Baryonic Dark Matter and Big Bang Nucleosynthesis

The recently observed Deuterium abundance in a low- metallicity high-redshift hydrogen cloud, which is about ten times larger than that observed in the near interstellar medium, is that expected from the Standard Big Bang Nucleosynthesis theory and the observed abundances of $^4$He and $^7$Li extrapolated to their primordial values. The inferred cosmic baryon to photon ratio, $\eta=(1.60\pm 0.1)\times 10^{-10},$ yields a mean cosmic baryon density, in critical mass units, of $\Omega_b\approx (0.6\pm 0.1)\times 10^{-2}h^{-2},$ with $h$ being the Hubble constant in units of $100 km~s^{-1} Mpc^{-1}$. This baryon density is consistent with the mean cosmic density of matter visible optically and in X-rays. It implies that most of the baryons in the Universe are visible and are not dark. Combined with the observed ratio of baryons to light in X-ray emitting clusters, it yields the value $\Omega \approx 0.15$ for the mean mass density of the Universe, which is consistent with that obtained from the mass to light ratio in clusters. This mass density is about ten times larger than the mean baryon mass density. It indicates that most of the matter in the Universe consists of nonbaryonic dark matter.Comment: Invited talk to be published in the proceedings of ``Dark Matter In Cosmology'' (Villars sur Ollon, Switzerland January 21-28, 1995