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{{Short description|Interferometer at Llano de Chajnantor Observatory in Chile}}
{{Infobox telescope
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[[Image:Cosmic Background Imager- CMB.png|right|220px|thumb|CMB as measured by the CBI experiment]]

The '''Cosmic Background Imager''' (or '''CBI''') was a 13-element [[interferometer]] perched at an elevation of 5,080 metres (16,700 feet) at [[Llano de Chajnantor Observatory]] in the [[Chile]]an [[Andes]].  It started operations in 1999 to study the [[cosmic microwave background radiation]] and ran until 2008.

CBI conducted measurements at frequencies between 26 and 36 [[GHz]] in ten bands of 1&nbsp;GHz [[Bandwidth (signal processing)|bandwidth]]. It had a resolution of better than 1/10 of a degree. (In comparison, the pioneering [[Cosmic Background Explorer|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.  More technically, CBI was the first experiment to detect intrinsic anisotropy in the [[microwave]] background on mass scales of galaxy clusters; it provided the first detection of the [[Joseph Silk|Silk damping]] tail; it found a hint of excess power at high-l multipoles (CBI-excess) than expected from the [[Lambda-CDM model|ΛCDM]] model; and it detected fluctuations in the [[Polarization (waves)|polarization]] of the microwave background obtaining the first detailed E-mode polarization spectrum providing evidence that it is out of phase with the total intensity mode spectrum.

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 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.

CBI was a collaboration among a number of institutions in the US and Europe. It still closely collaborates with Chilean institutions [[Universidad de Chile]] and [[University of Concepción|Universidad de Concepción]] through the Chajnantor Observatory.

In 2006, new 1.4 m antennas replaced the old 0.9 m dishes for more high-resolution studies in total intensity mode. During this stage, CBI was called '''CBI-2'''.

In June 2008, CBI-2 stopped the observations and the 13-antenna instrument was removed from its mount. The new [[QUIET telescope]] instrument was installed in August 2008 on the CBI mount, replacing CBI-2 .

== See also ==
{{commons category|Cosmic Background Imager}}
* [[List of telescope types]]
* [[Very Small Array]]
* [[Wilkinson Microwave Anisotropy Probe]]
* [[Degree Angular Scale Interferometer]]

== External links ==
* [http://www.astro.caltech.edu/chajnantor/ Caltech Chajnantor Observatory]
* [http://pr.caltech.edu/periodicals/EandS/articles/LXVII4/chile.html The Caltech-Chile connection]
* [http://www.astro.caltech.edu/~tjp/CBI/ Official Web Page]

{{CMB experiments}}
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[[Category:Radio telescopes]]
[[Category:Astronomical observatories in Chile]]
[[Category:Interferometric telescopes]]
[[Category:Buildings and structures in Antofagasta Region]]
[[Category:Cosmic microwave background experiments]]
[[Category:Science and technology in Chile]]