Cosmic microwave background radiation

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Template:Cosmology In cosmology, the cosmic microwave background radiation (CMB) is a form of electromagnetic radiation discovered in 1964 that radiates throughout the universe in the microwave range.

1 Features
2 CMB and the Big Bang
3 Detection, prediction and discovery
4 Experiments
- 4.1 List of experiments in approximate chronological order
5 See also
6 Bibliography
7 References and external links

Features

The principal feature of the CMB is how closely it matches a black body spectrum. Although the temperature of the CMB varies from point to point (i.e. it contains small anisotropies), the spectrum in a particular direction almost exactly resembles a black body. The CMB radiation frequency ranges from 0.3 GHz to 630 GHz, and peaks at 160.4 GHz, corresponding to a temperature of 2.725 kelvins.

There are however very small yet significant variations (anisotropies) from the black body spectrum. The most pronounced is the dipole anisotropy (180 degree scales) which is at a level of about 10⁻³ of the monopole. This feature is consistent with the Earth moving at some 600 km/s relative to the CMB towards the constellation Virgo. This does not vary from season to season.

Variations due to external physics also exist; the Sunyaev-Zel'dovich effect is one of the major factors here, in which a cloud of high energy electrons scatters the radiation, transferring some energy to the CMB photons.

Even more interesting are anisotropies at a level of roughly 10⁻⁵ on scales of roughly tens of arcminutes to several degrees. These very small variations are the result of the Sachs-Wolfe effect which causes photons from the cosmic microwave background to be gravitationally redshifted. According to inflationary theory, the origin of the variations is quantum fluctuations which expand during inflation and result in primordial fluctuations. The angular power spectrum of these variations (in terms of amplitudes of component multipole moments) can be calculated and produces a number of peaks and valleys. The location of these peaks and valleys can be correlated with cosmological parameters such as the Hubble constant, and the geometry of the universe.

CMB and the Big Bang

This radiation, as well as the red shift, are regarded as the best available evidence of the Big Bang (BB) theory. The discovery of this radiation in the mid-1960s curtailed interest for alternatives such as the steady state theory. The CMB gives a snapshot of the Universe when, according to standard cosmology, the temperature dropped enough to allow electrons and protons to form hydrogen atoms, thus making the universe transparent to radiation. When it originated some 400,000 years after the Big Bang -- this time period is generally known as the "time of last scattering" or the period of recombination or decoupling -- the temperature of the Universe was about 3000 K. Since then the temperature of the radiation has dropped by a factor of roughly 1100 due to the expansion of the Universe. As the universe expands, the CMB photons are redshifted, cooling the radiation inversely proportional to the Universe's scale length. For details on reasoning that the radiation is used as evidence of the Big Bang, see Cosmic background radiation of the Big Bang.

After the creation of the CMB, there are a number of important events. After the emission of the CMB, ordinary matter in the universe was mostly in the form of neutral hydrogen and helium atoms, but from observations of galaxies, it seems that most of the volume of the intergalactic medium (IGM) today consists of ionized material (since there are few absorption lines due to hydrogen atoms). This implies a period of reionization in which the material of the universe breaks down into hydrogen ions.

The CMB photons scatter off free charges such as electrons not bound in atoms. In an ionized universe such electrons have been liberated from neutral atoms by ionizing (ultraviolet) radiation. Today these free charges are at sufficiently low density in most of the volume of the Universe that they do not measureably affect the CMB. However, if the IGM was ionized at very early times, when the universe was still denser, then there are two main effects on the CMB: 1) small scale anisotropies are erased (just as looking at an object through fog, details of the object appear fuzzy) and 2) the physics of how photons scatter off of free electrons (Thomson scattering) induce polarization anisotropies on large angular scales. This large angle polarization is correlated with the large angle temperature perturbation.

Both of these effects have been observed by the WMAP satellite, providing evidence that the universe was ionized at very early times, at a redshift of larger than 17. The detailed provenance of this early ionizing radiation is still a matter of scientific debate. It may have included starlight from the very first population of stars (population III stars), supernovae when these first stars reached the end of their lives, or the ionizing radiation produced by the accretion disks of massive black holes.

