PRECISION MEASURES OF THE PRIMORDIAL ABUNDANCE OF DEUTERIUM

Abstract

We report the discovery of deuterium absorption in the very metal-poor ([Fe/H] = −2.88) damped Lyα system at z abs = 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 Lyα 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 100 Ω b, 0 h 2 = 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, N eff = 3.28 ± 0.28. We therefore rule out the existence of an additional (sterile) neutrino (i.e., N eff = 4.046) at 99.3% confidence (2.7σ), provided that the values of N eff and of the baryon-to-photon ratio (η 10 ) did not change between BBN and recombination. We also place a strong bound on the neutrino degeneracy parameter, independent of the 4 He primordial mass fraction, Y P : ξ D = +0.05 ± 0.13 based only on the CMB+(D/H) p observations. Combining this value of ξ D with the current best literature measure of Y P , we find a 2σ upper bound on the neutrino degeneracy parameter, |ξ| ⩽ +0.062.