Not all spins of black-hole binaries formed in clusters are isotropic and not all spins of black-hole binaries formed in isolation are aligned

We kept on saying that measuring spins with gravitational-wave data is good for black-hole binary formation channels. The key idea there is that black holes formed in isolation (“the fields”) should have spins aligned with the binary angular momentum, while binaries that formed in crowded places (“the clusters”) should instead have spins that are isotropically distributed everywhere. In this “not-all-not-all” paper by finishing MSc student Sofia, we tried to go a bit beyond this simple idea and (analytically!) work out corrections in both cases. So actually, not all spins of black-hole binaries formed in clusters are isotropic and not all spins of black-hole binaries formed in isolation are aligned.

S. Dossena, D. Gerosa, T. Bruel.
arXiv:2609.31879 [astro-ph.HE].