We have used recent observations of helium-4, nitrogen, and oxygen from some four dozen, low-metallicity, extragalactic H II regions to define mean N vs. O, 4He vs. N, and 4He vs. O relations which are extrapolated to zero metallicity to determine the primordial 4He mass fraction YP. The data and various subsets of the data, selected on the basis of nitrogen and oxygen, are all consistent with YP = 0.232 ± 0.003. For the 2 σ (statistical) upper bound we find YP2 σ ≤ 0.238. Estimating a 2% systematic uncertainty (σsyst = ± 0.005) leads to a maximum upper bound to the primordial helium mass fraction: YPMAX = YP2 σ + σsyst ≤ 0.243. We compare these upper bounds to YP with recent calculations of the predicted yield from big bang nucleosynthesis to derive upper bounds to the nucleon-to-photon ratio η(η10 ≡ 1010η) and the number of equivalent light (≲10 MeV) neutrino species. For Yp ≤ 0.238 (0.243), we find η10 ≤ 2.5(3.9) and Nv ≤ 2.7(3.1). If indeed YP ≤ 0.238, then BBN predicts enhanced production of deuterium and helium-3 which may be in conflict with the primordial abundances inferred from model-dependent (chemical evolution) extrapolations of solar system and interstellar observations. Better chemical evolution models and more data - especially D-absorption in the QSO Ly-α clouds - will be crucial to resolve this potential crisis for BBN. The larger upper bound, YP ≤ 0.243, is completely consistent with BBN which, now, bounds the universal density of nucleons ( for Hubble parameter 40 ≤ Ho ≤ 100 km s-1 Mpc-1 and cosmic background radiation temperature T = 2.726 ± 0.010) to lie in the range 0.01 ≤ ΩBBN ≤ 0.09 (for Ho = 50 h50 km s-1 Mpc-1, 0.04 ≤ ΩBBN h502 ≤ 0.06).
- Cosmology: theory
- Nuclear reactions, nucleosynthesis, abundances