Black hole demography in the era of gravitational-wave astronomy
The era of gravitational-wave astronomy has just begun and the formation channels of binary black holes (BBHs) are one of its most debated questions. In my talk, I will discuss what we learned and what we still do not understand about the formation of black holes from the collapse of massive stars, focusing on the impact of pair instability and stellar rotation. Based on novel population-synthesis calculations, I will show that massive binary stars can evolve into BBHs with total mass up to ~130 Msun, but only BBHs with total mass <~90 Msun coalesce within a Hubble time, unless dynamics comes into play. Massive stars that form in dense star clusters undergo several close encounters, and so do the black holes formed from these stars: dynamical exchanges in young star clusters can trigger exotic mergers of BBHs with large total mass (>90 Msun) and with small mass ratios (<0.5). Finally, I will discuss the evolution of BBHs across cosmic time and its importance to interpret gravitational-wave observations.