2021ApJ...908L..34G

Gravitational-wave observations can be used to accurately measure the Hubble constant H₀ 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 quantify the implied H₀ 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. As the electromagnetic association is currently uncertain, our main goal here is to determine the effect of eccentricity on the estimated H₀. We obtain H₀=68.8_–25.5^+45.7 km s^–1 Mpc^–1. Our results indicate that future H₀ computations using black hole mergers will need to account for possible eccentricity. For extreme cases, the orbital velocity of binaries in active galactic nucleus disks can represent a significant systematic uncertainty.