Casimir energy in finite expanding universes: a prelude
  We present the Green's functions that are the solutions of the massive Klein-Gordon equation for a scalar field with non-minimal coupling to gravitation for several static and expanding cosmological models. An important feature of such functions is that they can be used to study the appearence of a Casimir energy in models of the universe with compact spatial sections. 
Sandro Silva e Costa, Reuven Opher (IAG-USP)