  For a brief time in its early evolution the Universe was a cosmic nuclear
reactor. The expansion and cooling of the Universe limited this epoch to the
first few minutes, allowing time for the synthesis in astrophysically
interesting abundances of only the lightest nuclides (D, 3He, 4He, 7Li). For
big bang nucleosynthesis (BBN) in the standard models of cosmology and particle
physics (SBBN), the SBBN-predicted abundances depend on only one adjustable
parameter, the baryon density parameter (the ratio by number of baryons
(nucleons) to photons). The predicted and observed abundances of the relic
light elements are reviewed, testing the internal consistency of primordial
nucleosynthesis. The consistency of BBN is also explored by comparing the
values of the cosmological parameters inferred from primordial nucleosynthesis
for the standard model and for models with non-standard early Universe
expansion rates with those determined from studies of the cosmic background
radiation, which provides a snapshot of the Universe some 400 thousand years
after BBN ended.
