Matter instabilities in general Gauss-Bonnet gravity
  We study the evolution of cosmological perturbations in f(G) gravity, where
the Lagrangian is the sum of a Ricci scalar R and an arbitrary function f in
terms of a Gauss-Bonnet term G. We derive the equations for perturbations
assuming matter to be described by a perfect fluid with a constant equation of
state w. We show that density perturbations in perfect fluids exhibit negative
instabilities during both the radiation and the matter domination, irrespective
of the form of f(G). This growth of perturbations gets stronger on smaller
scales, which is difficult to be compatible with the observed galaxy spectrum
unless the deviation from General Relativity is very small. Thus f(G)
cosmological models are effectively ruled out from this Ultra-Violet
instability, even though they can be compatible with the late-time cosmic
acceleration and local gravity constraints.
