Future Cosmic Microwave Background Constraints to the Baryon Density
  We discuss what can be learned about the baryon density from an all-sky map of the cosmic microwave background (CMB) with sub-degree angular resolution. With only minimal assumptions about the primordial spectrum of density perturbations and the values of other cosmological parameters, such a CMB map should be able to distinguish between a Universe with a baryon density near 0.1 and a baryon-dominated Universe. With additional reasonable assumptions, it is conceivable that such measurements will constrain the baryon density to an accuracy similar to that obtained from BBN calculations. 
Marc Kamionkowski (Columbia), Gerard Jungman (Syracuse), Arthur   Kosowsky (Harvard), and David N. Spergel (Princeton and Ma...