Vacuum structure for scalar cosmological perturbations in Modified
  Gravity Models
  We have found for the general class of Modified Gravity Models f(R,G) a new
instability which can arise in vacuum for the scalar modes of the cosmological
perturbations if the background is not de Sitter. In particular, the
short-wavelength modes, if stable, in general have a group velocity which
depends linearly in k, the wave number. Therefore these modes will be in
general superluminal. We have also discussed the condition for which in general
these scalar modes will be ghost-like. There is a subclass of these models,
defined out of properties of the function f(R,G) and to which the f(R) and f(G)
models belong, which however does not have this feature.
