COSMIC LITHIUM: GOING UP OR COMING DOWN ?

Observations of interstellar lithium provide a valuable complement to studies of lithium in Pop I and Pop II stars. Large corrections for unseen LiII and for non-gas phase lithium have provided obstacles to using interstellar data for abundance determinations. An approach to surmounting these difficulties is proposed and is applied to the Galaxy and the LMC. The key is that since potassium and lithium behave similarly regarding ionization and depletion, their observed ratio (LiI/KI) can be used to probe the abundance and evolution of lithium. For ten lines-of-sight in the interstellar medium of the Galaxy (ISM) the Li/K ratio observed $(\log (N_{Li}/N_K)_{ISM} = - 1.88 \pm 0.09)$ is entirely consistent with the solar system value $(\log (N_{Li}/N_{K})_{\odot} = - 1.82 \pm 0.05)$. The absence of LiI in front of SN87A in the LMC, coupled with the observed KI, corresponds to an upper bound (at \rsim 95 \% \ CL) of $\log (N_{Li}/N_K)_{LMC} < - 0.3 + \log (N_{Li}/N_K)_{ISM}.$ This low upper bound to LMC lithium suggests that cosmic lithium is on its way up from a primordial abundance lower, by at least a factor of two, than the present Pop I value of $[Li]_{PopI} \equiv 12 + \log (Li/H)_{PopI} = 3.2 \pm 0.1$.Comment: Submitted to the ApJ (Part 1), April 14, 199

Neutrinos And Big Bang Nucleosynthesis

According to the standard models of particle physics and cosmology, there should be a background of cosmic neutrinos in the present Universe, similar to the cosmic microwave photon background. The weakness of the weak interactions renders this neutrino background undetectable with current technology. The cosmic neutrino background can, however, be probed indirectly through its cosmological effects on big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) radiation. In this BBN review, focused on neutrinos and, more generally on dark radiation, the BBN constraints on the number of "equivalent neutrinos" (dark radiation), on the baryon asymmetry (baryon density), and on a possible lepton asymmetry (neutrino degeneracy) are reviewed and updated. The BBN constraints on dark radiation and on the baryon density following from considerations of the primordial abundances of deuterium and helium-4 are in excellent agreement with the complementary results from the CMB, providing a suggestive, but currently inconclusive, hint of the presence of dark radiation and, they constrain any lepton asymmetry. For all the cases considered here there is a "lithium problem": the BBN-predicted lithium abundance exceeds the observationally inferred primordial value by a factor of ~3.Comment: Invited Review article for the Special Issue on Neutrino Physics, Advances in High Energy Physics, 25 pages, 10 figure

Light Element Nucleosynthesis

An introductory review of the early evolution of the Universe relevant to the primordial synthesis of the light nuclides deuterium, helium-3, helium-4 and lithium-7. The predictions of the element abundances in the "standard", hot, big bang cosmological model (SBBN) are described. After descriptions of the evolution of the primordial abundances from "there and then" to "here and now", the SBBN predictions are compared to current observational data. The implications for the standard model and for physics beyond the standard model are discussed.Comment: 12 pages, 5 postscript figures; To appear in the Encyclopedia of Astronomy and Astrophysics (Institute of Physics) December, 200