%0 Book Section
%T Cosmological density perturbations in modified gravity theories
publisher Amer Inst Physics
%D 2009
%U 978-0-7354-0658-2
%@ https://hdl.handle.net/20.500.14352/53241
%X 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 quasi-static 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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