Editing Cosmic microwave background (section)

====Late time anisotropy====
Since the CMB came into existence, it has apparently been modified by several subsequent physical processes, which are collectively referred to as late-time anisotropy, or secondary anisotropy. When the CMB photons became free to travel unimpeded, ordinary matter in the universe was mostly in the form of neutral hydrogen and helium atoms. However, observations of galaxies today seem to indicate that most of the volume of the [[intergalactic medium]] (IGM) consists of ionized material (since there are few absorption lines due to hydrogen atoms). This implies a period of [[reionization]] during which some of the material of the universe was broken into hydrogen ions.

The CMB photons are scattered by free charges such as electrons that are not bound in atoms. In an ionized universe, such charged particles have been liberated from neutral atoms by ionizing (ultraviolet) radiation. Today these free charges are at sufficiently low density in most of the volume of the universe that they do not measurably affect the CMB. However, if the IGM was ionized at very early times when the universe was still denser, then there are two main effects on the CMB:
# Small scale anisotropies are erased. (Just as when looking at an object through fog, details of the object appear fuzzy.)
# The physics of how photons are scattered by free electrons ([[Thomson scattering]]) induces polarization anisotropies on large angular scales. This broad angle polarization is correlated with the broad angle temperature perturbation.

Both of these effects have been observed by the WMAP spacecraft, providing evidence that the universe was ionized at very early times, at a [[redshift]] around 10.<ref name="WMAP9Cosmo"/> The detailed provenance of this early ionizing radiation is still a matter of scientific debate. It may have included starlight from the very first population of stars ([[population III]] stars), supernovae when these first stars reached the end of their lives, or the ionizing radiation produced by the accretion disks of massive black holes.

The time following the emission of the cosmic microwave background—and before the observation of the first stars—is semi-humorously referred to by cosmologists as the [[Timeline of the Big Bang#Dark Ages|Dark Age]], and is a period which is under intense study by astronomers (see [[21 centimeter radiation]]).

Two other effects which occurred between reionization and our observations of the cosmic microwave background, and which appear to cause anisotropies, are the [[Sunyaev–Zeldovich effect]], where a cloud of high-energy electrons scatters the radiation, transferring some of its energy to the CMB photons, and the [[Sachs–Wolfe effect]], which causes photons from the Cosmic Microwave Background to be gravitationally redshifted or blueshifted due to changing gravitational fields.