Quantum gravity slows inflation

We consider the quantum gravitational back-reaction on an initially inflating, homogeneous and isotropic universe whose topology is T-3 x R. Although there is no secular effect at one loop, an explicit calculation shows that two-loop processes act to slow the rate of expansion by an amount which becomes non-perturbatively large at late times. By exploiting Feynman's tree theorem we show that all higher loops act in the same sense.