Non-standard gravitational waves imply gravitational slip: on the
  difficulty of partially hiding new gravitational degrees of freedom
  In many generalized models of gravity, perfect fluids in cosmology give rise
to gravitational slip. Simultaneously, in very broad classes of such models,
the propagation of gravitational waves is altered. We investigate the extent to
which there is a one-to-one relationship between these two properties in three
classes of models with one extra degree of freedom: scalar (Horndeski and
beyond), vector (Einstein-Aether) and tensor (bimetric).
  We prove that in bimetric gravity and Einstein-Aether, it is impossible to
dynamically hide the gravitational slip on all scales whenever the propagation
of gravitational waves is modified. Horndeski models are much more flexible,
but it is nonetheless only possible to hide gravitational slip dynamically when
the action for perturbations is tuned to evolve in time toward a divergent
kinetic term. These results provide an explicit, theoretical argument for the
interpretation of future observations if they disfavoured the presence of
gravitational slip.
