BIG-BANG NUCLEOSYNTHESIS REVISITED

We compute the homogeneous big-bang nucleosynthesis yields of D, 3He, 4He, and 7Li, taking into account recent measurements of the neutron mean-life as well as updates of several nuclear reactions rates which primarily affect the production of 7Li. We discuss the extraction of primordial abundances from observation and the likelihood that the primordial mass fraction of 4He, Yp, is less than or equal to 0.24. Using the primordial abundances of D+3He and 7Li we limit the baryon-to-photon ratio (η in units of 10-10): 2.6 ≤ η10 ≤ 4.3 (or, in terms of the present mass density in baryons, 1.8×10-31 ≤ ρ{variant}B ≤ 3.0×10-31g/cm3, for a microwave background temperature of 2.75°) which we use to argue that baryons contribute between 0.02 and 0.11 to the critical energy density of the universe. An upper limit to Yp of 0.24 constrains the number of light neutrinos to Nν ≤ 3.4, in excellent agreement with the LEP and SLC collider results. We turn this argument around to show that the collider limit of 3 neutrino species can be used to bound the primordial abundance of 4He: 0.235 ≤ Yp ≤ 0.245. © 1990.