Big Bang nucleosynthesis, microwave anisotropy, and the light element abundances

摘要

From the observations of the anisotropies of the Cosmic Microwave Background (CMB) radiation, the WMAP satellite has provided a determination of the baryonic density of the Universe, with an unprecedented precision: 4%. This imposes a careful reanalysis of the standard Big-Bang Nucleosynthesis (SBBN) calculations. In a recent paper, we used the R-matrix theory to fit S-factor data on nuclear reactions involved in Big Bang nucleosynthesis. We derived the reaction rates with associated uncertainties, which were evaluated on statistical grounds (available at http://pntpm3.ulb.ac.be/bigbang). Combining these BBN results with the Omega(b)h(2) value from WMAP, we deduced the light element (He-4, D, He-3 and Li-7) primordial abundances and compare them with spectroscopic observations. There is a very good agreement with deuterium observed in cosmological clouds, which strengthens the confidence on the estimated baryonic density of the Universe. However, there is a discrepancy between the deduced Li-7 abundance and the one observed in halo stars of our Galaxy, supposed, until now, to represent the primordial abundance of this isotope. The origin of this discrepancy, observational, nuclear or more fundamental remains to be clarified. The possible role of the up to now neglected Be-7(d,p)2 alpha and 'Be(d,a)5Li reactions is considered and we present here a dedicated experiment performed at Louvain-la-Neuve to measure these cross sections.