What is "recombination"

Recombination \Re*com`bi*na"tion\ (r?*k?m`b?*n?"sh?n), n. Combination a second or additional time.

1791, from re- + combination.

n. 1 combination a second or subsequent time. 2 (context genetics English) The formation of genetic combinations in offspring that are not present in the parents 3 (context chemistry English) The reverse of dissociation 4 (context astrophysics English) The process by which the plasma of electrons and protons produced after the Big Bang condensed into hydrogen, or the epoch in which this process occurred.

1. n. (genetics) a combining of genes or characters different from what they were in the parents

2. (physics) a combinng of charges or transfer of electrons in a gas that results in the neutralization of ions; important for ions arising from the passage of high-energy particles

Recombination may refer to:

• Genetic recombination, the process by which genetic material is broken and joined to other genetic material
• Recombination (physics), in semiconductors, the elimination of mobile charge carriers (electrons and holes)
• Crossover (genetic algorithm), also called recombination
• Plasma recombination, the formation of neutral atoms from the capture of free electrons by the cations in a plasma
• Recombination (cosmology), the time at which protons and electrons formed neutral hydrogen in the timeline of the Big Bang
• Recombination (chemistry), the opposite of dissociation

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 = ).

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. 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 to the microwave spectrum).