Neutron star merger GW170817 strongly constrains doubly coupled
  bigravity
  We study the implications of the recent detection of gravitational waves
emitted by a pair of merging neutron stars and their electromagnetic
counterpart, events GW170817 and GRB170817A, on the viability of the doubly
coupled bimetric models of cosmic evolution, where the two metrics couple
directly to matter through a composite, effective metric. We demonstrate that
the bounds on the speed of gravitational waves place strong constraints on the
doubly coupled models, forcing either the two metrics to be proportional at the
background level or the models to become singly coupled. Proportional
backgrounds are particularly interesting as they provide stable cosmological
solutions with phenomenologies equivalent to that of $\Lambda$CDM at the
background level as well as for linear perturbations, while nonlinearities are
expected to show deviations from the standard model.
