A Hubble constant measurement from superluminal motion of the jet in
  GW170817
  The Hubble constant ($H_0$) 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 $H_0$. Notably, Planck cosmic
microwave background (CMB) and the local Cepheid-supernovae distance ladder
measurements determine $H_0$ with a precision of $\sim 1\%$ and $\sim 2\%$
respectively. A $3$-$\sigma$ 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 $H_0$, 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 $H_0=74^{+16}_{-8}$ 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
$H_0$ 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 $H_0=68.9^{+4.7}_{-4.6}$ 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.
