When Matter Matters
  We study a recently proposed scenario for the early universe: Subluminal
Galilean Genesis. We prove that without any other matter present in the
spatially flat Friedmann universe, the perturbations of the Galileon scalar
field propagate with a speed at most equal to the speed of light. This proof
applies to all cosmological solutions -- to the whole phase space.
  However, in a more realistic situation, when one includes any matter which is
not directly coupled to the Galileon, there always exists a region of phase
space where these perturbations propagate superluminally, indeed with
arbitrarily high speed. We illustrate our analytic proof with numerical
computations. We discuss the implications of this result for the possible UV
completion of the model.
