Immediately after the
Big Bang, the
Universe was extremely hot and dense. Under these conditions, atoms could not exist, and the matter was distributed as a highly ionised plasma. As the
Universe expanded, however, its
density and temperature dropped until (after about 300,000 years) the conditions were such that
ions and
electrons could 'recombine' to form atoms (mostly
hydrogen and
helium). This is known as the '
epoch of recombination'.
We cannot actually observe the
Universe before the
epoch of recombination, since the ionised plasma that existed before this time was very efficient at scattering radiation. The result is that information about the early
Universe (information
astronomers detect in the form of
photons) was not able to escape, hiding the early
Universe from current observational strategies.
At the
epoch of recombination, however, the ionised plasma gave way to neutral atoms, which did not scatter the
photons but allowed them to travel freely. All
photons emitted since this time have also been able to travel unimpeded, allowing
astronomers to study objects all the way back to recombination. The oldest
photons detected - those that were emitted at the
epoch of recombination - make up the cosmic microwave radiation.