"First Order" Polarization of the CMB


Density perturbations at last scattering produce anisotropy of the Total Intensity - T spectrum

Material 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!