CMB Polarization: Physical Mechanisms

E modes have gradient-like polarization geometry and are produced strongly by ordinary scalar density perturbations. B modes have curl-like geometry and can arise from gravitational lensing of E modes; primordial gravitational waves could also create large-scale B modes.

Thomson scattering polarizes radiation when incoming intensity has an angular quadrupole. At recombination and later reionization, this converts local radiation anisotropy into linear polarization. Gravitational lensing by intervening structure remaps polarization on its way to Earth.

E-mode polarization and lensing-generated B modes have been detected. Searches for a primordial B-mode component continue and must carefully separate Galactic dust and other foregrounds.

Thomson scattering creates linear polarization when the incoming radiation field has a quadrupole anisotropy. Gravitational lensing later converts some E-mode power into B modes. The generation mechanism is selective: Thomson scattering produces polarization only when the incident radiation has a quadrupole anisotropy. An exactly isotropic radiation field would remain unpolarized after scattering. This is why polarization is a particularly clean probe of velocity gradients and perturbation geometry near recombination.