Epoch of recombination
In cosmology, recombination refers to the epoch at which charged electrons and protons first became bound to form electrically neutral hydrogen atoms. Recombination occurred about 378,000 years after the Big Bang (at a redshift of z = 1100). The word "recombination" is misleading, since protons and electrons had never been previously combined, but the name was given before the Big Bang hypothesis had been proven, and is retained for historical reasons.
Immediately after the Big Bang, the universe was a hot, dense plasma of photons, electrons, and protons. The 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.
Shortly after, photons decoupled from matter in the universe, 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 black-body radiation emitted when the universe was at a temperature of some 4000 K, redshifted by a factor of 1100 from the visible spectrum to the microwave spectrum).
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