Superluminality in dilatationally invariant generalized Galileon
  theories
  We consider small perturbations about homogeneous backgrounds in
dilatationally-invariant Galileon models. The issues we address are stability
(absence of ghosts and gradient instabilities) and superluminality. We show
that in Minkowski background, it is possible to construct the Lagrangian in
such a way that any homogeneous Galileon background solution is stable and
small perturbations about it are subluminal. On the other hand, in the case of
FLRW backgrounds, for any Lagrangian functions there exist homogeneous
background solutions to the Galileon equation of motion and time-dependence of
the scale factor, such that the stability conditions are satisfied, but the
Galileon perturbations propagate with superluminal speed. Thus, a popular class
of the generalized Galileon models is plagued by superluminality.
