Secondary-mass Features Improve Spectral-siren H 0 Constraints

This is the authors' abstract. We don't add key points for this paper.

Gravitational-wave (GW) signals from compact binary coalescences (CBCs) enable independent measurements of the Hubble constant H 0 via the spectral siren method, which critically depends on an accurate model of the source-frame mass distribution. While the primary-mass function has been extensively studied, the impact of the secondary-mass distribution on cosmological inference has been largely overlooked. Here, we perform a joint inference of population and cosmological parameters using 142 confident CBC detections from GWTC-4.0, adopting a new parametric model that flexibly describes features in both the component-mass spectrum and the pairing function, with particular emphasis on the secondary masses. We find H 0 = 71 . 4 − 13.4 + 13.8 km s − 1 Mpc − 1 (68% CL) from spectral sirens alone, and H 0 = 73 . 5 − 7.2 + 9.2 km s − 1 Mpc − 1 when combined with the bright siren GW170817. Compared to the standard LIGO–Virgo–KAGRA Collaboration Fullpop-4.0 analysis, these constraints represent improvements of ∼29.8% and ∼22.2% in H 0 uncertainty, respectively. The enhanced precision is driven by previously unmodeled features, including peaks near 18 M ⊙ and 65 M ⊙ as well as mass-dependent pairing transitions at 28 M ⊙ and 52 M ⊙ . Our results demonstrate that the secondary-mass function is also a key ingredient for precision standard-siren cosmology.