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 Cl in the range l = 400-1500. The broadband amplitudes we have measured are δTband = 58.7 μK for l = 603 and δTband = 29.7 μK for l = 1190, where these are half-power widths in l. Such a decrease in power at high l is one of the fundamental predictions of the standard cosmological model, and these are the first observations which cover a broad enough l range to show this decrease in a single experiment. The Cl we have measured enables us to place limits on the density parameter, Ωtot ≤ 0.4 or Ωtot ≥ 0.7 (90% confidence).