  A spatially flat Friedmann-Robertson-Walker background with a general scale
factor is considered. In this space-time, the energy-momentum tensor of the
scalar field with a general curvature coupling parameter is obtained. Using the
Thermo Field Dynamics (TFD) formalism the Stefan-Boltzmann law and the Casimir
effect at finite temperature are calculated. The Casimir effect at zero
temperature is also considered. The expansion of the universe changes these
effects. A discussion of these modifications is presented.
