ar X iv : 0 81 2 . 30 48 v 1 [ as tr o - ph ] 1 6 D ec 2 00 8 Cosmological density perturbations in modified gravity theories
نویسنده
چکیده
In the context of f (R) theories of gravity, we study the cosmological evolution of scalar perturbations by using a completely general procedure. We find that the exact fourth-order differential equation for the matter density perturbations in the longitudinal gauge, reduces to a second-order equation for sub-Hubble modes. This simplification is compared with the standard (quasi-static) equation used in the literature. We show that for general f (R) functions the quasistatic approximation is not justified. However for those f (R) adequately describing the present phase of accelerated expansion and satisfying local gravity tests, it does give a correct description for the evolution of perturbations.
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