Anisotropic stress as signature of non-standard propagation of gravitational waves

TL;DR

This paper clarifies that anisotropic stress indicates modified gravity and links it to altered gravitational wave propagation across various theories, enabling potential model-independent tests of gravity.

Contribution

It establishes a direct correspondence between anisotropic stress and non-standard gravitational wave propagation in broad classes of modified gravity models.

Findings

Anisotropic stress signals modifications in gravity.

Gravitational wave behavior can be constrained through anisotropic stress measurements.

Potential for model-independent tests of gravity using cosmological data.

Abstract

We make precise the heretofore ambiguous statement that anisotropic stress is a sign of a modification of gravity. We show that in cosmological solutions of very general classes of models extending gravity --- all scalar-tensor theories (Horndeski), Einstein-Aether models and bimetric massive gravity --- a direct correspondence exists between perfect fluids apparently carrying anisotropic stress and a modification in the propagation of gravitational waves. Since the anisotropic stress can be measured in a model-independent manner, a comparison of the behavior of gravitational waves from cosmological sources with large-scale-structure formation could in principle lead to new constraints on the theory of gravity.