  During its hot, dense, early evolution the Universe was a primordial nuclear
reactor, synthesizing the light nuclides D, 3He, 4He and 7Li in the first
thousand seconds. The presently observed abundances of these relic nuclides
provide a unique window on the early Universe. The implications of current
observations for cosmology (the universal density of nucleons) and for particle
physics (new particles beyond the standard model) will be reviewed. The present
data appear to be in rough agreement with the predictions of the standard, hot
big bang model for three species of light neutrinos, and a nucleon-to-photon
ratio restricted to a narrow range of 3-4 parts in 10 billion. On closer
inspection, however, a tension is revealed between the inferred primordial
abundances of deuterium and helium-4. Although observations of deuterium in
nearly primordial, high-redshift QSO absorbers may help to relieve this
tension, current data appear to exacerbate the crisis. Resolution of this
conflict may lie with the data (statistical uncertainties?), with the analysis
of the data (systematic uncertainties?), or with the fundamental physics
(massive, unstable, and/or degenerate neutrinos?). Independent (non-BBN)
evidence from cosmological observations of large-scale dynamics and structure
may be useful in deciding among the current options.
