Constraints from the damping tail

The detection of anisotropy in the cosmic microwave background (CMB) on arcminute scales by the Cosmic Background Imager (CBI) provides us with our first measurement of the damping tail and closes one chapter in the CMB story. We now have experimental verification for all of the features in the temperature anisotropy spectrum predicted theoretically two decades ago. The CBI result allows us to constrain both parameterized models based on the inflationary cold dark matter paradigm and to examine model-independent constraints on the matter content, the distance to last scattering, and the thickness of the last scattering surface. In particular, we show that recombination had to proceed slowly, with the surface of last scattering having a width Deltaz greater than or similar to 50. This provides strong constraints on nonstandard recombination scenarios. By providing a lower limit on the duration of recombination, it implies a lower limit on the polarization of the subdegree scale anisotropy, which is close to current experimental upper limits.