The Existence of Baryons at z=1000

Abstract

Fluctuations in the CMB have now been detected over a wide range of angular scales, and a consistent picture seems to be emerging. The data cannot currently constrain a large number of cosmological parameters, but it is clear that there is more information than just the normalization of the models. Here we use the data to constrain a second parameter, namely the amplitude of the Doppler peak, using a phenomenological approach to the power spectrum. We find that the data prefer a peak of height ~3, with a purely flat spectrum ruled out at the 95%CL. Although there are concerns about possible foregrounds and non-Gaussian fluctuations, we believe that the existence of a peak at degree-scales is established by the data. This immediately implies that reionization was unimportant for the CMB. It also potentially leads to difficulties for models where the fluctuations were produced by topological defects. Independent constraints on Omega_B, on the slope n, etc. will need to wait for further data. At the moment, the simple presence of a Doppler peak should be seen as strong supporting evidence for standard dark matter-dominated models with some few percent of baryons at z=1000.