On the electromagnetic nature of dark energy and the origin of cosmic
  magnetic fields
  In this work we consider quantum electromagnetic fields in an expanding
universe. We start by reviewing the difficulties found when trying to impose
the Lorenz condition in a time-dependent geometry. Motivated by this fact, we
explore the possibility of extending the electromagnetic theory by allowing the
scalar state which is usually eliminated by means of the Lorenz condition to
propagate, preserving at the same time the dynamics of ordinary transverse
photons. We show that the new state cannot be generated by charged currents,
but it breaks conformal invariance and can be excited gravitationally. In fact,
primordial quantum fluctuations produced during inflation can give rise to
super-Hubble temporal electromagnetic modes whose energy density behaves as a
cosmological constant. The value of the effective cosmological constant is
shown to agree with observations provided inflation took place at the
electroweak scale. The theory is compatible with all the local gravity tests
and is free from classical or quantum instabilities. Thus we see that, not only
the true nature of dark energy can be established without resorting to new
physics, but also the value of the cosmological constant finds a natural
explanation in the context of standard inflationary cosmology. On sub-Hubble
scales, the new state generates an effective charge density which, due to the
high electric conductivity of the cosmic plasma after inflation, gives rise to
both vorticity and magnetic fields. Present upper limits on vorticity coming
from CMB anisotropies are translated into lower limits on the present value of
cosmic magnetic fields. We find that magnetic fields $B_{\lambda}> 10^{-12}$ G
can be typically generated with coherence lengths ranging from sub-galactic
scales up to the present Hubble radius.
