Big bang nucleosynthesis in light of discordant deuterium measurements

Two-dimensional concordance plots involving the baryon-to-photon ratio, eta, and an effective number of light neutrinos, N-v, are used to discuss the overall consistency of standard big bang nucleosynthesis in light of recent determinations of the primordial deuterium (H-2) abundance. Observations of high-redshift Ly alpha clouds have provided discordant H-2/H determinations: one cloud with a high value of H-2/H compared with the previously accepted range of (H-2 + He-3)/H, and one system with a significantly lower value of H-2/H than those previously obtained. The high value of H-2/H agrees well with the current observationally inferred primordial abundances of He-4 and Li-7 for N-v = 3. The low value of H-2/H does not fit well with the current observationally inferred primordial abundance of He-4 for N-v = 3. In addition, if the low value of H-2/H is indicative of the primordial deuterium abundance, then significant depletion of Li-7 in old, hot Population II halo stars is probably required in order to obtain a concordant range of eta, for any effective number of neutrino flavors. The use of conservative ranges for the primordial abundances of H-2, He-4, and Li-7 allows the success of the standard picture for N-v = 3.