Challenges to the standard model of Big Bang nucleosynthesis.

Big Bang nucleosynthesis provides a unique probe of the early evolution of the Universe and a crucial test of the consistency of the standard hot Big Bang cosmological model. Although the primordial abundances of 2H, 3He, 4He, and 7Li inferred from current observational data are in agreement with those predicted by Big Bang nucleosynthesis, recent analysis has severely restricted the consistent range for the nucleon-to-photon ratio: 3.7 </= eta10 </= 4.0. Increased accuracy in the estimate of primordial 4He and observations of Be and B in Pop II stars are offering new challenges to the standard model and suggest that no new light particles may be allowed (N(BBN)nu </= 3.0, where Nnu is the number of equivalent light neutrinos).