Prospects for Measuring the Hubble Constant with Neutron-Star–Black-Hole Mergers

Gravitational wave (GW) and electromagnetic (EM) observations of neutron-star-black-hole (NSBH) mergers can provide precise local measurements of the Hubble constant ($H_0$), ideal for resolving the current $H_0$ tension. We perform end-to-end analyses of realistic populations of simulated NSBHs, incorporating both GW and EM selection for the first time. We show that NSBHs could achieve unbiased 1.5%–2.4% precision $H_0$ estimates by 2030. The achievable precision is strongly affected by the details of spin precession and tidal disruption, highlighting the need for improved modeling of NSBH mergers.

Figure 1

Distributions of a subset of parameters from our SEOBNR (top) and IMRPhenom (bottom) samples, as drawn from the prior (dotted), selected by GW SNR (dashed) and selected by GW and EM emission (colored histograms). The bins are colored by the fractional $H_0$ uncertainty the mergers within the bin achieve: the yellowest and lightest bins are most informative.

Figure 2

Left three panels: distance and inclination posteriors for a selection of mergers, simulated and sampled using the IMRPhenom waveform with precessing (gray filled) and aligned (dark red dashed) spins, and using SEOBNR with aligned spins (red). The selection includes the highest-SNR merger common to both catalogs (left) and the IMRPhenom merger whose BH spin is closest to being aligned (second from right). Right: distributions of fractional uncertainties on luminosity distance (dotted) and $H_0$ (solid) from individual mergers from our IMRPhenom (gray) and SEOBNR (red) NSBH catalogs.