1. Introduction

The simplicity of the standard model of Big Bang Nucleosynthesis (BBN) and its success when confronted with observations place the theory as one of the cornerstones of modern cosmology. BBN is based on the inclusion of an extended nuclear network into 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, . Other factors, such as the uncertainties in reaction rates, and the neutron mean-life can be treated by standard statistical and Monte Carlo techniques [2, 3, 4, 5]. The theory then allows one to make predictions (with specified uncertainties) of the abundances of the light elements, D, ³He, ⁴He, and ⁷Li. As there exist several detailed reviews on BBN, I will briefly summarize the key results and devote this contribution to the impact of recent observations of ⁴He and ⁷Li along with the related observations of Be and B. In referring to the standard model, I will mean homogeneous nucleosynthesis, with three neutrino flavors (N = 3), and a neutron mean life of 886.7 ± 1.9 s [6].

The dominant product of big bang nucleosynthesis is ⁴He, resulting in an abundance of close to 25% by mass. Lesser amounts of the other light elements are produced: D and ³He at the level of about 10^-5 by number, and ⁷Li at the level of 10^-10 by number. The resulting abundances of the light elements are shown in Figure 1, over the range in = 10¹⁰ between 1 and 10. The curves for the ⁴He mass fraction, Y, bracket the computed range based mainly on the uncertainty of the neutron mean-life. Uncertainties in the produced ⁷Li abundances have been adopted from the results in Hata et al. [3]. Uncertainties in D and ³He production are small on the scale of this figure. The dark shaded boxes correspond to the observed abundances and will be discussed below.

Figure 1. The light element abundances from big bang nucleosynthesis as a function o 10.

At present, there is a general concordance between the theoretical predictions and the observational data, particularly, for ⁴He and ⁷Li [7]. These two elements indicate that lies in the range 1.55 < < 4.45. There is limited agreement for D/H as well, as will be discussed below. High D/H narrows the range to 1.5 < < 3.4 and low D/H is compatible at the 2 level in the range 4.2 < < 5.3.