Big Bang Nucleosynthesis in Light of Discordant Deuterium Measurements

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Abstract
Two-dimensional concordance plots involving the baryon-to-photon ratio, g, and an e†ective number of light neutrinos, are used to discuss the overall consistency of standard big bang nucleosynthesis in N l , light of recent determinations of the primordial deuterium (2H) abundance. Observations of high-redshift Lya clouds have provided discordant 2H/H determinations : one cloud with a high value of 2H/H com- pared with the previously accepted range of (2H )3 He)/H, and one system with a signiÐcantly lower value of 2H/H than those previously obtained. The high value of 2H/H agrees well with the current observationally inferred primordial abundances of 4He and 7Li for The low value of 2H/H does N l \ 3. not Ðt well with the current observationally inferred primordial abundance of 4He for In addi- N l \ 3. tion, if the low value of 2H/H is indicative of the primordial deuterium abundance, then signiÐcant depletion of 7Li in old, hot Population II halo stars is probably required in order to obtain a concord- ant range of g, for any e†ective number of neutrino Ñavors. The use of conservative ranges for the pri- mordial abundances of 2H, 4He, and 7Li allows the success of the standard picture for N l \ 3. Subject headings : cosmology : observations È early universe È nuclear reactions, nucleosynthesis, abundances Considerations of big bang nucleosynthesis (BBN) com- prise an important probe of the physics of the early uni- verse. Primordial nucleosynthesis calculations predict successfully the relative abundances of the lightest elements over some 10 orders of magnitude. However, as claimed determinations of primordial abundances have grown in precision in the recent past, a number of di†erent points of view have arisen regarding the overall consistency of the standard big bang nucleosynthesis picture. The fact that proponents of these di†erent points of view sometimes ascribe high conÐdence levels to their conclusions makes the situation all the more confusing. Recent deuterium mea- surements in high-redshift Lya clouds Fan, & (Tytler,

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