Daria went through everything from beginning to end, coming up with the “one-stop solution for your AGN disc needs” (that was actually the working title of the paper…). So pip install pAGN and have fun.

Daria Gangardt, Alessandro Alberto Trani, Clément Bonnerot, Davide Gerosa.
Monthly Notices of the Royal Astronomical Society, 530 (2024) 3986–3997.
arXiv:2403.00060 [astro-ph.HE].

March 4, 2024



Probing AGN jet precession with LISA.

This is the first of two papers on the arxiv today: it’s fun when two long, very different projects by different people just happen to be done on the same day! This paper is by my former colleague Nate Steinle (now a postdoc in Manitoba, Canada). Here we connect the dynamics of jets in AGN disks to the spin of black holes observable by LISA. And show the latter is a diagnostic of the former! And it’s nice to see my disk-binary code being used for something I didn’t think of when I wrote it.

Nathan Steinle, Davide Gerosa, Martin G. H. Krause
arXiv:2403.00066 [astro-ph.HE].

February 28, 2024



Primordial black holes by Lisa (not LISA…)

Our student Lisa Merlo defended her BSc 3rd year project today! Lisa worked with Pippa Cole and me on computing rates for mergers of primordial black holes, also considering a new detector prototype that the experimental group here is developing (nickname BAUSCIA, from the Milan dialect). Short answer: the rate is low but now is more accurately low. Lisa’s presentation was amazing and working with her has been a real pleasure. Stay tuned for her future astro career!

February 21, 2024



We got (another!) Marie Curie Fellowship!

Huge huge congrats to Zacharias Roupas who was awarded a Marie Curie Fellowship with us! Zachos is currently based at the British University in Egypt and will be joining my group in Milan in the Fall of 2024. The Marie Curie Fellowship program is a prestigious postdoctoral scheme operating at the EU level and, together with Arianna, we’ll now have two Marie Curie grantees in the group. Zachos’ winning proposal is titled “Black hole spin and mass function in gaseous proto-clusters” (nickname: protoBH).

February 16, 2024



Astrophysical and relativistic modeling of the recoiling black-hole candidate in quasar 3C 186

Not sure what happened here, how the hell did I end up writing a paper with actual radio data that needed to be reduced… Call me an ambulance.

The guy here is 3C186 which is not a postcode but a quasar. A funny one because it’s not centered on the galaxy (it’s a bit off) and it’s also going at another velocity (ciao ciao). One of the leading explanations is that 3C186 is a recoiling black hole, the remnant of black-hole merger is being kicked away (yeah these things can happen). 3C186 also has a radio jet, and that should point in the direction of the black-hole spin. The funny thing is that spin and the kick appear perpendicular to each other, and this is fun because theory says they should actually be parallel. We looked into this a bit carefully and discovered it’s all a lie! The spin and the kick both point along the line of sight and appear perpendicular only because of a super strong projection effect. If this is true, the radio jet should also point straight to us! We then tried to test this with whatever ratio data we could grab (where is that ambulance) and found that… mmh, well, it’s a maybe.

Matteo Boschini, Davide Gerosa, Om Sharan Salafia, Massimo Dotti.
Astronomy & Astrophysics 686 (2024) A245.
arXiv:2402.08740 [astro-ph.GA].

January 23, 2024



Tenured professorship in Astrophysics at the University of Milano-Bicocca (Italy)

The University of Milano-Bicocca (Milan, Italy) will be opening a tenured professorship in astrophysics, with a focus on gravitational-wave data analysis and exploitation. With this notice, we invite expressions of interest from potential candidates.

Milano-Bicocca hosts a large group in gravitational astronomy, with activities covering all bands of the gravitational-wave spectrum and the related experiments (LIGO/Virgo, LISA, ET, PTA). Faculty members with matching interests include Bortolas, Colpi, Dotti, Gerosa, Giacomazzo, and Sesana. The group hosts two large ERC grants and currently counts about 10 PhD students and 15 postdocs. We are part of a wider astrophysics unit at Milano-Bicocca (with activities in large-scale structures and experimental cosmology) as well as a large Physics department with ~70 faculty members.

