| Abstract from DBPedia | In cosmology, recombination refers to the epoch during which charged electrons and protons first became bound to form electrically neutral hydrogen atoms. Recombination occurred about 370,000 years after the Big Bang (at a redshift of z = 1100). The word "recombination" is misleading, since the Big Bang theory doesn't posit that protons and electrons had been combined before, but the name exists for historical reasons since it was named before the Big Bang hypothesis became the primary theory of the creation of the universe. Immediately after the Big Bang, the universe was a hot, dense plasma of photons, leptons, and quarks: the quark epoch. At 10−6 seconds, the Universe had expanded and cooled sufficiently to allow for the formation of protons: the hadron epoch. 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. This is the current state of the interior of the Sun. 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. Recombination involves electrons binding to protons (hydrogen nuclei) to form neutral hydrogen atoms. Because direct recombinations to the ground state (lowest energy) of hydrogen are very inefficient, these hydrogen atoms generally form with the electrons in a high energy state, and the electrons quickly transition to their low energy state by emitting photons. Two main pathways exist: from the 2p state by emitting a Lyman-a photon - these photons will almost always be reabsorbed by another hydrogen atom in its ground state - or from the 2s state by emitting two photons, which is very slow. This production of photons is known as decoupling, which leads to recombination sometimes being called photon decoupling, but 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 is observed today as cosmic microwave background radiation (in that sense, the cosmic background radiation is infrared [and some red] black-body radiation emitted when the universe was at a temperature of some 3000 K, redshifted by a factor of 1100 from the visible spectrum to the microwave spectrum).宇宙の晴れ上がり（うちゅうのはれあがり、英: clear up of the Universe）は、ビッグバン理論において宇宙の始まり以来、初めて光子が長距離を進めるようになった時期を指す。ビッグバンからおよそ38万年後に宇宙の温度は約 3000 K まで低下し電子と原子核が結合して原子を形成すると、光子は電子との相互作用をせずに長距離を進めるようになる。つまり、初期宇宙は電離度が大きいため光子にとっては「霧がかった」状態にあるが、再結合により電離度が減少する結果として宇宙は透明になる（晴れ上がる）。 「宇宙の晴れ上がり」という用語は佐藤文隆によるもので、この言葉に直接対応する英語の定訳はなく、宇宙の再結合（recombination）などと呼ばれる。 (Source: http://dbpedia.org/resource/Recombination_(cosmology)) |
