A gravitational-wave standard siren measurement of the Hubble constant
  The detection of GW170817 in both gravitational waves and electromagnetic
waves heralds the age of gravitational-wave multi-messenger astronomy. On 17
August 2017 the Advanced LIGO and Virgo detectors observed GW170817, a strong
signal from the merger of a binary neutron-star system. Less than 2 seconds
after the merger, a gamma-ray burst (GRB 170817A) was detected within a region
of the sky consistent with the LIGO-Virgo-derived location of the
gravitational-wave source. This sky region was subsequently observed by optical
astronomy facilities, resulting in the identification of an optical transient
signal within $\sim 10$ arcsec of the galaxy NGC 4993. These multi-messenger
observations allow us to use GW170817 as a standard siren, the
gravitational-wave analog of an astronomical standard candle, to measure the
Hubble constant. This quantity, which represents the local expansion rate of
the Universe, sets the overall scale of the Universe and is of fundamental
importance to cosmology. Our measurement combines the distance to the source
inferred purely from the gravitational-wave signal with the recession velocity
inferred from measurements of the redshift using electromagnetic data. This
approach does not require any form of cosmic "distance ladder;" the
gravitational wave analysis can be used to estimate the luminosity distance out
to cosmological scales directly, without the use of intermediate astronomical
distance measurements. We determine the Hubble constant to be
$70.0^{+12.0}_{-8.0} \, \mathrm{km} \, \mathrm{s}^{-1} \, \mathrm{Mpc}^{-1}$
(maximum a posteriori and 68% credible interval). This is consistent with
existing measurements, while being completely independent of them. Additional
standard-siren measurements from future gravitational-wave sources will provide
precision constraints of this important cosmological parameter.
