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We evaluate the two-point functions of the electromagnetic field in (D+1)-dimensional spatially flat Friedmann-Robertson-Walker universes with apower-law scale factor, assuming that the field is prepared in the Bunch-Daviesvacuum state. The range of powers are specified in which the two-pointfunctions are infrared convergent and the Bunch-Davies vacuum for theelectromagnetic field is a physically realizable state. The two-point functionsare applied for the investigation of the vacuum expectation values of the fieldsquared and the energy-momentum tensor, induced by a single and two parallelconducting plates. Unlike to the case of conducting plates in the Minkowskibulk, in the problem under consideration the stresses along the directionsparallel to the plates are not equal to the energy density. We show that, inaddition to the diagonal components, the vacuum energy-momentum tensor has anonzero off-diagonal component which describes energy flux along the directionnormal 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. Atseparations between the plates smaller than the curvature radius of thebackground spacetime, to the leading order, we recover the corresponding resultin the Minkowski spacetime and in this case the forces are attractive. Atlarger separations, the influence of the curvature on the Casimir forces isessential with different asymptotic behavior for decelerated and acceleratedexpansions. In particular, for the latter case there is a range of powers ofthe expansion law in which the forces become repulsive at large separationsbetween the plates.

Electromagnetic two-point functions and the Casimir effect in Friedmann-Robertson-Walker cosmologies