Big-Bang Nucleosynthesis and the Baryon Density of the Universe

Big-bang nucleosynthesis is one of the cornerstones of the standard cosmology. For almost thirty years its predictions have been used to test the big-bang model to within a fraction of a second of the bang. The concordance that exists between the predicted and observed abundances of D, $^3$He, $^4$He and $^7$Li provides important confirmation of the standard cosmology and leads to the most accurate determination of the baryon density, between $1.7 \times 10^{-31}\gcmm3$ and $4.1\times 10^{-31}\gcmm3$ (corresponding to between about 1\% and 14\% of critical density). This measurement of the density of ordinary matter is crucial to almost every aspect of cosmology and is pivotal to the establishment of two dark-matter problems: (i) most of the baryons are dark, and (ii) if the total mass density is greater than about 14\% of the critical density as many determinations now indicate, the bulk of the dark matter must be ``nonbaryonic,'' comprised of elementary particles left from the earliest moments. We critically review the present status of primordial nucleosynthesis and discuss future prospects.