Big Bang Nucleosynthesis in Crisis?

A new evaluation of the constraint on the number of light neutrino species (N_ν) from big bang nucleosynthesis suggests a discrepancy between the predicted light element abundances and those inferred from observations, unless the inferred primordial ⁴He abundance has been underestimated by 0.014±{}0.004 (1σ) or less than 10% (95% C.L.) of ³He 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_ν=2.1±0.3 (1σ) and the upper limit is N_ν<2.6 (95% C.L.). The data are inconsistent with the standard model (N_ν=3) at the 98.6% C.L.