First intrinsic anisotropy observations with the cosmic background imager

We present the first results of observations of the intrinsic anisotropy of the cosmic microwave background radiation with the Cosmic Background Imager from a site at 5080 m altitude in northern Chile. Our observations show a sharp decrease in C-iota in the range iota = 400-1500. The broadband amplitudes we have measured are deltaT(band) = 58.7(-6.3)(+7.7) muK for iota = 603(-166)(+180) and deltaT(band) = 29.7(-4.2)(+4.8) muK for iota = 1190(-224)(+261), where these are half-power widths in iota. Such a decrease in power at high iota is one of the fundamental predictions of the standard cosmological model, and these are the first observations which cover a broad enough iota range to show this decrease in a single experiment. The C-iota we have measured enables us to place limits on the density parameter, Omega (tot) less than or equal to 0.4 or Omega (tot) greater than or equal to 0.7 (90% confidence).