Cosmological perturbation in f(R, G) theories with a perfect fluid

In order to classify modified gravity models according to their physical properties, we analyze the cosmological linear perturbations for f(R, G) theories (R being the Ricci scalar and G, the Gauss-Bonnet term) with a minimally coupled perfect fluid. For the scalar-type perturbations, we identify in general six degrees of freedom. We find that two of these physical modes obey the same dispersion relation as the one for a nonrelativistic de Broglie wave. This means that spacetime is either highly unstable or its fluctuations undergo a scale-dependent superluminal propagation. Two other modes correspond to the degrees of freedom of the perfect fluid, and propagate with the sound speed of such a fluid. The remaining two modes correspond to the entropy and temperature perturbations of the perfect fluid, and completely decouple from the other modes for a barotropic equation of state. We then provide a concise condition on f(R, G) theories, which both f(R) and R + f(G) do fulfill, to avoid the de Broglie-type dispersion relation. For the vector-type perturbation, we find that the perturbations decay in time. For the tensor-type perturbation, the perturbations can be either superluminal or subluminal, depending on the model. No-ghost conditions are also obtained for each type of perturbation.