LIGO's latest run tightens the leash on primordial black holes

Combining the first part of LIGO-Virgo-KAGRA's fourth observing run with detailed models of how primordial black hole (PBH) binaries form and merge, this analysis constrains PBH abundance across 0.6–100 solar masses more tightly than any previous gravitational-wave study. Individual resolvable merger events drive the constraints, while the stochastic gravitational-wave background adds only secondary limits. Allowing some cataloged events to be PBH mergers softens the bounds slightly, but a joint fit with astrophysical black holes finds no statistical evidence that any observed events require a primordial origin.