The Deuteron confronts big bang nucleosynthesis

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 N_v). 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 N_v = 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 N_v 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 N_v = 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.