The period after the emission of the CMB and the observation of the first stars is semi-humorously referred to by cosmologists as the dark age, and is a period which is under intense study by astronomers.

Image:WMAP power spectrum.jpg

The power spectrum of the cosmic microwave background radiation anisotropy in terms of the multipole moment (top). Data from WMAP have extended the accuracy of the spectrum far beyond what was known from earlier measurements.

Detection, prediction and discovery

Image:WMAP.jpg

WMAP image of the CMB anisotropy,Cosmic microwave background radiation(June 2003)

Main article: Discovery of cosmic microwave background radiation

The CMB was predicted by George Gamow, Ralph Alpher, and Robert Hermann in the 1940s and was accidentally discovered in 1964 by Arno Penzias and Robert Woodrow Wilson, who received a Nobel Prize in Physics in 1978 for this discovery. The interpretation of the CMB was a very controversial issue in the 1960s with some proponents of the steady state theory arguing that the CMB was the result of scattered starlight from distant galaxies. Using this model, and based on the study of narrow absorption line features in the spectra of stars, the astronomer Andrew McKellar wrote in 1941: "It can be calculated that the 'rotational' temperature of interstellar space is 2 K." However, during the 1970s the consensus view moved to the point of view that the CBR was the remnant of the big bang. Among the observations that swung the astronomical community toward this point of view were the fact that the CBR was much smoother than would be expected from scattered star light.

Because water absorbs microwave radiation, a fact that is used to build microwave ovens, it is rather difficult to observe the CMB with ground-based instruments. CMB research therefore makes increasing use of air and space-borne experiments. Ground-based observations of the CMB are usually made from high altitude locations such as the Chilean Andes and the South Pole.

Experiments

Of these experiments, the Cosmic Background Explorer (COBE) satellite that was flown in 1989-1996 is probably the most famous and which made the first detection of 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 scale of the first acoustic peak, which COBE did not have sufficient resolution to resolve. These measurements were able to rule out cosmic strings as a theory of cosmic structure formation, and suggested cosmic inflation was the right theory. The first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment reported that the highest power fluctions occur at one degree scales. 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 and the Cosmic Background Imager. In fact, the Cosmic Background Imager has made the first detection of the CMB polarization.

In June 2001, NASA launched a second CBR space mission, WMAP, to make much more accurate measurements of the large scale anisotropies over the full sky. Results from this mission disclosed in 2003 provided a detailed measurement of the angular power spectrum down to 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 small 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 CLOVER array and South Pole Telescope in Antarctica and the Atacama Cosmology Telescope in Chile will provide additional data not available from satellite observations, possibly including B-mode polarization component.

List of experiments in approximate chronological order

Each experiment provided improved data quality when compared with previous experiments.

Cosmic Background Explorer - measured the very large scale fluctuations
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
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-Zeldovich 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 - (2008?) - improved precision for small scale fluctuations and B-mode polarization measurements
Planck - (2009?) - will give improved precision and polarization data at all scales

Bibliography

Seife, Charles (2003). Breakthrough of the Year: Illuminating the Dark Universe. Science 302 2038–2039.
Partridge, R. B. (1995). 3K: The Cosmic Microwave Background Radiation. New York: Cambridge University Press.
R. A. Alpher and R. Herman, "On the Relative Abundance of the Elements," Physical Review 74 (1948), 1577. This paper contains the first estimate of the present temperature of the universe.
A. A. Penzias and R. W. Wilson, "A Measurement of Excess Antenna Temperature at 4080 Mc/s," Astrophysics Journal 142 (1965), 419. The paper describing the discovery of the cosmic microwave background.
R. H. Dicke, P. J. E. Peebles, P. G. Roll and D. T. Wilkinson, "Cosmic Black-Body Radiation," Astrophysics Journal 142 (1965), 414. The theoretical interpretation of Penzias and Wilson's discovery.