Why there is nothing rather than something: A theory of the cosmological constant

Wormholes are topology-changing configurations in euclidean quantum gravity whose importance has recently been advocated by several authors. I argue here that if wormholes exist, they have the effect of making the cosmological constant vanish. The argument involves approximations in dealing with physics at the wormhole energy scale (assumed to be somewhat below the Planck mass) but is exact in all interactions at all lower energies.