We consider Bayesian constraints on standard isocurvature baryon models from the slope and normalization of the anisotropy power spectrum detected by the COBE DMR experiment in their 2 year maps. In conjunction with either the amplitude of matter fluctuations σ_8 or its slope, all open models are ruled out at greater than 95% confidence, whereas cosmological-constant dominated models are constrained to be highly ionized. By including the COBE FIRAS 95% confidence upper limit on spectral distortion under the assumption of collisional ionization, we further reduce the available parameter space for A models by excluding these highly ionized models. These constraints define a single remaining class of standard models which makes definite and testable predictions for degree-scale anisotropies and large-scale structure.
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