Big Bang Nucleosynthesis in Crisis?

Abstract

A new evaluation of the constraint on the number of light neutrino species (${N}_{\ensuremath{\nu}}$) 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\ifmmode\pm\else\textpm\fi{}0.004 ($1\ensuremath{\sigma}$) 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 ${N}_{\ensuremath{\nu}}=2.1\ifmmode\pm\else\textpm\fi{}0.3$ ($1\ensuremath{\sigma}$) and the upper limit is ${N}_{\ensuremath{\nu}}<2.6$ (95% C.L.). The data are inconsistent with the standard model (${N}_{\ensuremath{\nu}}=3$) at the 98.6% C.L.