Cosmic microwave background experiments

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See also: Discovery of the cosmic microwave background

Cosmic background radiation temperature on the celestial sphere as determined with the COBE satellite, (top) uncorrected, (middle) corrected for the dipole term due to our peculiar velocity, (bottom) corrected for contributions from the dipole term and from our galaxy.

Observation of the cosmic microwave background (CMB) were first made by Arno Penzias and Robert Woodrow Wilson at Bell Telephone Laboratories in 1964. Subsequently, hundreds of cosmic microwave background experiments have been conducted to measure and characterize the signatures of the radiation. The most famous experiment is probably the NASA Cosmic Background Explorer (COBE) satellite that orbited in 1989–1996 and which detected and quantified the large scale anisotropies at the limit of its detection capabilities. Inspired by the initial COBE results of an extremely isotropic and homogeneous background, a series of ground- and balloon-based experiments quantified 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 CMB 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 (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] Design

The design of cosmic microwave background experiments is a very challenging task. The greatest problems are:

• Detectors The challenge of observing differences of a few microkelvins on top of a 2.7 K signal is difficult. Many improved microwave detector technologies have been designed for microwave background applications. Some technologies used are HEMT, MMIC, SIS (Superconductor-Insulator-Superconductor) and bolometers. Experiments generally use elaborate cryogenic systems to keep the detectors cool. Often, experiments are interferometers which only measure the spatial fluctuations in signals on the sky, and are insensitive to the average 2.7 K background. Another problem is the 1/f noise intrinsic to all detectors. Usually the experimental scan strategy is designed to minimize the effect of such noise.
• Optics To minimize side lobes, microwave optics usually utilize elaborate lenses and feed horns.
• Water vapor Because water absorbs microwave radiation (a fact utilized in the operation of microwave ovens), it is rather difficult to observe the microwave background with ground-based instruments. CMB research therefore makes increasing use of air and space-borne experiments. Ground-based observations are usually made from dry, high altitude locations such as the Chilean Andes and the South Pole.

[edit] Analyses

The analysis of cosmic microwave background data to produce maps, an angular power spectrum and ultimately cosmological parameters is a complicated, computationally difficult problem. Although computing a power spectrum from a map is in principle a simple Fourier transform, decomposing the map of the sky into spherical harmonics, in practice it is hard to take the effects of noise and foregrounds into account. Constraints on many cosmological parameters can be obtained from their effects on the power spectrum, and results are often calculated using Markov Chain Monte Carlo sampling techniques.

[edit] Low multipoles

With the increasingly precise data provided by WMAP, there have been a number of claims that the CMB suffers from anomalies, such as non-gaussianity. The most longstanding of these is the low-l multipole controversy. Even in the COBE map, it was observed that the quadrupole (l = 2 spherical harmonic) has a low amplitude compared to the predictions of the big bang. Some observers have pointed out that the anisotropies in the WMAP data did not appear to be consistent with the big bang picture. In particular, the quadrupole and octupole (l = 3) modes appear to have an unexplained alignment with each other and with the ecliptic plane.^[1] A number of groups have suggested that this could be the signature of new physics at the largest observable scales. Ultimately, due to the foregrounds and the cosmic variance problem, the largest modes will never be as well measured as the small angular scale modes. The analyses were performed on two maps that have had the foregrounds removed as best as is possible: the "internal linear combination" map of the WMAP collaboration and a similar map prepared by Max Tegmark and others.^[2] Later analyses have pointed out that these are the modes most susceptible to foreground contamination from synchrotron, dust and free-free emission, and from experimental uncertainty in the monopole and dipole. While the low quadrupole does appear to be robust (The measured value has a likelihood of roughly 2–4% in the Lambda-CDM model.), carefully accounting for the procedure used to remove the foregrounds from the full sky map reduces the significance of the alignment, and may suggest that it is due to foreground contamination.^[3]

[edit] List of experiments in approximate chronological order

The power spectrum of the cosmic microwave background radiation anisotropy in terms of the angular scale (or multipole moment). The data shown come from the WMAP (2006), Acbar (2004) Boomerang (2005), CBI (2004) and VSA (2004) instruments. Also shown is a theoretical model (solid line).

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

[edit] See also

• List of experiments at NASA's LAMBDA archive
• Space missions at LAMBDA

[edit] References

1. ^ A. de Oliveira-Costa, M. Tegmark, M. Zaldarriga and A. Hamilton (2004). "The significance of the largest scale CMB fluctuations in WMAP". Phys. Rev. D69: 063516.  arXiv:astro-ph/0307282. D. J. Schwarz, G. D. Starkman, D. Huterer and C. J. Copi (2004). "Is the low-l microwave background cosmic?". Phys. Rev. Lett. 93: 221301.  arXiv:astro-ph/0403353. P. Bielewicz, K. M. Gorski and A. J. Banday (2004). "Low-order multipole maps of CMB anisotropy derived from WMAP". Mon. Not. Roy. Astron. Soc. 355: 1283.  arXiv:astro-ph/0405007.
2. ^ C. L. Bennett et al. (WMAP collaboration) (2003). "First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: preliminary maps and basic results". Astrophysical Journal Supplement 148: 1.  arXiv:astro-ph/0302207. G. Hinshaw et al. (WMAP collaboration) (March 2006). "Three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: temperature analysis". preprint.  M. Tegmark, A. de Oliveira-Costa and A. Hamilton (2003). "A high resolution foreground cleaned CMB map from WMAP". Phys. Rev. D68: 123523.  arXiv:astro-ph/0302496. The first year WMAP paper warns: "the statistics of this internal linear combination map are complex and inappropriate for most CMB analyses." The third year paper states: "Not surprisingly, the two most contaminated multipoles are [the quadrupole and octopole], which most closely trace the galactic plane morphology."
3. ^ A. Slosar and U. Seljak (2004). "Assessing the effects of foregrounds and sky removal in WMAP". Phys. Rev. D70: 083002.  arXiv:astro-ph/0404567. P. Bielewicz, H. K. Eriksen, A. J. Banday, K. M. Gorski and P. B. Lilije (2005). "Multipole vector anomalies in the first-year WMAP data: a cut-sky analysis". Astrophys. J. 635: 750–60.  arXiv:astro-ph/0507186. C. J. Copi, D. Huterer, D. J. Schwarz and G. D. Starkman (2006). "On the large-angle anomalies of the microwave sky". Mon. Not. Roy. Astron. Soc. 367: 79–102.  arXiv:astro-ph/0508047. A. de Oliveira-Costa and M. Tegmark (2006). "CMB multipole measurements in the presence of foregrounds". preprint.  arXiv:astro-ph/0603369.