Do both black holes spin in merging binaries? Evidence from GWTC-4 and astrophysical implications

Angular momentum transport in high-mass stars is commonly modeled by extrapolating the behavior of better-observed low-mass stars. According to the conventional picture, the cores of most black hole progenitors lose almost all of their angular momentum when their outer layers are ejected before core collapse. Accordingly, most black holes are expected to be born with dimensionless spin magnitudes of $\chi \lesssim 0.01$, even if some black holes are born with non-negligible spin due to tidal interactions in a progenitor binary. One might therefore expect to find a large fraction of $\chi \lesssim 0.01$ black holes in merging binary black hole (BBH) systems. We find that the conventional picture of angular momentum transport is in tension with data from LIGO--Virgo--KAGRA's fourth gravitational-wave transient catalog. We find no support for a sub-population of BBH systems with $\chi \lesssim 0.01$. Neither do we find support for a sub-population with only one spinning black hole as expected for tidal spin-up scenarios. Instead, we find evidence for two subpopulations in which both black holes have non-negligible spin. Approximately 84% of BBH systems contain two black holes with modest spins $\chi \approx 0.1$ and approximately 16% contain two black holes with large spins $\chi \approx 0.8$. These estimates come from our best-fit model, which is favored with natural log Bayes factors $\ln B \gtrsim 3$ over models that require a sub-population of $\chi \lesssim 0.01$ black holes, and models that do not contain multiple spin sub-populations. These results are difficult to reconcile with our current understanding of angular momentum transport.