The Dark Magnetism of the Universe
  Despite the success of Maxwell's electromagnetism in the description of the
electromagnetic interactions on small scales, we know very little about the
behaviour of electromagnetic fields on cosmological distances. Thus, it has
been suggested recently that the problems of dark energy and the origin of
cosmic magnetic fields could be pointing to a modification of Maxwell's theory
on large scales. Here, we review such a proposal in which the scalar state
which is usually eliminated be means of the Lorenz condition is allowed to
propagate. On super-Hubble scales, the new mode is essentially given by the
temporal component of the electromagnetic potential and contributes as an
effective cosmological constant to the energy-momentum tensor. The new state
can be generated from quantum fluctuations during inflation and it is shown
that the predicted value for the cosmological constant agrees with observations
provided inflation took place at the electroweak scale. We also consider more
general theories including non-minimal couplings to the space-time curvature in
the presence of the temporal electromagnetic background. We show that both in
the minimal and non-minimal cases, the modified Maxwell's equations include new
effective current terms which can generate magnetic fields from sub-galactic
scales up to the present Hubble horizon. The corresponding amplitudes could be
enough to seed a galactic dynamo or even to account for observations just by
collapse and differential rotation in the protogalactic cloud.
