Speed of gravitational waves and black hole hair
  The recent detection of GRB 170817A and GW170817 constrains the speed of
gravity waves $c_T$ to be that of light, which severely restricts the landscape
of modified gravity theories that impact the cosmological evolution of the
universe. In this work, we investigate the presence of black hole hair in the
remaining viable cosmological theories of modified gravity that respect the
constraint $c_T=1$. We focus mainly on scalar-tensor theories of gravity,
analyzing static, asymptotically flat black holes in Horndeski, Beyond
Horndeski, Einstein-Scalar-Gauss-Bonnet, and Chern-Simons theories. We find
that in all of the cases considered here, theories that respect $c_T=1$ do not
allow for hair, or have negligible hair. We further comment on vector-tensor
theories including Einstein Yang-Mills, Einstein-Aether, and Generalised Proca
theories, as well as bimetric theories.
