MeV-scale reheating and neutrino thermalization

We study how the oscillations of the neutrinos affect their thermalization process during the reheating period with temperature O(1) MeV in the early universe. We follow the evolution of the neutrino density matrices and investigate how the predictions of big bang nucleosynthesis vary with the reheating temperature. For the reheating temperature of several MeV, we find that including the oscillations makes different predictions, especially for 4He abundance. Also, the effects on the lower bound of the reheating temperature from cosmological observations are discussed. (author)