<P> 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 . </P> <P> 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: </P> <Ol> <Li> Small scale anisotropies are erased . (Just as when looking at an object through fog, details of the object appear fuzzy .) </Li> <Li> 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 . </Li> </Ol> <Li> Small scale anisotropies are erased . (Just as when looking at an object through fog, details of the object appear fuzzy .) </Li>

When do astronomers think cosmic background radiation formed