Measurements of the deuterium abundance in quasar absorption systems

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Observational constraints on the primordial deuterium-to-hydrogen ratio (D/H) can test theories of the early universe and provide constraints on models of big bang nucleosynthesis (BBN). We measure deuterium absorption in high-redshift, metal-poor QSO absorption systems and directly infer the value of primordial D/H. We present two measurements of D/H, and find D/H = 3.3 ± 0.3 × 10⁻⁵ at z=3.572 towards QSO 1937-1009 and D/H = 4.0 ± 0.7 × 10⁻⁵ at z=2.504 towards QSO 1009+2956. Both measurements use multiple-component Voigt profile analysis of high resolution, high signal-to-noise spectra and determinations of the Lyman continuum optical depth in low resolution spectra to constrain the column densities of deuterium and hydrogen. The measurements are consistent with a single primordial value of D/H = 3.4 ± 0.3 × 10⁻⁵. This is a relatively low value, which supports homogeneous models of BBN and standard models of galactic chemical evolution. With standard BBN, we find a cosmological baryon-to-photon ratio, η= 5.1 ± 0.3 × 10⁻¹⁰, and a present-day baryon density in units of the critical density, Ωb h₁₀₀² = 0.019 ± 0.001.