An interacting dark sector and the implications of the first gravitational-wave standard siren detection on current constraints

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, characterized by a conformal interaction between dark matter and dark energy, by combining current data from: 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 oscillation 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.