Challenges for the Statistical Gravitational-Wave Method to Measure the Hubble Constant

Abstract

Gravitational waves (GW) can be employed as standard sirens that will soon measure the Hubble constant with sufficient precision to weigh in on the $\sim 5\sigma$ Hubble tension. Most GW sources will have no identified electromagnetic counterpart, leading to uncertainty in the redshift of the source, and in turn a degeneracy between host distance, redshift, and $H_0$. In the case where no electromagnetic counterparts are identified, it has been proposed that a statistical canvassing of candidate GW hosts, found in a large survey for example, can be used to accurately constrain the Hubble constant. We study and simulate this galaxy voting method to compute $H_0$. We find that the Hubble constant posterior is in general biased relative to the true value even when making optimistic assumptions about the statistical properties of the sample. This bias is caused by the fundamental degeneracy between redshift and $H_0$, and is effectively irreducible without accurate information about the redshift distribution from which the GW sources come.