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In cosmology, recombination refers to the epoch at which charged electrons and protons first became bound to form electrically neutral hydrogen atoms.〔Note that the term recombination is a misnomer, considering that it represents the ''first'' time that electrically neutral hydrogen formed.〕 Recombination occurred about 378,000 years after the Big Bang (at a redshift of ''z'' = ). Immediately after the Big Bang, the universe was a hot, dense plasma of photons, electrons, and protons. This plasma was effectively opaque to electromagnetic radiation due to Thomson scattering by free electrons, as the mean free path each photon could travel before encountering an electron was very short. As the universe expanded, it also cooled. Eventually, the universe cooled to the point that the formation of neutral hydrogen was energetically favored, and the fraction of free electrons and protons as compared to neutral hydrogen decreased to a few parts in 10,000. Shortly after, photons decoupled from matter in the universe, which leads to recombination sometimes being called photon decoupling, although recombination and photon decoupling are distinct events. Once photons decoupled from matter, they traveled freely through the universe without interacting with matter, and constitute what we observe today as cosmic microwave background radiation. == The recombination history of hydrogen == The cosmic ionization history is generally described in terms of the free electron fraction ''x''e as a function of redshift. It is the ratio of the abundance of free electrons to the total abundance of hydrogen (both neutral and ionized). Denoting by ''n''e the number density of free electrons, ''n''H that of atomic hydrogen and ''n''p that of ionized hydrogen (i.e. protons), ''x''e is defined as : Since hydrogen only recombines once helium is fully neutral, charge neutrality implies ''n''e = ''n''p, i.e. ''x''e is also the fraction of ionized hydrogen. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Recombination (cosmology)」の詳細全文を読む スポンサード リンク
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