A large scale coherent magnetic field: interactions with free streaming
  particles and limits from the CMB
  We study a homogeneous and nearly-isotropic Universe permeated by a
homogeneous magnetic field. Together with an isotropic fluid, the homogeneous
magnetic field, which is the primary source of anisotropy, leads to a
plane-symmetric Bianchi I model of the Universe. However, when free-streaming
relativistic particles are present, they generate an anisotropic pressure which
counteracts the one from the magnetic field such that the Universe becomes
isotropized. We show that due to this effect, the CMB temperature anisotropy
from a homogeneous magnetic field is significantly suppressed if the the
neutrino masses are smaller than 0.3 eV.
