  We present a multi-messenger measurement of the Hubble constant H_0 using the
binary-black-hole merger GW170814 as a standard siren, combined with a
photometric redshift catalog from the Dark Energy Survey (DES). The luminosity
distance is obtained from the gravitational wave signal detected by the
LIGO/Virgo Collaboration (LVC) on 2017 August 14, and the redshift information
is provided by the DES Year 3 data. Black-hole mergers such as GW170814 are
expected to lack bright electromagnetic emission to uniquely identify their
host galaxies and build an object-by-object Hubble diagram. However, they are
suitable for a statistical measurement, provided that a galaxy catalog of
adequate depth and redshift completion is available. Here we present the first
Hubble parameter measurement using a black-hole merger. Our analysis results in
$H_0 = 75.2^{+39.5}_{-32.4}~{\rm km~s^{-1}~Mpc^{-1}}$, which is consistent with
both SN Ia and CMB measurements of the Hubble constant. The quoted 68% credible
region comprises 60% of the uniform prior range [20,140] ${\rm
km~s^{-1}~Mpc^{-1}}$, and it depends on the assumed prior range. If we take a
broader prior of [10,220] ${\rm km~s^{-1}~Mpc^{-1}}$, we find $H_0 = 78^{+
96}_{-24}~{\rm km~s^{-1}~Mpc^{-1}}$ ($57\%$ of the prior range). Although a
weak constraint on the Hubble constant from a single event is expected using
the dark siren method, a multifold increase in the LVC event rate is
anticipated in the coming years and combinations of many sirens will lead to
improved constraints on $H_0$.
