  The annihilation of electron-positron pairs around one second after the big
bang distorts the Fermi-Dirac spectrum of neutrino energies. We determine the
distortions assuming neutrino mixing with an inverted neutrino-mass hierarchy.
Nonequilibrium thermodynamics, the Boltzmann equation, and numerical
integration are used to achieve the results. The various types of neutrino
behavior are established as a function of masses and mixing angles.
