The anisotropies of the cosmic microwave background radiation provide valuable diagnostics of the conditions in the early universe. The intensity fluctuations of the CMB have been measured with many instruments and provide one of the foundations of the standard Lambda-CDM model of cosmology. But recently a few instruments have successfully measured the much weaker polarization fluctuations of the CMB on scales of a few minutes of arc. I will present new polarization observations made with the Cosmic Background Imager (CBI), a 30-GHz interferometer array in the Chilean Andes.
The CBI results provide the most precise measurements yet of the angular power spectrum of polarization anisotropy. The power spectrum shows peaks and valleys that are shifted in phase by half a cycle relative to those of the total intensity spectrum. This key agreement between the phase of the observed polarization spectrum and that predicted on the basis of the total intensity spectrum provides additional support for the standard model of cosmology, in which dark matter and dark energy are the dominant constituents, the geometry is close to flat, and primordial density fluctuations are predominantly adiabatic with a matter power spectrum commensurate with inflationary cosmological models.