Neutrino Decoupling   /H = ( k B T/ 1MeV) 3 ~ 1 T(Neutrinos) decoupl ~ 1MeV ~ Kelvin; today: 1.95 K Time after Big Bang: 1 Second T(Photons) decoupling = 3000 Kelvin; today: K Time(Photons) decoupling = years Below T = 1 MeV: