  Prior to photon decoupling magnetic random fields of comoving intensity in
the nano-Gauss range distort the temperature and the polarization anisotropies
of the microwave background, potentially induce a peculiar B-mode power
spectrum and may even generate a frequency-dependent circularly polarized
V-mode. We critically analyze the theoretical foundations and the recent
achievements of an interesting trialogue involving plasma physics, general
relativity and astrophysics.
