Abundances of light nuclei

An overview of the standard model of big bang nucleosynthesis (BBN) in the post-WMAP era is presented. In this context, the theoretical prediction for the abundances of D, ³He, ⁴He, and ⁷Li is discussed. The observational determination of the light nuclides is also discussed. While, the D and ⁴He observations are concordant with BBN predictions, ⁷Li remains discrepant with the CMB-preferred baryon density and possible explanations are reviewed. The standard model [1] of big bang nucleosynthesis (BBN) is based on an extended nuclear network in a homogeneous and isotropic cosmology. Apart from the input nuclear cross sections, the theory contains only a single parameter, namely the baryon-to-photon ratio, η ≡ n_B/n_γ. The theory then allows one to make predictions (with specified uncertainties) of the abundances of the light elements, D, ³He, ⁴He, and ⁷Li. There have been many improvements over the last few years in the state of the theory, particularly in the treatment of the nuclear cross-sections. However, perhaps the most important new input is the WMAP determination of the baryon density [2], Ω_Bh², or equivalently η. Thus one is now able to make very precise predictions of the light element isotopes, which can be individually compared with observation [3]. The predictions span some nine orders of magnitude in abundance. The major uncertainties in BBN calculations come from the thermonuclear reaction rates. There are 11 key strong rates (as well as the neutron lifetime) that dominate the uncertainty budget [4, 5]. Recently the input nuclear data have been carefully reassessed [4–7], leading to improved precision in the abundance predictions. The NACRE collaboration presented a larger focus nuclear compilation [6].