Neutrino decoupling

Neutrino decoupling occurs when weak interactions become slower than cosmic expansion. The neutrino distribution then free streams and cools as $T_{\nu }\propto a^{-1}$.

After neutrino decoupling, electron-positron annihilation transfers entropy to photons but not to neutrinos. Comparing the electromagnetic entropy degrees of freedom before and after annihilation gives $T_{\nu }/T_{\gamma }=(4/11{)}^{1/3}$.