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Since the baryon-to-photon ratio η10 is in some doubt at present, we ignore the constraints on η10 from big bang nucleosynthesis (BBN) and fit the three key cosmological parameters (h, ΩM, η10) to four other observational constraints: Hubble parameter (ho), age of the universe (to), cluster gas (baryon) fraction (fo ≢ fGh3/2), and effective shape parameter (Γo). We consider open and flat CDM models and flat ΛCDM models, testing goodness of fit and drawing confidence regions by the Δχ2 method. CDM models with ΩM = 1 (SCDM models) are accepted only because we allow a large error on ho, permitting h < 0.5. Open CDM models are accepted only for ΩM ≳ 0.4. ΛCDM models give similar results. In all of these models, large η10 (≳ 6) is favored strongly over small η10 (≲ 2), supporting reports of low deuterium abundances on some QSO lines of sight, and suggesting that observational determinations of primordial 4He may be contaminated by systematic errors. Only if we drop the crucial Γo constraint are much lower values of ΩM and η10 permitted.

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