Electromagnetic two-point functions and the Casimir effect in
  Friedmann-Robertson-Walker cosmologies
  We evaluate the two-point functions of the electromagnetic field in (D+1)
-dimensional spatially flat Friedmann-Robertson-Walker universes with a
power-law scale factor, assuming that the field is prepared in the Bunch-Davies
vacuum state. The range of powers are specified in which the two-point
functions are infrared convergent and the Bunch-Davies vacuum for the
electromagnetic field is a physically realizable state. The two-point functions
are applied for the investigation of the vacuum expectation values of the field
squared and the energy-momentum tensor, induced by a single and two parallel
conducting plates. Unlike to the case of conducting plates in the Minkowski
bulk, in the problem under consideration the stresses along the directions
parallel to the plates are not equal to the energy density. We show that, in
addition to the diagonal components, the vacuum energy-momentum tensor has a
nonzero off-diagonal component which describes energy flux along the direction
normal to the plates. For a single plate this flux is directed from the plate.
The Casimir forces are investigated in the geometry of two plates. At
separations between the plates smaller than the curvature radius of the
background spacetime, to the leading order, we recover the corresponding result
in the Minkowski spacetime and in this case the forces are attractive. At
larger separations, the influence of the curvature on the Casimir forces is
essential with different asymptotic behavior for decelerated and accelerated
expansions. In particular, for the latter case there is a range of powers of
the expansion law in which the forces become repulsive at large separations
between the plates.
