BIG-BANG NUCLEOSYNTHESIS AND THE DENSITY OF BARYONS IN THE UNIVERSE

Now that extragalactic deuterium observations are being made, Big Bang Nucleosynthesis (BBN) is on the verge of undergoing a transformation. Previously, the emphasis was on demonstrating the concordance of the Big Bang Nucleosynthesis model with the abundances of the light isotop es extrapolated back to their primordial values using stellar and Gala ctic evolution theories. Once the primordial deuterium abundance is co nverged upon, the nature of the field will shift to using the much mor e precise primordial D/H to constrain the more flexible stellar and Ga lactic evolution models (although the question of potential systematic error in He-4 abundance determinations remains open). The remarkable success of the theory to date in establishing the concordance has led to the very robust conclusion of BBN regarding the baryon density. The BBN constraints on the cosmological baryon density are reviewed and d emonstrate that the bulk of the baryons are dark and also that the bul k of the matter in the universe is non-baryonic. Comparison of baryoni c density arguments from Lyman-alpha clouds, x-ray gas in clusters, an d the microwave anisotropy are made and shown to be consistent with th e BBN value.