Assessing Big-Bang Nucleosynthesis

Systematic uncertainties in the light-element abundances and their evolution make a rigorous statistical assessment difficult. However, using Bayesian methods we show that the following statement is robust: the predicted and measured abundances are consistent with 95% credibility only if the baryon-to-photon ratio is between $2\times {10}^{-10}$ and $6.5\times {10}^{-10}$ and the number of light neutrino species is less than 3.9. Our analysis suggests that the ${}^4$He abundance may have been systematically underestimated.