Neutrino collective effects during their decoupling era in the early
  universe
  There is an accepted approach to calculation of the neutrino flavor
density-matrix in the halo of a supernova, in which neutrino amplitudes, not
cross-sections, need to be followed carefully in the region above the region of
frequent scatterings. The same reasoning and techniques, applied to the
evolution of neutrino flavors and energy distributions in the early universe in
the era of neutrino decoupling, leads to radical changes in the predictions of
the effects of the neutrino-neutrino interaction. Predictions for the
production of sterile neutrinos, should they exist, will also be changed.
