UC San Diego

Recent determinations of the deuterium abundance, 2H/H, in high redshift Lyman limit hydrogen clouds challenge the usual picture of primordial nucleosynthesis based on "concordance" of the calculated light element (2H, 3He, 4He, 7Li) nucleosynthesis yields with the observationally-inferred abundances of these species. Concordance implies that all light element yields can be made to agree with the observationally-inferred abundances (within errors) for single global specifications of the baryon-to-photon ratio, η; lepton number; neutron lifetime; and expansion rate (or equivalently, effective number of light neutrino degrees of freedom Nv). Though one group studying Lyman limit systems obtains a high value of 2H/H (∼ 2 × 10-4), another group finds consistently low values (∼ 2 × 10-5). In the former case, concordance for Nv = 3 is readily attained for the current observationally-inferred abundances of 4He and 7Li. But if the latter case represents the primordial deuterium abundance, then concordance for any Nv is impossible unless the primordial value of 7Li/H is considerably larger than the abundance of lithium as measured in old, hot Pop II halo stars. Furthermore, concordance with Nv = 3 is possible for low 2H/H only if either (1) the primordial 4He abundance has been significantly underestimated, or (2) new neutrino sector physics is invoked. We argue that systematic underestimation of both the 7Li and 4He primordial abundances is the likely resolution of this problem, a conclusion which is strengthened by new results on 4He.