  Within the context of hot big-bang cosmology, a cosmic background of
presently low energy neutrinos is predicted to exist in concert with the
photons of the cosmic background radiation. The number density of the
cosmological neutrinos is of the same order as that of the photons of the
cosmic background radiation. That makes neutrinos the second most abundant
particle species in the universe. In the early universe, when these neutrinos
were highly relativistic, their effects in determining the ultimate structure
and evolution of the universe were significant.
