Do evolving black hole masses mess up Hubble constant measurements?

Spectral sirens infer the Hubble constant by combining gravitational-wave distance measurements with the population properties of merging black holes—using their mass distribution as a cosmic ruler. The team checked whether allowing black hole masses to evolve with redshift corrupts this measurement using 94 black hole mergers from GWTC-4.0. They found no significant evidence for such evolution, and any systematic error from ignoring it stays smaller than uncertainties from other population assumptions. Future detectors may change this story.