  The emerging structure of the neutrino mass matrix, when combined with the
primordial element abundances, places the most stringent constraint on the
flavor asymmetries in the cosmological neutrino background and therefore its
energy density. I review the mechanism of synchronized neutrino oscillations in
the an early universe with degenerate (asymmetric) neutrino and antineutrino
densities and the implications of refined measurements of neutrino parameters.
