A Hubble constant measurement from superluminal motion of the jet in GW170817 (Letter to the Editor)

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 (refs. 1–10). Gravitational wave (GW) sources accompanied by electromagnetic (EM) counterparts offer an independent standard siren measurement of H0 (refs. 11–13), as demonstrated following the discovery of the neutron star merger, GW170817 (refs. 14–16). 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=74-8+16kms-1Mpc-1 (median and symmetric 68% credible interval)13. In this analysis, the degeneracy in the GW signal between the source distance and the observing angle dominated the H0 measurement uncertainty. Recently, tight constraints on the observing angle using high angular resolution imaging of the radio counterpart of GW170817 have been obtained17. Here, we report an improved measurement H0=70.3-5.0+5.3kms-1Mpc-1 by using these new radio observations, combined with the previous GW and EM data. We estimate that 15 more GW170817-like events, having radio images and light curve data, as compared with 50–100 GW events without such data18,19, will potentially resolve the tension between the Planck and Cepheid–supernova measurements.