Cosmic microwave background radiation

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.

CMB and the Big Bang

This radiation, as well as the red shift, are regarded as the best

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available evidence of the Big Bang (BB) theory. The discovery of this

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radiation in the mid-1960s curtailed interest for

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alternatives such as the steady state theory.

Related Topics:
Alternatives - Steady state theory

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The CMB gives a snapshot of the Universe when, according to standard

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cosmology, the temperature dropped enough to allow electrons and

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protons to form hydrogen atoms, thus making the universe transparent

Related Topics:
Proton - Hydrogen

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to radiation. When it originated some 400,000 years after the Big

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Bang -- this time period is generally known as the "time of last scattering"

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or the period of recombination or decoupling -- the temperature of the

Related Topics:
Recombination - Decoupling

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Universe was about 3000 K. Since then the temperature of the

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radiation has dropped by a factor of roughly 1100 due to the

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expansion of the Universe. As the universe expands, the CMB photons are

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redshifted, cooling the radiation inversely proportional to the

Related Topics:
Redshift - Inversely proportional

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Universe's scale length. For details on reasoning

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that the radiation is used as evidence of the Big Bang, see

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[[Big Bang#Cosmic_microwave_background_radiation|Cosmic background radiation

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of the Big Bang]].

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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.

Related Topics:
Intergalactic medium - Reionization

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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.

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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.

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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.

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