Bayesian analysis of the spin distribution of LIGO/Virgo black holes

Gravitational wave detection from binary black hole (BBH) inspirals has become routine thanks to the LIGO/Virgo interferometers. The nature of these black holes remains uncertain. We study here the spin distributions of LIGO/Virgo black holes from the first catalog GWTC-1 and the first four published BBH events from run O3. We compute the Bayes evidence for several independent priors: flat, isotropic, spin-aligned and anti-aligned. We find strong evidence for low spins in all of the cases, and significant evidence for small isotropic spins versus any other distribution. When considered as a homogeneous population of black holes, these results give support to the idea that LIGO/Virgo black holes are primordialJGB and ERM thank the Theory Division for their generous hospitality during their Sabbatical at CERN in 2018, when this work originated. The authors acknowledge support from the Research Project PGC2018-094773-B-C32 (MINECO-FEDER, Spain) and the Centro de Excelencia Severo Ochoa Program, Spain SEV-2016-059