(time when the universe's ionized hydrogen atoms neutralized)
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 Compton scattering by electrons,
which had made space opaque. This lengthening of the photon
mean free path is known as the photon decoupling,
when one of the conditions of thermodynamic equilibrium
(photon interactions with matter the same energy distribution of photons
as are destroyed) was removed.
Recombination was the beginning of the dark age:
the universe was transparent but there were not yet stars.
The freed photons traveled freely,
generally with no interaction with matter,
and now constitute the cosmic microwave background.
The term recombination is also used for the pairing
of ions with electrons in other circumstances, which
happens in many astronomical phenomena, e.g., the interior of stars.
The resulting emission is called free-bound emission.
(Big Bang,cosmology,hydrogen,ionization,event,CMB,early universe)
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Referenced by pages:
baryon acoustic oscillations (BAO)
cosmic microwave background (CMB)
epoch of reionization (EOR)
helium 1083 nm line
mean free path
surface of last scattering
Sachs-Wolfe effect (SWE)
weak lensing (WL)