Imprints of a primordial magnetic field upon the cosmic microwave background anisotropy and polarization

We review the imprints that a primordial magnetic field may have left upon the cosmic microwave background (CMB) anisotropy and polarization through Faraday rotation around the time of decoupling. Differential Faraday rotation reduces the degree of linear polarization acquired through anisotropic Thomson scattering. Depolarization reduces the damping due to photon diffusion, which results in an increase of the anisotropy on small angular scales. The effect is significant at frequencies around and below 10 GHz {ie2513-1} whereB ₀ is the present strength of the primordial magnetic field.