In physics and astronomy, recombination refers to the absorption of one
or more electrons by ionized atoms in a gas. In cosmology,
recombination refers to an important phase of the universe's history -- the time
when the early universe cooled enough to allow electromagnetic radiation to
flow almost freely throughout the universe.
This heralded the decoupling of matter and
radiation and the point at which the cosmic
as we see it today was created. Although the rough
shape of the CMB spectrum was set much earlier, the
faint anisotropies observed in the microwave background were
generated at the time of recombination.
After atomic nuclei first formed, the cosmos was
a soup of hydrogen and helium atoms, and electromagnetic
radiation. The atoms and the radiation were "coupled" -- electrons could not
remain attached to atoms because they'd be knocked off by the radiation, and
radiation could not flow freely through the universe because it would be
absorbed by atoms. This situation lasted from just a few minutes after the
big bang until hundreds of thousands of years later. However, as the
universe expanded it began to cool, and the
average energy of a photon fell below that needed
to ionize a hydrogen atom.
When this happened, photons wouldn't necessarily ionize any
neutral atom they encountered, and could thus begin to travel unimpeded
through the universe. This is the epoch of recombination -- when
electrons could finally recombine with atomic nuclei and form neutral
Recombination is considered an epoch rather than an event
because it didn't occur all at once everywhere in the universe. This is
because the density of matter and radiation wasn't the same throughout
the universe -- it was
anisotropic. These differences in density resulted in different
opacities to radiation, so different parts of the universe
became transparent at slightly different times.
In terms of redshift, recombination occurred on average at
z = 1100 ± 100, or about 300,000 years after the
big bang. (For reference, the most distant galaxies observed are at z = 6.7 or so.) This redshift is considered the "surface" which radiates the
cosmic microwave background that we observe, and is known as the last scattering
surface. The slight differences in the temperature of this last scattering
surface represent different matter densities, and these differences say
something about what the universe was like at the time of recombination.
When astronomers measure the slight anisotropies in the CMB
(as with the WMAP and COBE satellites) what
they're really looking for are the faint ripples of matter density at the
time of recombination.
I made use of the excellent http://nedwww.ipac.caltech.edu/level5/ while writing