Cosmology with Binary Neutron Stars: Does Mass-Redshift Correlation Matter?

TL;DR

This paper investigates whether assuming a non-evolving mass distribution for binary neutron star mergers affects cosmological parameter estimates, finding minimal bias and supporting their use as reliable spectral sirens for future gravitational wave cosmology.

Contribution

The study demonstrates that the joint mass-redshift distribution of BNS mergers can be factorized, enabling unbiased cosmological inferences even with simplified models.

Findings

Mass-redshift distribution factorization reduces bias in Hubble constant estimation.

Non-evolving BNS mass model introduces less than 0.5% bias at redshift 0.4.

BNS mergers are promising for spectral siren cosmology with next-generation detectors.

Abstract

Next-generation gravitational wave detectors are expected to detect millions of compact binary mergers across cosmological distances. The features of the mass distribution of these mergers, combined with gravitational wave distance measurements, will enable precise cosmological inferences, even without the need for electromagnetic counterparts. However, achieving accurate results requires modeling the mass spectrum, particularly considering possible redshift evolution. Binary neutron star (BNS) mergers are thought to be less influenced by changes in metallicity compared to binary black holes (BBH) or neutron star-black hole (NSBH) mergers. This stability in their mass spectrum over cosmic time reduces the chances of introducing biases in cosmological parameters caused by redshift evolution. In this study, we use the population synthesis code COMPAS to generate astrophysically motivated catalogs of BNS mergers and explore whether assuming a non-evolving BNS mass distribution with redshift could introduce biases in cosmological parameter inference. Our findings show that despite significant variations in the BNS mass distribution across binary physics assumptions and initial conditions in COMPAS, the joint mass-redshift population can be expressed as the product of the mass distribution marginalized over redshift and the redshift distribution marginalized over masses. This enables a 2% unbiased constraint on the Hubble constant-sufficient to address the Hubble tension. Additionally, we show that in the fiducial COMPAS setup, the bias from a non-evolving BNS mass model is less than 0.5% for the Hubble parameter measured at redshift 0.4. These results establish BNS mergers as strong candidates for spectral siren cosmology in the era of next-generation gravitational wave detectors.