#### June 28, 14:15 in A315 (Thursday --> SPECIAL DATE CosmoSeminar)

#### Hayato MOTOHASHI (Tokyo U., Japan)

#### Cosmic evolution in f(R) gravity

f(R) gravity is a simple and nontrivial extension of General Relativity which has recently received much attention to explain the accelerating expansion of the Universe without cosmological constant. In order to test the theory observationally, we have studied the evolution of background universe and perturbations. I will present the results of analytical and numerical analysis of equation-of-state (EoS) parameter of effective dark energy and matter density fluctuations in f(R) gravity. It is found that EoS parameter exhibits phantom crossing and the growth index of density fluctuations evolves nonmonotonically with time and depends on scale. Furthermore, the matter power spectrum is enhanced in f(R) gravity compared with LCDM model. Since this enhancement cancels the suppression by free streaming of massive neutrinos, the allowed range of neutrino mass is relaxed in f(R) gravity. We also found that the larger sterile neutrino mass is allowed in f(R) gravity and it can resolve the tension between recent constraints from neutrino oscillation experiments and cosmological data.