We are targeting the opening of a faculty position on a timescale of a few months, with a prospective starting date in the early fall of 2024. Onboarding will be at the associate professor level (“professore associato” in the Italian system), which is a tenured appointment. Formal application requirements include holding either the Italian national habilitation (ASN) or a comparable position abroad for at least 3 years. We are happy to assist potential candidates with their ASN application.

Current strategic interests include the development of gravitational-wave data-analysis pipelines for the LISA space mission. At the same time, we are open to all strong candidates willing to bring their ambitious research programs in relativistic astrophysics and/or gravitational-wave astronomy to Milan.

Interested applicants are encouraged to send their CVs and a short cover letter to [email protected] by February 15th, 2024. The CV should include the names and email addresses of three referees who might be approached for references.

January 17, 2024



In memory of Chris Belczynski

Hey Chris, just wanted to say thanks because you wanted to understand what was going on, for that ski run down the Highland Bowl in Aspen, for sending me yet another version of those StarTrack files I had to postprocess, for those obscure code comments in Polish, for that last chat in Japan last month (I’ll finish that calculation about tides we sketched at the board!), and for the energy. I’m sure you’re on a beautiful mountain.

mykeeper.com/profile/KrzysztofBelczynski

December 31, 2023



2023 Wrapped!

Much like Spotify, here is our group “Wrapped”, 2023 edition!

Some of the group highlights include…  We welcomed Pippa, Nick, Arianna, Sshorab, and Matteo. We said bye to Matt who moved to MIT and Nate who moved to Canada, while Daria remains our UK stronghold. Michele got a faculty job, Viola got a postdoc, Davide got a PRIN grant, and Giulia got a SigmaXi grant. We graduated something like 12 BSc students and 4 MSc students (and all 4 of them now have PhD positions). A few long-term visitors (Francesco, Giulia, Harrison) made the group even better for a while. We wrote lots of papers, gave lots of talks, and ate lots of cakes. LIGO is taking data, LISA is being adopted, Virgo has seen better days, and GR is still true. Arianna was in the newspaper, Sshorab broke Davide’s ribs, Alice danced Greek dances, and Costantino got his first American coffee ever. Our gwpopnext conference was a blast and we discussed too much, thunderstorms included.

… now get ready for all the 2024 surprises!

November 22, 2023



Calibrating signal-to-noise ratio detection thresholds using gravitational-wave catalogs

In the gravitational-wave world, we usually say a binary merger is detected if it has a sufficiently large SNR (signal-to-noise ratio). But is that true? Detection pipelines are far more complicated than that. Here we try to figure out a section threshold from what’s detected. That is: (some) people agree that these guys are GWs, so what’s your SNR threshold for detectability? It’s like reading in the minds of a GW data analyst…

Matthew Mould, Christopher J. Moore, Davide Gerosa.
Physical Review D 109 (2023) 063013.
arXiv:2311.12117 [gr-qc].

November 21, 2023



Starting a new class! “Scientific computing” for PhD students

I’m teaching the first lecture of a new class today. This is “Scientific computing with Python,” a 16h module for PhD students. To the (many) PhD students who signed up: thanks for your interest, hope you’ll like this. BTW the title says Python but there will also be some Mathematica and some git, just for fun. My material is online at

github.com/dgerosa/scientificcomputing_bicocca_2023

Have a look if you want and please do give feedback if you do 🙂

November 20, 2023



November graduations: 4 Bsc projects with us

We had another graduation session in November, and a whopping 4 people graduated with research projects in our group. Here are the new BSc physicists who just defended:

  • Matteo Falcone worked with the other Matteo (Boschini) on a simulation design strategy for machine learning;
  • Serena Caslini worked with Nick on a new strategy to classify burst GW signals;
  • Marco “104” Bianchi worked with Giulia and put together a neural network for black-hole binary spin precession using his gaming GPU 🙂
  • Martin “Top” Gerini was supervised by Alice on supermassive black holes, LISA, and glitches.

