  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.
