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. The idea is that the bare cosmological constant is actually GUT scale and that this causes inflation during the early universe. Inflation slows due to the accumulated gravitational attraction between virtual pairs of gravitons (and possibly also scalars) which are ripped apart by the rapid expansion. In this talk I first review work on the mechanism and then give simple physical answers to the following three questions:

1. Why does the induced energy density go like -H²/G (G H²)² (H t)^power ?
2. Why does the induced pressure go like minus the induced energy density?
3. How can the effect be causal when it involves super-horizon modes?

Graphics version of abstract

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