Cosmic microwave background constraints on a baryonic dark matter-dominated universe

In recent years, upper limits on the cosmic microwave background (CMB) anis otropies, combined with predictions made by theories of galaxy formation, h ave been extremely powerful in ruling out purely baryonic dark matter (BDM) universes. However, it has recently been argued that the absence of a prom inent second peak in the anisotropy spectrum measured by the BOOMERanG-98 a nd MAXIMA-1 experiments seems to favor a Lambda BDM model when combined wit h standard big bang nucleosynthesis (BBN) constraints. In this Letter, we f urther investigate this result, showing that by using the CMB data alone, a purely baryonic adiabatic model of structure formation seems unlikely if t he universe is flat (Omega = 1). Combining the CMB data with Type Ia supern ova data renders purely baryonic models inconsistent with flatness at high significance and more than 3 sigma away from both the BBN constraints and t he Hubble Space Telescope Key Project result of h = 0.72 +/- 0.08. These re sults indicate that only a radical revision of cosmology with ad hoc proper ties could bring baryonic models such as those advocated by modified Newton ian dynamics enthusiasts back into agreement with current observations.