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A new evaluation of the constraint on the number of light neutrino species (N_) 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. 0140. 004 (1) 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_=2. 10. 3 (1) and the upper limit is N_<2. 6 (95% C. L. ). The data are inconsistent with the standard model (N_=3) at the 98. 6% C. L.
Hata et al. (Mon,) studied this question.