GWTC-4.0: population properties of merging compact binaries

We detail the population properties of binary neutron star, neutron star–black hole binary, and binary black hole mergers using 158 events from the cumulative Gravitational-Wave Transient Catalog 4.0. The black hole primary mass distribution consists of a power-law-like continuum that steepens above 35 M_⊙ with overdensities at 10 M_⊙ and 35 M_⊙. Binary black holes with primary masses near 10 M_⊙ are more likely to have less massive secondaries, with a mass ratio distribution peaking at q=0.71^+0.12_-0.11 , potentially a signature of stable mass transfer during binary evolution. Black hole spins are inferred to be nonextremal, with 90% of black holes having χ < 0.6, and preferentially aligned with binary orbits, implying many merging binaries form in isolation. However, we find that a significant fraction, 0.23–0.41, of binaries have negative effective inspiral spins, suggesting many could be formed dynamically in gas-free environments. We find evidence for correlation between effective inspiral spin and mass ratio, driven either by variation in the mode of the distribution or the width. The binary black hole merger rate increases with redshift, consistent with the cosmic star formation density. While there is no evidence of the mass spectrum evolving with redshift, the distribution of effective inspiral spin is found to broaden as redshift increases out to z ≈ 1. We infer the local merger rates (at z = 0) to be 8.8–250 Gpc⁻³ yr⁻¹ for binary neutron stars, 9.3–86 Gpc⁻³ yr⁻¹ for neutron star–black hole binaries, and 13–26 Gpc⁻³ yr⁻¹ for binary black holes; all values reflect central 90% credible intervals.