Cosmic microwave background (CMB) determinations of the baryon-to-photon ratio η ∝ Ωbaryonh2 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, Nv,eff. We show how the CMB can improve this limit, given current light element data. Moreover, it will become possible to constrain Nv,eff independent of 4He, by using other elements, notably deuterium; this will allow for sharper limits and tests of systematics. For example, a 3% measurement of η, together with a 10% (3%) measurement of primordial D/H, can measure Nv,eff to a 95% confidence level of σ95%(Nv,eff) = 1.8 (1.0) if η ∼ 6.0 × 10-10. If instead, one adopts the standard model value Nv,eff = 3, then one can use η (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.
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We are grateful to the anonymous referee for suggestions which helped to clarify and improve this paper. The work of K.A.O. was partially supported by DOE grant DE-FG02-94ER-40823.