Cosmic microwave background experiments

[
Cosmic background (CMB) 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.]

There have been a variety of experiments to measure the Cosmic microwave background radiation anisotropies and polarization since its first observation in 1964 by Penzias and Wilson. These include a mix of ground-, balloon- and space-based receivers. The most notable of these are COBE, which first detected the temperature anisotropies of the CMB, and showed that it had a black body spectrum; CBI, DASI, which first detected the polarization signal from the CMB and WMAP, which has provided the best full-sky CMB maps to date. Planned future experiments include the Planck satellite, which aims to produce high-resolution all-sky maps of both the temperature anisotropies and polarization signals, and various ground-based experiments primarily intended to investigate small-scale anisotropies and trying to detect the polarization caused by gravitational waves in the early universe.

History

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 in 1999 experimenters working at Cerro Toco in Chile 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 (CBI). DASI made the first detection of the polarization of the CMB and the CBI provided the first E-mode spectrum with compelling evidence that it is out of phase with the T-mode spectrum..

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 July 31, 2008 [http://www.rssd.esa.int/index.php?project=PLANCK&page=index] . 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, Atacama Cosmology Telescope and the QUIET telescope in Chile will provide additional data not available from satellite observations, possibly including the B-mode polarization.

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.

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.

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. [cite journal | author = A. de Oliveira-Costa, M. Tegmark, M. Zaldarriga and A. Hamilton | title = The significance of the largest scale CMB fluctuations in WMAP | journal = Phys. Rev. | volume = D69 | year=2004|pages=063516 arxiv|archive=astro-ph|id=0307282. cite journal | author = D. J. Schwarz, G. D. Starkman, D. Huterer and C. J. Copi |title = Is the low-"l" microwave background cosmic?|journal=Phys. Rev. Lett.|volume=93|year=2004|pages=221301 |doi = 10.1103/PhysRevLett.93.221301 arxiv|archive=astro-ph|id=0403353. cite journal | author = P. Bielewicz, K. M. Gorski and A. J. Banday | title = Low-order multipole maps of CMB anisotropy derived from WMAP | journal = Mon. Not. Roy. Astron. Soc. | volume = 355 | year = 2004 | pages =1283 | doi = 10.1111/j.1365-2966.2004.08405.x arxiv|archive=astro-ph|id=0405007.] 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. [cite journal|author=C. L. Bennett "et al." (WMAP collaboration)|title=First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: preliminary maps and basic results|journal=Astrophysical Journal Supplement|volume=148|pages=1|year=2003|doi=10.1086/377253 arxiv|archive=astro-ph|id=0302207. cite journal|author =G. Hinshaw "et al." (WMAP collaboration)|title = Three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: temperature analysis|year=2006|month=March|journal=preprint|url=http://lambda.gsfc.nasa.gov/product/map/current/map_bibliography.cfm cite journal|author=M. Tegmark, A. de Oliveira-Costa and A. Hamilton|title=A high resolution foreground cleaned CMB map from WMAP|journal=Phys. Rev.|volume=D68|pages=123523|year=2003 arxiv|archive=astro-ph|id=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."] 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. A full Bayesian analysis of the WMAP power spectrum demonstrates that the quadrupole prediction of Lambda-CDM cosmology is consistent with the data at the 10% level and that the octupole is not remarkable [cite journal | author = I. O'Dwyer et al. | title = Bayesian Power Spectrum Analysis of the First-Year Wilkinson Microwave Anisotropy Probe Data | journal = Astrophys. J. Lett | volume = 617 | pages = L99–L102 | year = 2004 | doi = 10.1086/427386 arxiv | archive = astro-ph | id=0407027] . Carefully accounting for the procedure used to remove the foregrounds from the full sky map further reduces the significance of the alignment by ~5%. [cite journal | author = A. Slosar and U. Seljak | title = Assessing the effects of foregrounds and sky removal in WMAP | journal = Phys. Rev. | volume = D70 | pages = 083002 | year = 2004 arxiv|archive=astro-ph|id=0404567. cite journal |author = P. Bielewicz, H. K. Eriksen, A. J. Banday, K. M. Gorski and P. B. Lilije | title = Multipole vector anomalies in the first-year WMAP data: a cut-sky analysis | journal = Astrophys. J. | volume = 635 | year = 2005 | pages = 750–60 | doi = 10.1086/497263 arxiv|archive=astro-ph|id=0507186. cite journal | author = C. J. Copi, D. Huterer, D. J. Schwarz and G. D. Starkman | title = On the large-angle anomalies of the microwave sky | journal = Mon. Not. Roy. Astron. Soc. | volume = 367 | pages = 79–102 | year = 2006 | doi = 10.1111/j.1365-2966.2005.09980.x arxiv|archive=astro-ph|id=0508047. cite journal| author = A. de Oliveira-Costa and M. Tegmark | title = CMB multipole measurements in the presence of foregrounds | year = 2006 | journal = preprint arxiv|archive=astro-ph|id=0603369.]

List of experiments

The following is a list of all past, and currently planned (as of 2007), experiments to measure the Cosmic Microwave Background.cite web | url=http://lambda.gsfc.nasa.gov/links/experimental_sites.cfm | title=LAMBDA - CMB Experiment Sites | accessdate=2007-10-04] In general, each experiment provided improved data quality when compared with previous experiments, or looked at a different component of the CMB.

Colour
:legend|#FFFFCC|Space based experiments|border=1px solid #AAAAAA:legend|#CCFFCC|Balloon based experiments|border=1px solid #AAAAAA:legend|#f9f9f9|Ground based experiments|border=1px solid #AAAAAA

References