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The Cosmic Background Imager (or CBI) is a 13-element interferometer perched at an elevation of 5,000 metres at Llano de Chajnantor Observatory in the Chilean Andes. It is used to study the cosmic microwave background radiation. It was one of the first experiments to accurately measure small angular-scale fluctuations in the microwave background; it has also detected fluctuations in the polarization of the microwave background. Its measurements are conducted at frequencies between 26 and 36 GHz in ten bands of 1Ghz bandwidth. It has a resolution of better than 1/10 of a degree, in comparison the pioneering COBE satellite, which produced the first detection of fluctuations in the microwave background in 1992, had a resolution of about 7 degrees. Among the key findings of the CBI is the fact that fluctuations which have a small size on the sky are weaker than fluctuations which have a large size on the sky, which confirmed earlier theoretical predictions.

The CBI was built at the California Institute of Technology, and employed sensitive radio amplifiers from the National Radio Astronomy Observatory; two similar experiments are the Very Small Array, operated on the island of Tenerife, and the Degree Angular Scale Interferometer, operated in Antarctica. Both of these experiments used radio interferometry to measure CMB fluctuations at lower resolution over larger areas of the sky. Another experiment operated from Antarctica, the Arcminute Cosmology Bolometer Array Receiver, used total power (bolometric) detection and a single antenna at higher frequency and similar angular resolution to obtain results comparable results to the CBI. The confluence of these and other CMB experiments employing different measurement techniques in recent years is a great triumph of observational cosmology.

• Very Small Array
• Wilkinson Microwave Anisotropy Probe
• Degree Angular Scale Interferometer

• The Caltech-Chile connection
• Official CBI Web Page