Within the standard models of particle physics and cosmology we have calculated the big-bang prediction for the primordial abundance of 4 He to a theoretical uncertainty of less than 0.1 % (δY P < ±0.0002), improving the current theoretical precision by a factor of 10. At this accuracy the uncertainty in the abundance is dominated by the experimental uncertainty in the neutron mean lifetime, τ n = 885.4±2.0 sec. The following physical effects were included in the calculation: the zero and finite-temperature radiative, Coulomb and finite-nucleon-mass corrections to the weak rates; order-α quantum-electrodynamic correction to the plasma density, electron mass, and neutrino temperature; and incomplete neutrino decoupling. New results for the finite-temperature radiative correction and the QED plasma correction were used. In addition, we wrote a new and independent nucleosynthesis code designed to control numerical errors to be less than 0.1%. Our predictions for the 4 He abundance are presented in the form of an accurate fitting formula. Summarizing our work in one number, Y P (η = 5 × 10 −10 ) = 0.2462 ± 0.0004 (expt) ± < 0.0002 (theory). Further, the baryon density inferred from the Burles-Tytler determination of the primordial D abundance, Ω B h 2 = 0.019 ± 0.001, leads to the prediction: Y P = 0.2464 ± 0.0005 (D/H) ± < 0.0002 (theory) ± 0.0005 (expt). This "prediction" and an accurate measurement of the primeval 4 He abundance will allow an important consistency test of primordial nucleosynthesis.

In the standard Big Bang cosmology the canonical value for the ratio of relic neutrinos to CMB photons is 9/11. Within the framework of the Standard Model of particle physics there are small corrections, in sum about 1%, due to slight heating of neutrinos by electron/positron annihilations and finite-temperature QED effects. We show that this leads to changes in the predicted cosmic microwave background (CMB) anisotropies that might be detected by future satellite experiments. NASA's MAP and ESA's PLANCK should be able to test the canonical prediction to a precision of 1% or better and could confirm these corrections.Comment: 8 pages + 3 figure

We compute primordial light-element abundances for cases with fine structure constant α different from the present value, including many sources of α dependence neglected in previous calculations. Specifically, we consider contributions arising from Coulomb barrier penetration, photon coupling to nuclear currents, and the electromagnetic components of nuclear masses. We find the primordial abundances to depend more weakly on α than previously estimated, by up to a factor of 2 in the case of 7 Li. We discuss the constraints on variations in α from the individual abundance measurements and the uncertainties affecting these constraints. While the present best measurements of primordial D/H, 4 He/H, and 7 Li/H may be reconciled pairwise by adjusting α and the universal baryon density, no value of α allows all three to be accommodated simultaneously without consideration of systematic error. The combination of measured abundances with observations of acoustic peaks in the cosmic microwave background favors no change in α within the uncertainties.PACS numbers: 26.35.+c,98.80Ft,98.80.Es

A massive neutrino which decays after recombination (tу10 13 sec) into relativistic decay products produces an enhanced integrated Sachs-Wolfe effect, allowing constraints to be placed on such neutrinos from present cosmic microwave background anisotropy data. Previous treatments of this problem have approximated the decay products as an additional component of the neutrino background. This approach violates energy-momentum conservation, and we show that it leads to serious errors for some neutrino masses and lifetimes. We redo this calculation more accurately, by correctly incorporating the spatial distribution of the decay products. For low neutrino masses and long lifetimes, we obtain a much smaller distortion in the CMB fluctuation spectrum than have previous treatments. We combine these new results with a recent set of CMB data to exclude the mass and lifetime range m h Ͼ100 eV, Ͼ10 12 sec. Masses as low as 30 eV are excluded for a narrower range in lifetime. ͓S0556-2821͑99͒04824-9͔