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CMB polarization is caused by Thomson scattering and by gravitational lensing of cosmic microwave background (CMB) photons. The CMB electromagnetic radiation (EMR), as we receive it, is largely unpolarized, somewhat under 10% being polarized. The patterns formed of its polarization as found across regions of the sky can be characterized as a combination of two underlying types of pattern, termed polarization modes, E-mode and B-mode (so-named in analogy to the divergence and curl of electric and magnetic fields as expressed in Maxwell's equations, with no relation to these other than by analogy), with unpolarized EMR referred to as T-mode (T for temperature: the EMR has a black-body spectrum corresponding to a specific temperature). They are also classified as primary modes or secondary modes, based on whether the polarization occurred at/before recombination, when the photons began their free streaming, or subsequent to it, such as through gravitational lensing. The primary modes included E-modes from Thomson scattering, from features in the density distribution due to initial fluctuations and baryon acoustic oscillations (BAOs), and theoretically include B-modes resulting from gravitational waves (GWs, specifically primordial gravitational waves, a theorized product of initial fluctuations). If the latter were found, they would support inflation-cosmology theory. They would also independently support the existence of gravitational waves themselves, though since the direct GW detections, this has become a less-momentous possibility. Secondary modes include B-modes created by gravitational lensing of the primary E-modes.
Angular power spectrums can be produced for cross-correlations of pairs of these, correlations known as BB, EE, TT, TE, TB, and EB. The latter two are zero on every scale. TT is basically the usual power spectrum (i.e., ignoring polarization) and is the strongest since only a small fraction of the EMR is polarized. EE results from Thomson scattering, and BB results from gravitational lensing of the EE polarized EMR. The TE detections confirm the existence of the E-mode and have provided a useful tool for study in some cases where the BB measurements are insufficiently prominent to be of use.