Recombination (cosmology)

Recombination was the early-universe transition in which electrons joined nuclei to form neutral atoms, making the universe transparent to radiation. The photons released from matter then survive as the cosmic microwave background.

Cosmic microwave background: The relic radiation from the early universe, now observed as a nearly uniform microwave glow. Its photons last scattered when recombination made the universe transparent.
Cosmic expansion: The increase over time of distances between unbound regions of the universe. Expansion diluted and cooled the plasma, driving it toward neutral atoms.
Cosmic dawn: The era when the first stars and galaxies began illuminating the previously dark universe. It followed the long interval after recombination, when matter cooled before the first stars formed.
Planck (spacecraft): The European Space Agency mission that mapped the cosmic microwave background from 2009 to 2013. Its precise background measurements constrain the timing and duration of recombination.
Thomson scattering: The scattering of electromagnetic radiation by free charged particles at energies well below their rest-mass energy. Before recombination, free electrons repeatedly scattered photons through this process.
Big Bang nucleosynthesis: The formation of light atomic nuclei during the universe’s first few minutes. It established the primordial nuclei that later captured electrons.
Baryon acoustic oscillations: A preferred scale in the clustering of matter, inherited from sound waves in the early photon-baryon plasma. The end of photon coupling froze these plasma sound waves into later matter patterns.
CosmoRec: A numerical code that calculates the primordial hydrogen and helium recombination history. It models the detailed atomic processes behind the ionization history.
Hydrogen atom: The simplest atom, consisting of one proton bound to one electron. Most primordial electrons became bound to protons as neutral hydrogen formed.
Ionization: The process by which an atom or molecule loses or gains electrons and becomes charged. Recombination reversed the highly ionized state left by the hot early universe.

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