Instability of de Sitter space on short time scales

We consider the Einstein gravitational field equations coupled to the expectation value of the stress tensor in a real scalar field theory with arbitrary mass and coupling to curvature. We consider small oscillations of the metric and linearize the response of the quantum wave functional and the stress-tensor expectation value. In a small-time approximation we find unstable oscillations for a minimally coupled field and stable oscillations in the case of conformal coupling