title: Anisotropic stress as a signature of nonstandard propagation of gravitational waves.
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.
authors.affiliation: School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom.
authors.forename: Ippocratis D
authors.initials: ID
authors.lastname: Saltas
authors.affiliation: Départment de Physique Théorique and Center for Astroparticle Physics, Université de Genève, Quai Ernest-Ansermet 24, CH-1211 Genève 4, Switzerland and African Institute for Mathematical Sciences, 6 Melrose Road, Muizenberg 7945, South Africa.
authors.forename: Ignacy
authors.initials: I
authors.lastname: Sawicki
authors.affiliation: Intitut für Theoretische Physik, Ruprecht-Karls-Universität Heidelberg, Philosophenweg 16, 69120 Heidelberg, Germany.
authors.forename: Luca
authors.initials: L
authors.lastname: Amendola
authors.affiliation: Départment de Physique Théorique and Center for Astroparticle Physics, Université de Genève, Quai Ernest-Ansermet 24, CH-1211 Genève 4, Switzerland and African Institute for Mathematical Sciences, 6 Melrose Road, Muizenberg 7945, South Africa.
authors.forename: Martin
authors.initials: M
authors.lastname: Kunz