  Out of the four components of the electromagnetic field, Maxwell's theory
only contains two physical degrees of freedom. However, in an expanding
universe, consistently eliminating one of the "unphysical" states in the
covariant (Gupta-Bleuler) formalism turns out to be difficult to realize. In
this work we explore the possibility of quantization without subsidiary
conditions. This implies that the theory would contain a third physical state.
The presence of such a new (temporal) electromagnetic mode on cosmological
scales is shown to generate an effective cosmological constant which can
account for the accelerated expansion of the universe. This new polarization
state is completely decoupled from charged matter, but can be excited
gravitationally. In fact, primordial electromagnetic quantum fluctuations
produced during electroweak scale inflation could naturally explain the
presence of this mode and also the measured value of the cosmological constant.
The theory is compatible with all the local gravity tests, it is free from
classical or quantum instabilities and reduces to standard QED in the flat
space-time limit. 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. Possible signals, discriminating this model from LCDM,
are also discussed.