Congrats all (and twice congrats to Marco and Serena, who graduated with full marks and honors). It was great working with you. Matteo and Martin are now enrolled in an MSc degree in Artificial Intelligence (good luck!), while Marco and Serena are starting our MSc degree in Astrophysics.

October 31, 2023



Spin-eccentricity interplay in merging binary black holes

I’m obsessed with spinning black-hole binaries but, guys, spinning and eccentric black holes are even better! This is the first first-author paper by Giulia, who is not only a rising GW astronomer but also a semi-professional baker… So take two spoons of black holes, one spoon of spin dynamics, some eccentricity (but less than 0.6 ounces), and a pinch of maths. Put this in a bowl, mix it thoroughly with numerical integrations …and the result is very tasty! Spins and eccentricity shape the dynamics of black-hole binaries together, which means one can hope to measure eccentricity indirectly from the spins, but also that if you forget about eccentricity then your spin inference will be crap. Buon appetito.

Giulia Fumagalli, Davide Gerosa.
Physical Review D 108 (2023) 124055.
arXiv:2310.16893 [gr-qc].

October 28, 2023



2 Masters + 2 Undegrads

We’ve had four amazing research students graduating with us in October!

  • Alessandro Santini defended his MSc project, which was actually completed in part at Johns Hopkins University (USA) with R. Cotesta and E. Berti. Alessandro worked our a possible astrophysical model to explain the mass-spin correlation observed by LIGO. We’ve published this already! Alessandro is moving on with a PhD at the AEI in Potsdam, Germany.
  • Francesco Nobili also got his MSc degree. His project was completed with S. Baghwat at the University of Birmingham as is about fitting ringdown amplitudes. I discovered other students call him “Brock” from the Pokemon character, so I started doing the same… Brock is starting a PhD in computational astrophysics at the University of Insubria in Como, Italy.
  • Federico Ravelli. Completed a shorter BSc project with Viola De Renzis on spin effects in LIGO/Virgo data…
  • … and Simone Sferlazzo also got his Physics BSc degree. Simone worked with Michele Mancarella on “the use and abuse” (cit.) of Fisher matrices in GWs.

After the Master’s defenses, students turned the graduation party into a football supporter thing, with chants and all the rest!

October 14, 2023



Arianna on a national newspaper

Arianna Renzini (Marie Curie Fellow in my group) is on the Italian national newspaper “Corriere della Sera” today!

Here is the online version: Arianna Renzini, la studiosa di Astrofisica corteggiata da tutto il mondo: «Basta cervelli in fuga, lavorerò a Milano»

But it was also on the printed thing distributed everywhere!

(That picture was taken in my office… So my office is also in the national newspaper today! Wooo!)

October 11, 2023



Catalog variance of testing general relativity with gravitational-wave data

…and we’re back to testing GR. We’ve got many gravitational-wave events and would like to use them all together to figure out if our equations for gravity are correct. And here is the issue: there’s only one set (aka catalog) of black holes that contains all the black holes we’ve observed. Now that’s obvious you’d say, and you would be right!, much like we have a single Universe to observe (I’m not a language guy but indeed “Universe” means like “the whole thing”). This effect is known in cosmology (think those low-order multiples in the usual CMB plot), so we called it “the catalog variance of testing GR”. It’s bad, but the Baron Munchauseen tells us we can bootstrap.

Costantino Pacilio, Davide Gerosa, Swetha Bhagwat.
Physical Review D Letters 109 (2024) L081302.
arXiv:2310.03811 [gr-qc].

