<P> 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 more than 17 . 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 . </P> <P> 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 dark age, and is a period which is under intense study by astronomers (see 21 centimeter radiation). </P> <P> Two other effects which occurred between reionization and our observations of the cosmic microwave background, and which appear to cause anisotropies, are the Sunyaev--Zel'dovich 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 . </P> <P> The cosmic microwave background is polarized at the level of a few microkelvin . There are two types of polarization, called E-modes and B - modes . This is in analogy to electrostatics, in which the electric field (E-field) has a vanishing curl and the magnetic field (B - field) has a vanishing divergence . The E-modes arise naturally from Thomson scattering in a heterogeneous plasma . The B - modes are not produced by standard scalar type perturbations . Instead they can be created by two mechanisms: the first one is by gravitational lensing of E-modes, which has been measured by the South Pole Telescope in 2013; the second one is from gravitational waves arising from cosmic inflation . Detecting the B - modes is extremely difficult, particularly as the degree of foreground contamination is unknown, and the weak gravitational lensing signal mixes the relatively strong E-mode signal with the B - mode signal . </P>

What information would we need in order to calculate the current temperature of the cmb