A Hubble constant measurement from superluminal motion of the jet in GW170817

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 data^18,19, will potentially resolve the tension between the Planck and Cepheid–supernova measurements.