Precision Measures of the Primordial Abundance of Deuterium

We report the discovery of deuterium absorption in the very metal-poor ([Fe/H]=−2.88) damped Lyman-α system at zabs = 3.06726 toward the QSO SDSS J1358+6522. On the basis of 13 resolved D i absorption lines and the damping wings of the H i Lyman α transition, we have obtained a new, precise measure of the primordial abundance of deuterium. Furthermore, to bolster the present statistics of precision D/H measures, we have reanalyzed all of the known deuterium absorption-line systems that satisfy a set of strict criteria. We have adopted a blind analysis strategy (to remove human bias), and developed a software package that is specifically designed for precision D/H abundance measurements. For this reanalyzed sample of systems, we obtain a weighted mean of (D /H)p = (2.53 ± 0.04) × 10−5, corresponding to a universal baryon density 100Ωb,0 h2= 2.202 ± 0.046 for the standard model of big bang nucleosynthesis (BBN). By combining our measure of (D /H)p with observations of the cosmic microwave background (CMB), we derive the effective number of light fermion species, Neff= 3.28±0.28. We therefore rule out the existence of an additional (sterile) neutrino (i.e. Neff= 4.046) at 99.3 per cent confidence (2.7σ), provided that the values of Neff and of the baryonto-photon ratio (η10) did not change between BBN and recombination. We also place a strong bound on the neutrino degeneracy parameter, independent of the 4He primordial mass fraction, YP: ξD = +0.05± 0.13 based only on the CMB+(D /H)p observations. Combining this value of ξD with the current best literature measure of YP, we find a 2σ upper bound on the neutrino degeneracy parameter, |ξ| ≤ +0.062. Subject headings: quasars: absorption lines – quasars: individual: J1358+6522 – cosmology: observations.