  We show that the presence of a temporal electromagnetic field on cosmological
scales generates an effective cosmological constant which can account for the
accelerated expansion of the universe. Primordial electromagnetic quantum
fluctuations produced during electroweak scale inflation could naturally
explain the presence of this field and also the measured value of the dark
energy density. The behavior of the electromagnetic field on cosmological
scales is found to differ from the well studied short-distance behavior and, in
fact, the presence of a non-vanishing cosmological constant could be signalling
the breakdown of gauge invariance on cosmological scales. 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. This mechanism could be discriminated from a
true cosmological constant by upcoming observations of CMB anisotropies and
large scale structure.
