Gravitational wave constraints on dark sector models
  We explore the constraints on dark sector models imposed by the recent
observation of coincident gravitational waves and gamma rays from a binary
neutron star merger, GW170817. Rather than focusing on specific models as has
been considered by other authors, we explore this in the context of the
equation of state approach of which the specific models are special cases.
After confirming the strong constraints found by others for Horndeski,
Einstein-Aether and massive gravity models, we discuss how it is possible to
construct models which might evade the constraints from GW170817 but still
leading to cosmologically interesting modifications to gravity. Possible
examples are ``miracle cancellations" such as in $f(R)$ models, nonlocal models
and higher-order derivatives. The latter two rely on the dimensionless ratio of
the wave number of the observed gravitational waves to the Hubble expansion
rate being very large ($\sim10^{19}$) which is used to suppress modifications
to the speed of gravitational waves.
