A Hubble constant measurement from superluminal motion of the jet in GW170817
Abstract
The Hubble constant (H0) measures the current expansion rate of the Universe, and plays a fundamental role in cosmology. Tremendous effort has been dedicated over the past decades to measure H0. Notably, Planck cosmic microwave background (CMB) and the local Cepheid-supernovae distance ladder measurements determine H0 with a precision of 1\% and 2\% respectively. A 3-σ level of discrepancy exists between the two measurements, for reasons that have yet to be understood. Gravitational wave (GW) sources accompanied by electromagnetic (EM) counterparts offer a completely independent standard siren (the GW analogue of an astronomical standard candle) measurement of H0, as demonstrated following the discovery of the neutron star merger, GW170817. This measurement does not assume a cosmological model and is independent of a cosmic distance ladder. The first joint analysis of the GW signal from GW170817 and its EM localization led to a measurement of H0=74+16-8 km/s/Mpc (median and symmetric 68\% credible interval). In this analysis, the degeneracy in the GW signal between the source distance and the weakly constrained viewing angle dominated the H0 measurement uncertainty. Recently, Mooley et al. (2018) obtained tight constraints on the viewing angle using high angular resolution imaging of the radio counterpart of GW170817. Here we obtain a significantly improved measurement H0=68.9+4.7-4.6 km/s/Mpc by using these new radio observations, combined with the previous GW and EM data. We estimate that 15 more localized GW170817-like events (comparable signal-to-noise ratio, favorable orientation), having radio images and light curve data, will potentially bring resolution to the tension between the Planck and Cepheid-supernova measurements, as compared to 50-100 GW events without such data.