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We investigate the evolution of the linear cosmological perturbations in f(R)gravity, an alternative to dark energy for explaining the late-time cosmicacceleration. We numerically calculate the early-time evolution with anapproximation we contrive to solve a problem that commonly appears when onesolves the full evolution equations. With the approximate evolution equationswe can fairly assess the effect of the gravity modification on the early-timeevolution, thereby examining the validity of the general-relativity (GR)approximation that is widely used for the early universe. In particular, wecompare the CMB photon density perturbation and the matter density perturbationobtained respectively by our approximation and the conventional GRapproximation. We find that the effect of the gravity modification at earlytimes in f(R) gravity may not be negligible. We conclude that to beself-consistent, in the f(R) theory one should employ the approximationpresented in this paper instead of that of GR in the treatment of theearly-time evolution.

The Early-Time Evolution of the Cosmological Perturbations in f(R) Gravity