The natal spins of binary black holes after LIGO/Virgo’s O3a observing run

In October 2019, the Laser Interferometer Gravitational Wave Observatory (LIGO) and Virgo finished the first part of their third observing run (O3a). From the host of new binary black hole (BBH) detections in O3a, we have learned a great deal about both individual and ensemble properties of BBH spins. Drawing on the forthcoming O3a data, we summarize the latest measurements of BBH spins and discuss their astrophysical implications for natal spins and binary formation channels. In particular, it is commonly predicted that black holes in isolated BBH systems have vanishingly small natal spins, with all spin present due to tidal torques; we test the compatibility of the O3a data with this model.