"First Order" Polarization of the CMB


Density perturbations at last scattering produce T anisotropy.

Material is flowing along gradients in the density field:
Resulting polarization pattern aligned with its gradient (E-modes) and has zero curl (B-modes).
Since density perturbations produce the motions there is TE cross correlation.

Given T spectrum and standard cosmological model can predict expected E and TE spectra.
...if measurements don't match the whole framework falls apart! - Critical test!