  Polarization observations of the cosmic microwave background with the Cosmic
Background Imager from September 2002 to May 2004 provide a significant
detection of the E-mode polarization and reveal an angular power spectrum of
polarized emission showing 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
based on the total intensity spectrum provides 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.
