Constraining the early-Universe baryon density and expansion rate

We explore the constraints on those extensions to the standard models of cosmology and particle physics which modify the early-Universe, radiation dominated, expansion rate S equivalent to H '/H ( parameterized by the effective number of neutrinos N-nu). The constraints on S( N-nu) and the baryon density parameter eta B equivalent to ( n(B)/ n(gamma)) = 10(-10) eta(10), derived from big bang nucleosynthesis ( BBN, t similar to 20 min), are compared with those inferred from the cosmic microwave background anisotropy spectrum ( CMB, t similar to 400 kyr) and large scale structure ( LSS, t similar to 14 Gyr). At present, BBN provides the strongest constraint on N-nu ( N-nu = 2.4 +/- 0.4 at 68% confidence), but a weaker constraint on the baryon density. In contrast, while the CMB/LSS data best constrain the baryon density (eta(10) = 6.1(-0.1)(+ 0.2) at 68% confidence), independent of N-nu, at present they provide a relatively weak constraint on N-nu which is, however, consistent with the standard value of N-nu = 3. When the best. t values and the allowed ranges of these CMB/LSS-derived parameters are used to calculate the BBN-predicted primordial abundances, there is excellent agreement with the observationally inferred abundance of deuterium and good agreement with He-4, confirming the consistency between the BBN and CMB/LSS results. However, the BBN-predicted abundance of Li-7 is high, by a factor of 3 or more, if its observed value is uncorrected for possible dilution, depletion, or gravitational settling. We comment on the relation between the value of N-nu and a possible anomaly in the matter power spectrum inferred from observations of the Ly-alpha forest. Comparing our BBN and CMB/LSS results permits us to constrain any post-BBN entropy production as well as the production of any non-thermalized relativistic particles. The good agreement between our BBN and CMB/LSS results for N-nu and eta(B) permits us to combine our constraints finding, at 95% confidence, 1.8 < N-nu < 3.2 and 5.9 < eta(10) < 6.4.