Anisotropic stress as signature of non-standard propagation of   gravitational waves
  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. 
Ippocratis D. Saltas (Nottingham U.), Ignacy Sawicki (Geneva U., Dept.   Theor. Phys. and African Inst. Math. Sci., Cape Tow...
astro-ph.CO gr-qc hep-th