Big Bang Nucleosynthesis in Crisis?

A new evaluation of the constraint on the number of light neutrino species ($N_{\nu }$) from big bang nucleosynthesis suggests a discrepancy between the predicted light element abundances and those inferred from observations, unless the inferred primordial ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ abundance has been underestimated by 0.014±0.004 ($1\sigma$) or less than 10% (95% C.L.) of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ survives stellar processing. With the quoted systematic errors in the observed abundances and a conservative chemical evolution parametrization, the best fit to the combined data is $N_{\nu }=2.1\mathrm{}\pm 0.3$ ($1\sigma$) and the upper limit is $N_{\nu }<2.6\mathrm{}$ (95% C.L.). The data are inconsistent with the standard model ($N_{\nu }=3\mathrm{}$) at the 98.6% C.L.