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

TL;DR

This paper explores the relationship between gravitational slip and gravitational wave propagation in various modified gravity models, showing that in most cases, these features cannot be hidden simultaneously, which has implications for future observations.

Contribution

It provides a theoretical analysis demonstrating the difficulty of hiding gravitational slip alongside altered gravitational wave propagation in several classes of modified gravity models.

Findings

In bimetric and Einstein-Aether models, gravitational slip cannot be hidden if gravitational wave propagation is modified.

Horndeski models can hide gravitational slip only with fine-tuned, time-evolving actions.

Results support using gravitational slip observations to test and constrain gravity theories.

Abstract

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.