Modified Gravity As An Alternative To Dark Energy

We consider general curvature-invariant modifications of the Einstein-Hilbert action that become important only in regions of extremely low space-time curvature. We investigate the late-time evolution of the universe in such models, examining the possibilities for cosmic acceleration and other ultimate destinies. The models generically possess de Sitter space as an unstable solution and exhibit an interesting set of attractor solutions which, in some cases, provide alternatives to dark energy models. We show that modifications of the form f( R) are ruled out by solar system tests of gravitation. In addition, we also review the Palatini method of variation for such theories and contrast it with the metric variation approach