Perturbative analysis of generalized Einstein theories

The hypothesis that the energy-momentum tensor of ordinary matter is not conserved separately, leads to a nonadiabatic expansion and, in many cases, to an Universe older than usual. This may provide a solution for the entropy and age problems of the Standard Cosmological Model. We consider two different theories of this type, and we perform a perturbative analysis, yielding analytical expressions for the evolution of gravitational waves, rotational modes and density perturbations. One of these theories exhibits satisfactory properties at this level, while the other one should be discarded.