Ruling out a critical density baryonic universe

It has been suggested by Bartlett and Hall that our universe may have the critical density in baryons by virtue of specific interactions with a `shadow' world. We show that this possibility is severely constrained by primordial nucleosynthesis, stellar evolution and the thermalization of the cosmic microwave background. In particular, recent observations of small angular-scale anisotropy in the cosmic microwave background conclusively rule out all such baryon-dominated cosmologies.