PRIMORDIAL NUCLEOSYNTHESIS

With the advent of the new extragalactic deuterium observations, Big B ang nucleosynthesis (BBN) is on the verge of undergoing a transformati on. In the past, the emphasis has been on demonstrating the concordanc e of the BBN model with the abundances of the light isotopes extrapola ted back to their primordial values by using stellar and galactic evol ution theories, As a direct measure of primordial deuterium is converg ed upon, the nature of the field will shift to using the much more pre cise primordial D/H to constrain the more flexible stellar and galacti c evolution models (although the question of potential systematic erro r in He-4 abundance determinations remains open). The remarkable succe ss of the theory to date in establishing the concordance has led to th e very robust conclusion of BBN regarding the baryon density. This rob ustness remains even through major model variations such as an assumed first-order quark-hadron phase transition. The BBN constraints on the cosmological baryon density are reviewed and demonstrate that the bul k of the baryons are dark and also that the bulk of the matter in the universe is nonbaryonic. Comparison of baryonic density arguments from Lyman-alpha clouds, x-ray gas in clusters, and the microwave anisotro py are made.