Cosmic microwave background (CMB) determinations of the baryon-to-photon ratio eta infinity Omega(baryon)h(2) will remove the last free parameter from (standard) big bang nucleosynthesis (BBN) calculations. This will make BBN a much sharper probe of early universe physics, for example, greatly refining the BBN measurement of the effective number of light neutrino species, We show how the CMB can improve this limit, given current light element data. Moreover, it will become possible to constrain N-r,N-eff independent of He-4, by using other elements, notably deuterium; this will allow for sharper limits and tests of systematics. For example, a 3% measurement of eta, together with a 10% (3%) measurement of primordial D/H, can measure N-v,N-eff to a 95% confidence level of sigma(95%)(N-v,N-eff) = 1.8 (1.0) if eta similar to 6.0 x 10(-10). If instead, one adopts the standard model value N-v,N-eff = 3, then one can use eta (and its uncertainty) from the CMB to make accurate predictions for the primordial abundances. These determinations can in turn become key inputs in the nucleosynthesis history (chemical evolution) of galaxies thereby placing constraints on such models. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 48]
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