Microwave anisotropy constraints on isocurvature baryon models

We calculate the microwave background anisotropies expected in baryon-dominated universes with power law initial entropy perturbations. If recombination of the Universe proceeds in the standard way, only models with $${\Omega }_\text{B}\sim 1$$ are compatible with the small angle anisotropy constraints. If the Universe remained ionized for an extended period, the constraints at $$\theta \sim {6}^{^{\circ} }$$ limit the initial spectral index to n> – 2. A modest improvement in the anisotropy limits at small angular scales could exclude all reasonable isocurvature baryon models whatever the recombination history. The coherence scale for the linear polarization of the temperature fluctuations would provide a test of early re-ionization.