Cosmic microwave background experiments
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The discovery of the cosmic microwave background was the serendipitous result of an experiment by Arno Penzias and Robert Woodrow Wilson at Bell Telephone Laboratories in 1964. See discovery of the cosmic microwave background. The most famous experiment is probably the NASA Cosmic Background Explorer (COBE) satellite that orbited in 1989–1996 and which first detected the large scale anisotropies (other than the dipole). Inspired by the COBE results, a series of ground- and balloon-based experiments measured CMB anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the angular scale of the first acoustic peak, for which COBE did not have sufficient resolution. These measurements were able to rule out cosmic strings as the leading theory of cosmic structure formation, and suggested cosmic inflation was the right theory. During the 1990's, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment reported that the highest power fluctuations occur at scales of apporoximately one degree. Together with other cosmological data, these results implied that the geometry of the Universe is flat. A number of ground-based interferometers provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array, Degree Angular Scale Interferometer (DASI) and the Cosmic Background Imager. In fact, DASI made the first detection of the polarization of the CMB.
In June 2001, NASA launched a second CBR space mission, WMAP, to make much more precise measurements of the large scale anisotropies over the full sky. The first results from this mission, disclosed in 2003, were detailed measurements of the angular power spectrum to below degree scales, tightly constraining various cosmological parameters. The results are broadly consistent with those expected from cosmic inflation as well as various other competing theories, and are available in detail at NASA's data center for Cosmic Microwave Background (CMB) (see links below). Although WMAP provided very accurate measurements of the large angular-scale fluctuations in the CMB (structures about as large in the sky as the moon), it did not have the angular resolution to measure the smaller scale fluctuations which had been observed using previous ground-based interferometers.
A third space mission, the Planck Surveyor, is to be launched in 2007. Planck employs both HEMT radiometers as well as bolometer technology and will measure the CMB on smaller scales than WMAP. Unlike the previous two space missions, Planck is a collaboration between NASA and ESA (the European Space Agency). Its detectors got a trial run at the Antarctic Viper telescope as ACBAR (Arcminute Cosmology Bolometer Array Receiver) experiment – which has produced the most precise measurements at small angular scales to date – and at the Archeops balloon telescope.
Additional ground-based instruments such as the South Pole Telescope in Antarctica and the proposed Clover Project and Atacama Cosmology Telescope in Chile will provide additional data not available from satellite observations, possibly including the B-mode polarization.
[edit] List of experiments in approximate chronological order
Each experiment provided improved data quality when compared with previous experiments.
- RELIKT-1 - a Russian CMB anisotropy experiment onboard the Prognoz 9 satellite (launched 1 July 1983) first gave only upper limits on the large-scale anisotropy, but reanalysis of the data in 1992 claimed a signal roughly compatible with the later experiments
- Cosmic Background Explorer - measured the very large scale fluctuations
- Tenerife Experiment - A set of three intermediate scale microwave radiometers based in Tenerife.
- Saskatoon experiment - an experiment in Saskatchewan
- Cosmic Anisotropy Telescope - measured the very small scale fluctuations in small regions of the sky
- MAXIMA - measured intermediate scale fluctuations with improved precision
- BOOMERanG experiment - measured intermediate scale fluctuations with improved precision
- BEAST - A ground-based single dish CMB observatory at the University of California's White Mountain Research station.
- Archeops - measured large and intermediate scale with improved precision at the larger scales
- Cosmic Background Imager - measured the very small scale fluctuations with improved precision in small regions of the sky
- COSMOSOMAS - Circular scanning experiments for CMB and foregrounds in Tenerife.
- Very Small Array - measured intermediate and small scale fluctuations with improved precision in small regions of the sky
- Degree Angular Scale Interferometer - a temperature and polarization telescope at the South Pole
- Arcminute Cosmology Bolometer Array Receiver - measured intermediate and small scale fluctuations with improved precision
- Wilkinson Microwave Anisotropy Probe - measured intermediate and large scale fluctuations with improved precision
- QuaD (ongoing) - measured intermediate scale polarization with improved precision (South Pole).
- [Robinson Gravitational Wave Background Telescope (formerly BICEP)] (dec 2005) - measured large scale polarization with improved precision (South Pole).
- Atacama Pathfinder Experiment /SZ - (2005/2006) new telescope, prototype of ALMA, will be used partly to measure small scale fluctuations -part of the APEX experiment which will measure the CMB small scale fluctuations, mainly produce by Sunyaev-Zel'dovich effect (SZ effect), for more information see http://bolo.berkeley.edu/apexsz
- Atacama Cosmology Telescope - (2006) new telescope for measuring the small scale fluctuations being built in the Atacama Desert in Chile
- South Pole Telescope - (2006) a new telescope for measuring the small scale fluctuations and polarization, located at the South Pole
- SPIDER (2009?) - balloon-borne, will measure very large scale polarization.
- Clover Project - (2008?) - improved precision for small scale fluctuations and B-mode polarization measurements
- Planck - (2009?) - will give improved precision and polarization data at all scales
