Relaxation time of neutrinos in the early universe

The relaxation time of neutrinos and antineutrinos interacting with electrons and positrons in the early universe is calculated using the relativistic version of the two-temperature formalism, and the cross sections predicted by the Weinberg-Salam theory. The results indicate that primordial neutrinos decoupled from the rest of matter at a temperature around 2 x 10 to the 10th K. It is also shown that, as a consequence of this result, the ratio of the radiation temperature to the neutrino temperature becomes 0.2 percent smaller than the standard value (11/4) 1/3 and that the He-4 abundance resulting from primeval nucleosynthesis will differ, also from the standard value, by an amount of -0.000188.