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Cosmic microwave background radiation

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WMAP image of the CMB anisotropy
Cosmic microwave
background radiation

June 2001

The Cosmic Microwave Background Radiation is a form of electromagnetic radiation that fills the whole of the universe. It has the characteristics of black body radiation at a temperature of 2.726 kelvins. It has a frequency in the microwave range.

Table of contents
1 CMR and the Big Bang
2 Features
3 Detection, Prediction and Discovery
4 Experiments
5 CBR and Non-Standard Cosmologies
6 See also
7 Bibliography
8 References and external links

CMR and the Big Bang

This radiation is regarded as the best available evidence of the Big Bang (BB) theory and its discovery in the mid-1960s marked the end of general interest for alternatives such as the steady state theory. The CMR 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 300,000 years after the Big Bang -- this point in time is generally known as the "last scattering surface" -- the temperature of the Universe was about 6000 K. Since then it has dropped because of the expansion of the Universe, which cools radiation inversely proportional to the fourth power of the Universe's scale length.


One of the microwave background's most salient features is a high degree of isotropy. There are some anisotropies, the most pronounced of which is the dipole anisotropy at a level of about 10-4 at a scale of 180 degrees of arc. It is due to the motion of the observer against the CBR, which is some 700 km/s for the Earth.

Variations due to external physics also exist; the Sunyaev-Zel'dovich Effect is one of the major factors here, in which an 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 1/100000 and on a scale of a few arc minutes. Those very small variations correspond to the density fluctuations at the last scattering surface and give valuable information about the seeds for the large scale structures we observe now. These density fluctations arise because different parts of the universe are not in contact with each other.

In addition, the Sachs-Wolfe effect causes photons from the Cosmic microwave background to be gravitationally redshifted. These small-scale variations give observational constraints on the properties of universe, and are therefore one important test for cosmological models.

Detection, Prediction and Discovery

The CBR was predicted by George Gamow, Ralph Alpher, and Robert Hermann in the 1940s and was accidentally discovered in 1964 by Penzias and Wilson, who received a Nobel Prize in Physics in 1978 for this discovery. The CBR had, however, been detected and its temperature deduced in 1941, seven years before Gamow's prediction. Based on the study of narrow absorption line features in the spectra of stars, the astronomer Andrew McKellar wrote: "It can be calculated that the 'rotational' temperature of interstellar space is 2 K."

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.


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). In June 2001, NASA launched a second CBR space mission, WMAP, to make detailed measurements of the anisotropies over the full sky. Results from this mission provide a detailed measurement of the angular power spectrum down to degree scales, giving detailed constraints on 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) [ed. see links below],

A third space mission, Planck, is to be launched in 2007. Unlike the previous two space missions, Planck is a collaboration between NASA and ESA (the European Space Agency).

CBR and Non-Standard Cosmologies

During the mid-1990s, the lack of detection of anisotropies in the CBR led to some interest in nonstandard cosmologies (such as plasma cosmology) mostly as a backup in case detectors failed to find anisotropy in the CBR. The discovery of these anisotropies combined with a large amount of new data coming in has greatly reduced interest in these alternative theories.

Some supporters of non-standard cosmology argue that the primodorial background radiation is uniform (which is inconsistent with the big bang) and that the variations in the CBR are due to the Sunyaev-Zel'dovich effect mentioned above (among other effects).

See also

Main: Background radiation, COBE, Cosmic inflation, Cosmic background radiation, Gravity waves, Microwave, Unsolved problems in physics, WMAP

Physics and Astronomy: Anisotropy (or Anisotropic), Baryonic dark matter, Big bang nucleosynthesis, Blackbody, Black dwarf, Cold dark matter, Dark energy, Greisen-Zatsepin-Kuzmin limit, History of astronomy, Hubble's law, Integrated Sachs Wolfe effect, Nobel Prize in Physics, Observation, Olbers' paradox, Radio astronomy, Redshift

Theories: Big Bang, Big Crunch, Plasma cosmology, Reciprocal System of Theory

People: Arno Allan Penzias, Fred Hoyle, Georges Lemaître, Robert Wilson, Robert Woodrow Wilson

Timelines and lists: List of astronomical topics, List of famous experiments, Timeline of cosmic microwave background astronomy, Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure, Timeline of the Big Bang, Timeline of white dwarfs, neutron stars, and supernovae

Other: 1 E12 s, Background, Holmdel Township, New Jersey


References and external links