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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 Kelvin. It has a frequency in the microwave range.
This radiation is regarded as the best available evidence of the Big Bang (BB) theory and its discovery in the mid-1960s curtailed 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. 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 CMBR, there are a number of important events. The CMBR created hydrogen atoms, but from observations of galaxies, it seems that most of the intergalactic medium 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 favored explanation for this is that starlight causes reionization although there is evidence that reionization began before there were large numbers of stars.
The period after the emission of the CMBR and the observation of the first stars is semi-humorously refered to by cosmologists as the dark age, and is a period which is under intense study by astronomers.
One feature of the CMB is how closely it matches a black body. Although the temperature of the CMB varies from point to point contains small anisotropies, the spectrum in a particular direction very closely resembles a black body.
Another of the microwave background's salient features is a high degree of isotropy. There are however some anisotropies as well, 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 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 1/100000 and on a scale of a few arc minutes. Those very small variations are the result of the Sachs-Wolfe effect which causes photons from the Cosmic microwave background to be gravitationally redshifted. These density fluctations arise because different parts of the universe are not in contact with each other. The power spectrum of these variations can be calculated and produces a number of peaks and valleys. The location of these peaks and valleys can be coorelated with cosmological parameters such as the Hubble constant.
Main article: Discovery of cosmic microwave background radiation
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 interpretation of the CBR was a very controversial issue in the 1960's with some proponents of the steady state theory arguing that the CBR 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 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).
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). Conversely, supporters of 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, Black body, 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,
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
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