A calibrated dark-siren tension with General-Relativity propagation

The Hubble constant tension reflects a persistent mismatch between early-Universe and late-Universe inferences of cosmic expansion. Gravitational-wave “dark sirens” offer a complementary route to late-time inference because they carry an absolute distance scale without requiring an electromagnetic counterpart. Here we ask a sharper question than standard distance–redshift fitting: does the observed GWTC-3 catalogue prefer a non-General-Relativity propagation law in which gravitational-wave amplitudes decay differently with redshift? We report a calibrated propagation anomaly in 36 GWTC-3 dark sirens. Relative to an internal GR baseline, a fixed modified-propagation history yields a joint predictive-score shift of ∆LPDtot = +3.670 (a Bayes-factor proxy of exp(∆LPD) ≈ 39 in this fixed scoring framework), while 512 matched GR-consistent injection catalogues never approach the observed scale. If physical, this preference implies that GR-locked distance inference can act as a hidden systematic in late-time cosmology and must be tested as seriously as conventional selection and calibration effects.