Hubble Constant Measurement with GW190521 as an Eccentric Black Hole Merger

Abstract: Gravitational wave observations can be used to accurately measure the Hubble constant $H_0$ and could help understand the present discrepancy between constraints from Type Ia supernovae and the cosmic microwave background. Neutron star mergers are primarily used for this purpose as their electromagnetic emission can be used to greatly reduce measurement uncertainties. Here we estimate $H_0$ using the recently observed black hole merger GW190521 and its candidate electromagnetic counterpart found by ZTF using a highly eccentric explanation of the properties of GW190521. We find that the reconstructed distance of GW190521 and the redshift of the candidate host galaxy are more consistent with standard cosmology for our eccentric model than if we reconstruct the source parameters assuming no eccentricity. We obtain $H_0=88.6^{+17.1}_{-34.3}$\,km\,s$^{-1}$Mpc$^{-1}$ for GW190521, and $H_0=73.4^{+6.9}_{-10.7}$\,km\,s$^{-1}$Mpc$^{-1}$ in combination with the results of the neutron star merger GW170817. Our results indicate that future $H_0$ computations using black hole mergers will need to account for possible eccentricity. For extreme cases, the orbital velocity of binaries in AGN disks can represent a significant systematic uncertainty.