APJ

GWTC-4, the new amazing data release by LIGO/Virgo/KAGRA, just came out, and we of course started looking into it immediately. That was actually the perfect playground for a new approach we were developing for quite some time, namely using Delaunay triangulation to model black holes. This is part of a long research trend of developing “non-parametric” methods for gravitational-wave populations, where the goal is to fit the data without imposing a particular functional form. Ok, with new data and a new model, we looked at the correlation between masses and redshifts in merging black holes. You need to read the paper now, but the key result here is that the mass distribution does not have a peak at low redshifts.

R. Tenorio, A. Toubiana, T. Bruel, D. Gerosa, J. Gair.
Astrophysical Journal 994 (2025) L52. arXiv:2509.19466 [astro-ph.HE].

This paper came out of some discussions from our “Gravitational-wave snowballs” workshop in Sexten (Italy). We were discussing the good old problem of separating black-hole binary formation channels with spin measurements. Usually one says “aligned=isolated”, “isotropic=dynamical”. But then, some binaries that formed dynamically should also be eccentric. What we then realized is that, for those eccentric binaries and only for those, spin measurements can actually tell which of the dynamical channel (because there are many…) is at play.

J. Stegmann, D. Gerosa, I. Romero-Shaw, G. Fumagalli, H. Tagawa, L. Zwick.
Astrophysical Journal 994 (2025) L47. arXiv:2505.13589 [astro-ph.HE].

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

D. Gerosa, N. Giacobbo, A. Vecchio.
Astrophysical Journal 915 (2021) 56. arXiv:2104.11247 [astro-ph.HE].

The LISA data analysis problem is going to be massive: tons of simultaneous sources all together at the same time. In Birmingham we are developing a new scheme to tackle the problem, and here are the first outcomes. We populate satellite galaxies of the Milky Way with double white dwarfs and show that LISA… can actually do it! LISA will detect these guys, tell us which galaxies they come from, etc. It might even discover new small galaxies orbiting the Milky Way! Surprise, surprise, LISA is going to be amazing…

ps. Here is the first half of the story.

ps2. The code still needs a name. Suggestions?

E. Roebber, R. Buscicchio, A. Vecchio, C. J. Moore, A. Klein, V. Korol, S. Toonen, D. Gerosa, J. Goldstein, S. M. Gaebel, T. E. Woods.
Astrophysical Journal 894 (2020) L15. arXiv:2002.10465 [astro-ph.GA].