Surprises from gravitational-wave spin measurements

Hierarchical analysis of the binary black hole (BBH) detections by the Advanced LIGO and Virgo detectors has offered an increasingly clear picture of their mass, spin, and redshift distributions. Fully understanding the formation and evolution of BBH mergers will require not just the characterization of these marginal distributions, though, but the discovery of any correlations that exist between the properties of BBHs. Here, we present two analyses performed on recent LIGO-Virgo data. First, we show that the mass ratio and the effective inspiral spins of binary black holes are anti-correlated. While many proposed astrophysical formation channels predict some degree correlation between spins and mass ratio, these predicted correlations typically act in an opposite sense to the trend we observationally identify in the data. Then, we report tentative evidence - based on the first part of the third observing run - that the BBH spin vectors are oriented in resonant configurations. This conclusion is somewhat dependent on the prior used for the analysis, implying that more data will be necessary to confirm these findings.