Where do black holes come from? Sounds like a scify book title, but it’s real. These days, that’s actually the million dollar question in gravitational-wave astronomy. LIGO sees (lots of!) black holes in binaries, and those data encode information on how their stellar progenitors behave, what they like or did not like to do. This is paper is the latest attempt to understand if black holes formed alone (i.e. a single binary star forms a single binary black hole) or together (i.e. many stars exchange pairs in dense stellar environments).

Yann Bouffanais, Michela Mapelli, Davide Gerosa, Ugo N. Di Carlo, Nicola Giacobbo, Emanuele Berti, Vishal Baibhav.
Astrophysical Journal, 886 (2019) 25.
arXiv:1905.11054 [astro-ph.HE].