  The primordial abundances of the light nuclides produced by Big Bang
Nucleosynthesis (BBN) during the first 20 minutes in the evolution of the
Universe are sensitive to the universal density of baryons and to the expansion
rate of the early Universe. For example, while deuterium is an excellent
baryometer, helium-4 provides an accurate chronometer. Some 400 kyr later, when
the cosmic background radiation (CBR) was freed from the grasp of the ionized
plasma of protons and electrons, the spectrum of temperature fluctuations also
depended on (among other parameters) the baryon density and the density in
relativistic particles. The comparison between the constraints imposed by BBN
and those from the CBR reveals a remarkably consistent picture of the Universe
at two widely separated epochs in its evolution. Combining these two probes
leads to new estimates of the baryon density at present and tighter constraints
on possible physics beyond the standard model of particle physics. In this
review the consistency between these complementary probes of the universal
baryon density and the early-Universe expansion rate is exploited to provide
new constraints on any asymmetry between relic neutrinos and anti-neutrinos
(neutrino degeneracy).
