We have derived a primordial helium abundance of Y _ { p } = 0.2477 \pm 0.0029 , based on new atomic physics computations of the recombination coefficients of He i and of the collisional excitation of the H i Balmer lines together with observations and photoionization models of metal-poor extragalactic H ii regions .
The new atomic data increase our previous determination of Y _ { p } by 0.0086 , a very significant amount .
By combining our Y _ { p } result with the predictions made by the standard Big Bang nucleosynthesis model , we find a baryon-to-photon ratio , \eta , in excellent agreement both with the \eta value derived by the primordial deuterium abundance value observed in damped Lyman- \alpha systems and with the one obtained from the WMAP observations .