Gravitational wave speed: Implications for models without a mass scale
  The recent report that the gravitational wave speed equals the light speed
puts strong constraints on the anisotropic stress parameter of many modified
gravity models, a quantity that is directly observable through large-scale
structure. We show here that models without a mass scale completely escape
these constraints. We discuss a few relevant cases in detail: Brans-Dicke
theory, nonlocal models, and Galileon Lagrangian.
