Deuterium and the baryonic density of the universe

Big bang nucleosynthesis (BBN) is the creation of the light nuclei, deuteri um, He-3, He-4 and Li-7 during the first few minutes of the universe. Here we discuss recent measurements of the D to H abundance ratio, D/H, in our g alaxy and towards quasars. We have achieved an order of magnitude improveme nt in the precision of the measurement of primordial D/H, using the HIRES s pectrograph on the W. M. Keck telescope to measure D in gas with very nearl y primordial abundances towards quasars. From 1994 to 1996, it appeared tha t there could be a factor of 10 range in primordial D/H, but today four exa mples of low D are secure. High D/H should be much easier to detect, and si nce there are no convincing examples, it must be extremely rare or non-exis tent. All data are consistent with a single low value for D/H, and the exam ples which are consistent with high D/H are readily interpreted as H contam ination near the position of D. The new D/H measurements give the most accu rate value for the baryon-to-photon ratio, eta, and hence the cosmological baryon density. A similar density is required to explain the amount of Ly a lpha absorption from neutral hydrogen in the intergalactic medium (IGM) at redshift z similar or equal to 3, and to explain the fraction of baryons in local clusters of galaxies. The D/H measurements lead to predictions for the abundances of the other li ght nuclei, which generally agree with measurements. The remaining differen ces with some measurements can be explained by a combination of measurement and analysis errors or changes in the abundances after BBN. The measuremen ts do not require physics beyond the standard BBN model. Instead, the agree ment between the abundances is used to limit the non-standard physics. (C) 2000 Elsevier Science B.V. All rights reserved.