Quantum Gravity Slows Inflation
  We consider the quantum gravitational back-reaction on an initially inflating, homogeneous and isotropic universe whose topology is $T^3 \times \Re$. 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. 
N. C. Tsamis (Ecole Polytechnique and FO.R.T.H., Greece) and R. P.   Woodard (Univ. of Florida, Gainesville)