Anthony C.S. Readhead Timothy J. Pearson Steven Padin California Institute of Technology FY 98: $874,429 FY 99: $913,130 FY 00: $922,319 Cosmological Observations with the Cosmic Background Imager. The cosmic microwave background radiation (CMBR) is the relic radiation left over from the Big Bang with which the Universe began 15 billion years ago. Emitted just 300,000 years after the Big Bang, this radiation is the most ancient direct signal that we receive from the early Universe. It is believed that the important cosmological parameters - including the density parameter, the Hubble constant, and the cosmological constant - all left signatures on the CMBR and can be determined from sufficiently sensitive observations. In addition, the CMBR must contain clues to the origin of all structure in the Universe - from superclusters and clusters of galaxies down to galaxies themselves, and hence to stars and planets. The Cosmic Background Imager (CBI) is an instrument designed to make images of the CMBR and to measure its statistical properties on angular scales from 5 arc minutes to one degree (spherical harmonic scales from l = 3000 down to l = 300). Theoretical models suggest that these observations can be used to measure the fundamental cosmological parameters with some precision. The CBI is a 13-element interferometer mounted on a 6.5-meter platform operating in ten 1-GHz frequency bands from 26 GHz to 36 GHz. The CBI will be installed in Chile at an altitude of 5000 meters. A high, dry site is essential in order to reach the required sensitivity levels in a reasonable observing time. The instrument will be operated at the Chile site continuously over a 2 to 2.5 year period.
