Deducing cosmological observables from the S matrix

We study one loop quantum gravitational corrections to the long range force induced by the exchange of a massless scalar between two massive scalars. The various diagrams contributing to the flat space S matrix are evaluated in a general covariant gauge, and we show that dependence on the gauge parameters cancels at a point considerably before forming the full S matrix, which is unobservable in cosmology. It is possible to interpret our computation as a solution to the effective field equations - which could be done even in cosmology - but taking account of quantum gravitational corrections from the source and from the observer.