THE PRIMORDIAL DEUTERIUM ABUNDANCE OF THE MOST METAL-POOR DAMPED Lyα SYSTEM*

We report the discovery and analysis of the most metal-poor damped Ly alpha (DLA) system currently known, which also displays the Lyman series absorption lines of neutral deuterium. The average [O/H] abundance of this system is [O/H]. =. -2.804 +/- 0.015, which includes an absorption component with [O/H]. =. -3.07 +/- 0.03. Despite the unfortunate blending of many weak D I absorption lines, we report a precise measurement of the deuterium abundance of this system. Using the six highest-quality and self-consistently analyzed measures of D/H in DLAs, we report tentative evidence for a subtle decrease of D/H with increasing metallicity. This trend must be confirmed with future high-precision D/H measurements spanning a range of metallicity. A weighted mean of these six independent measures provides our best estimate of the primordial abundance of deuterium, 10(5) (D/H)(P) = 2.547 +/- 0.033 (log(10)(D/H)(P) = -4.5940 +/- 0.0056). We perform a series of detailed Monte Carlo calculations of Big Bang nucleosynthesis (BBN) that incorporate the latest determinations of several key nuclear reaction cross-sections, and propagate their associated uncertainty. Combining our measurement of (D/H) P with these BBN calculations yields an estimate of the cosmic baryon density, 100 Omega(B,0)h(2)(BBN) = 2.156 +/- 0.020, if we adopt the most recent theoretical determination of the d(p, gamma)He-3 reaction rate. This measure of Omega(B,0)h(2) differs by similar to 2.3 sigma from the Standard Model value estimated from the Planck observations of the cosmic microwave background. Using instead a d(p, gamma)He-3 reaction rate that is based on the best available experimental cross-section data, we estimate 100 Omega(B,0) h(2)(BBN) = 2.260 +/- 0.034, which is in somewhat better agreement with the Planck value. Forthcoming measurements of the crucial d(p, gamma)He-3 cross-section may shed further light on this discrepancy.