The behaviour of a massive, non-interacting and non-minimally coupled
quantised scalar field in an expanding de Sitter background is investigated by
solving the field evolution for an arbitrary initial state. In this approach
there is no need to choose a vacuum in order to provide a definition for
particle states. We conclude that the expanding de Sitter space is a stable
equilibrium configuration under small perturbations of the initial conditions.
Depending on the initial state, the energy density can approach its asymptotic
value from above or below, the latter of which implies a violation of the weak
energy condition. The backreaction of the quantum corrections can therefore
lead to a phase of super-acceleration also in the non-interacting massive case.