recombination

In cosmology, recombination is a point in the history of the universe around redshift 1090, at about 378k years after the Big Bang, when electrons and protons paired up to form neutral hydrogen atoms. In this usage, the term is a misnomer because this "recombination" is actually the first time the electrons and protons were together. This was the end of the photon epoch: when photons' mean free path (between electrons and protons) was short due to constant Compton scattering by electrons, which had made space opaque. This change is also termed the photon decoupling, removing one of the conditions maintaining the previous thermodynamic equilibrium, the previous balance of interactions between matter and photons that maintained some particular energy-distribution of each. Recombination was the beginning of the dark age: the universe was transparent but there were not yet stars. The freed photons traveled freely, many having no subsequent interaction with matter (thus they were free streaming), and now constitute the cosmic microwave background (CMB).

The term recombination has a more general meaning: the joining of positive ions and electrons into neutral atoms. This also occurs in other common situations, including many astronomical circumstances, such as the interior of stars. A resulting emission from such recombination is termed free-bound emission.