October 6, 2023



More people, more topics, more fun

Our group is getting some tremendous additions, with 5 people joining in the fall of 2023! The scope of our research is getting broader and broader 🙂

  • Pippa Cole is joining us as a postdoc from Amsterdam and she’s going to teach us fun things about dark matter, environmental effects on GW measurements, primordial black-holes etc.
  • Ssohrab Borhanian is also coming in as a postdoc (from Jena, Germany and Penn State before that), with all you can ever hope to know about 3G detectors.
  • Nick Loutrel is a new postdoc from Rome (and Princeton before, and Montana before) which strengthens the analytical / modeling side of the group.
  • Arianna Renzini is coming as a postdoc from sunny Caltech with her own Marie Curie Fellowship, ready to make a splash with stochastic gravitational-wave backgrounds!
  • Matteo Boschini is a new PhD student, after a successful MSc degree with a cool project on numerical-relativity surrogate models.

We’re soon going to have Giulia Capurri who will be visiting us for a few months from Trieste. Welcome aboard all! There are like 13 people at group meetings now…

September 20, 2023



Students going for 3G, ringdowns, and selection effects

Three of our BSc students graduated today.

  • Ludovica Carbone worked with Michele Mancarella and Francesco Iacovellie and has some nice forecasts for 3G detectors.
  • Riccardo Bosoni de Martini was supervised by Costantino Pacilio and checked super carefully their Fisher code for ringdowns.
  • Malvina Bellotti (who, I’m very envious, is from Cortina in the mountains!) worked with me on selection effects for GW surveys.

And, last but not least, let me add Simone Piscitelli, who last week defended his MSc degree at Milano Statale (“the other” University of Milan) supervised by Costantino Pacilio and myself. Simone worked on a cool test of GR. Stay tuned…

Congrats all!

September 17, 2023



Postdoc positions 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, with additional support from national grants. 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, as well as applications of statistical and machine-learning tools to gravity (although we are open to all candidates with a strong gravitational-wave and/or high-energy astrophysics background!). Candidates will have ample opportunities to kickstart new projects with group members and will be strongly encouraged to develop their own independent research lines.

We anticipate awarding up to three positions. Appointments will be for 2+1 years and come with a generous research and travel budget. The starting date is negotiable.

The astrophysics unit 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 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 15th, 2023 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 15th, 2023.

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]

August 28, 2023



Black-hole mergers in disk-like environments could explain the observed 𝑞−𝜒eff correlation

Gravitational-wave data keep on giving us surprises. The most outstanding one IMO is an observed correlation between mass ratios and spins of the black holes, which was first found by Tom Callister and friends. That is so, so weird… to the point that virtually zero astrophysical models so far can explain it fully and consistently. Well, we can’t either (at least not fully and consistently) but we think this paper is a nice attempt. The secret seems to be the symmetry of the astrophysical environment one considers, and data tends to prefer black holes assembled in cylindrical symmetry. That’s also weird to be honest, but there’s a candidate for this setup, namely accretion disks and their migration traps. Who knows, more data will tell.

… and huge congrats to my MSc student Alessandro who managed to publish a paper even before graduating!

Alessandro Santini, Davide Gerosa, Roberto Cotesta, Emanuele Berti
arXiv:2308.12998 [astro-ph.HE].
Physical Review D 108 (2023) 083033.
Other press coverage: astrobites.

July 19, 2023



New July physicists

Two students just completed their Bachelor’s degree with research projects in our group.

  • Leonardo Toti worked with myself and Giulia Fumagalli on exploring black-hole merger trees in dense clusters.
  • Simone Restuccia worked with Costantino Pacilio on applying dimensionality-reduction techniques to black-hole ringdowns.

I had the honor of heading their graduation committee and could call them “physicists” for the very first time (and the Italian ceremonial sentence is quite imposing: “coi poteri conferitami…“). Congrats Simone and Leonardo!

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]).