Quantum gravitational back-reaction offers a simultaneous explanation for why the cosmological constant is so small and a natural model of inflation in which scalars play no role. In this talk I review previous work and present a simple model of the mechanism in which the induced stress tensor behaves like negative vacuum energy with a density proportional to $-\Lambda/{ 8\pi G} \cdot (G \Lambda)^2 \cdot H t$. The model also highlights the essential role of causality in back-reaction.
Categories: gr-qc astro-ph hep-ph