The matter power spectrum in f(R) gravity
  Modified gravity can be considered as an alternative to dark energy. In a generalized theory of gravity, the universe may accelerate while containing only baryonic and dark matter. We study, in particular, the evolution of matter fluctuations in f(R) models within the Palatini approach, and find that the resulting matter power spectrum is sensitive to a nonlinear dependence on the curvature scalar in the gravitational action. The constraints that arise from comparison to the form of the observed matter power spectrum tighten the previous constraints derived from background expansion by several orders of magnitude. Models in the allowed parameter space are practically indistinguishable from general relativity with a cosmological constant when the backround expansion is considered. 
Tomi Koivisto