Astrophysics (Index)About


(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)
Further reading:
/Lookback Years

Referenced by pages:
baryon acoustic oscillations (BAO)
cosmic microwave background (CMB)
CMB anisotropies
CMB polarization
cosmic neutrino background (CNB)
continuum emission
dark age
diffusion damping
early universe
epoch of reionization (EOR)
free streaming
helium 1083 nm line
instability strip
mean free path
observable universe
Strömgren sphere
surface of last scattering
Sachs-Wolfe effect (SWE)
21-cm line
weak lensing (WL)