  We consider electromagnetic field quantization in an expanding universe. We
find that the covariant (Gupta-Bleuler) method exhibits certain difficulties
when trying to impose the quantum Lorenz condition on cosmological scales. We
thus explore the possibility of consistently quantizing without imposing such a
condition. In this case there are three physical states, which are the two
transverse polarizations of the massless photon and a new massless scalar mode
coming from the temporal and longitudinal components of the electromagnetic
field. An explicit example in de Sitter space-time shows that it is still
possible to eliminate the negative norm state and to ensure the positivity of
the energy in this theory. The new state is decoupled from the conserved
electromagnetic currents, but is non-conformally coupled to gravity and
therefore can be excited from vacuum fluctuations by the expanding background.
The cosmological evolution ensures that the new state modifies Maxwell's
equations in a totally negligible way on sub-Hubble scales. However, on
cosmological scales it can give rise to a non-negligible energy density which
could explain in a natural way the present phase of accelerated expansion of
the universe.
