  We reconsider the evolution of strongly degenerate neutrinos in the early
universe. Our chief concern is the validity of the entropy conservation after
the neutrino annihilation process has frozen out (so that the establishment of
chemical equilibrium is not trivial). We argue that the entropy indeed
conserves because elastic scattering keeps the neutrino and antineutrino
distribution functions in the equilibrium form and the sum of their chemical
potential keeps zero even after the neutrino annihilation freeze-out. We also
simulate the evolution of the degenerate neutrino spectrum to support the
argument. We conclude that the change in the neutrino degeneracy parameter when
the relativistic degrees of freedom in the universe decreases is calculated
using the entropy conservation and the lepton number conservation without
worrying about at what temperature the neutrino annihilation process freezes
out.
