An interacting dark sector and the first gravitational-wave standard
  siren detection
  After the first nearly simultaneous joint observations of gravitational-waves
and electromagnetic emission produced by the coalescence of a binary neutron
star system, another probe of the cosmic expansion which is independent from
the cosmic distance ladder, became available. We perform a global analysis in
order to constrain an interacting dark energy model, characterised by a
conformal interaction between dark matter and dark energy, by combining current
data from: $\textit{Planck}$ observations of the cosmic microwave background
radiation anisotropies, and a compilation of Hubble parameter measurements
estimated from the cosmic chronometers approach as well as from baryon acoustic
oscillations measurements. Moreover, we consider two measurements of the
expansion rate of the Universe today, one from the observations of the Cepheid
variables, and another from the merger of the binary neutron star system
GW170817. We find that in this interacting dark energy model, the influence of
the local measurement of the Hubble constant mostly affects the inferred
constraints on the coupling strength parameter between dark energy and dark
matter. However, the GW170817 Hubble constant measurement is found to be more
conservative than the Cepheid variables measurement, and in a better agreement
with the current high redshift cosmological data sets. Thus, forthcoming
gravitational-wave standard siren measurements of the Hubble constant would be
paramount for our understanding of the dark cosmic sector.
