Anisotropic Stress as a Signature of Nonstandard Propagation of Gravitational Waves

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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.
Funding Information
  • Deutsche Forschungsgemeinschaft
  • Swiss National Science Foundation
  • Science and Technology Facilities Council
  • Marie Skłodowska-Curie Intra-European Fellowship (DRKFRCS)

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