Non-standard gravitational waves imply gravitational slip: on the difficulty of partially hiding new gravitational degrees of freedom

摘要

In many generalized models of gravity, perfect fluids in cosmology give riseto gravitational slip. Simultaneously, in very broad classes of such models,the propagation of gravitational waves is altered. We investigate the extent towhich there is a one-to-one relationship between these two properties in threeclasses of models with one extra degree of freedom: scalar (Horndeski andbeyond), vector (Einstein-Aether) and tensor (bimetric). We prove that in bimetric gravity and Einstein-Aether, it is impossible todynamically hide the gravitational slip on all scales whenever the propagationof gravitational waves is modified. Horndeski models are much more flexible,but it is nonetheless only possible to hide gravitational slip dynamically whenthe action for perturbations is tuned to evolve in time toward a divergentkinetic term. These results provide an explicit, theoretical argument for theinterpretation of future observations if they disfavoured the presence ofgravitational slip.