The cosmological baryon density from the deuterium-to-hydrogen ratio in QSO absorption systems: D/H toward Q1243+3047

We report the detection of deuterium absorption at redshift 2.525659 toward Q1243+3047. We describe improved methods to estimate the deuterium to hydrogen abundance ratio (D/H) in absorption systems, including improved modeling of the continuum level, the Lyalpha forest, and the velocity structure of the absorption. Together with improved relative flux calibration, these methods give D/H = 2.42(-0.25)(+0.35) x 10(-5) from our Keck I HIRES spectra of Q1243+3047, where the error is from the uncertainty in the shape of the continuum level and the amount of D absorption in a minor second component. The measured D/H is likely the primordial value because [O/H] = -2.79 +/- 0.05. This absorption system has a neutral hydrogen column density log N-H (I) = 19.73 +/- 0.04 cm(-2), shows five D lines, and is mostly ionized. The best estimate of the primordial D/H is 2.78(-0.38)(+0.44) x 10(-5), from the log D/H-values toward five QSOs. The dispersion in the five values is larger than we expect from their individual measurement errors, and we suspect this is because some of these errors were underestimated. We observe a trend in D/H with log N-H (I) that we also suspect is spurious. The best value for D/H is 0.6 sigma smaller than we quoted in O'Meara et al. from three QSOs, and although we have more values, the error is similar because the dispersion is larger. In standard big bang nucleosynthesis (SBBN), the best D/H corresponds to a baryon-to-photon ratio eta = 5.9 +/- 0.5 x 10(-10) and gives precise predictions for the primordial abundances of the other light nuclei. We predict more He-4 than is reported in most measurements, although not more than allowed by some estimates of the systematic errors. We predict a He-3 abundance very similar to that reported by Bania et al., and we predict 3-4 times more Li-7 than is seen in halo stars. It is unclear if those stars could have destroyed this much of their Li-7. The eta-value from D/H corresponds to a cosmological baryon density Omega(b)h(2) = 0.0214 +/- 0.0020 (+/-9.3%), which agrees with the WMAP value of Omega(b)h(2) = 0.0224 +/- 0.001.