Finite temperature effects on the neutrino decoupling in the early Universe

Leading finite temperature effects on the neutrino decoupling temperature in the early Universe have been studied. We have incorporated modifications of the dispersion relation and the phase space distribution due to the presence of particles in the heat bath at temperature around MeV. Since both the expansion rate of the Universe and the interaction rate of a neutrino are reduced by finite temperature effects, it is necessary to calculate thermal corrections as precisely as possible in order to find the net effect on the neutrino decoupling temperature. We have performed such a calculation by using the finite temperature field theory. It has been shown that the finite temperature effects increase the neutrino decoupling temperature by 4.4%, the largest contribution coming from the modification of the phase space due to the thermal bath.