THE PHYSICAL BASIS FOR INFRARED DIVERGENCES IN INFLATIONARY QUANTUM-GRAVITY

We consider tree-order graviton scattering amplitudes on a (3 + 1)-dim ensional de Sitter background in conformally flat coordinates. Infra-r ed divergences which cannot be absorbed using conventional techniques are shown to arise because conformal factors from the vertices are not compensated either by propagators or by external wavefunctions. The p hysical problem at tree order is therefore late interaction times rath er than small spatial coordinate momenta, despite a mathematical probl em at small momenta in the naive mode expansion of the propagator. Eve n in loops, concern over small spatial momenta is physically irrelevan t because the chaotic conditions likely to prevail after the Big Bang could not have resulted in the simultaneous onset of inflation over a patch extending much beyond the Hubble radius. This motivates our prop osal for a propagator which can be used to compute expectation values well inside the de Sitter patch of a plausible initial state.