Binary black holes formed via different pathways are predicted to have distinct spin properties. Measuring these properties with gravitational waves provides an opportunity to unveil the origins of binary black holes. Recent work draws conflicting conclusions regarding the spin distribution observed by LIGO-Virgo-KAGRA (LVK). Some analyses suggest that a fraction of the observed black-hole spin vectors are significantly misaligned (by $>90°$) relative to the orbital angular momentum. This has been interpreted to mean that some binaries in the LVK dataset are assembled dynamically in dense stellar environments. Other analyses find support for a subpopulation of binaries with negligible spin and no evidence for significantly misaligned spin—a result consistent with the field formation scenario. In this work, we study the spin properties of binary black holes in the third LVK gravitational-wave transient catalog. We find that there is insufficient data to resolve the existence of a subpopulation of binaries with negligible black-hole spin (the presence of this subpopulation is supported by a modest Bayes factor of 1.7). We find modest support for the existence of mergers with extreme spin tilt angles $>90°$ (the presence of extreme-tilt binaries is favored by a Bayes factor of 10.1). Only one thing is clear based on gravitational-wave measurements of black hole spin: at least some of the LVK binaries formed in the field. At most 89% of binaries are assembled dynamically (99% credibility), though, the true branching fraction could be much lower, even negligible.

• Received 6 September 2022
• Accepted 12 October 2022

DOI:https://doi.org/10.1103/PhysRevD.106.103019