Causality and cosmic inflation - art. no. 023502

In the context of inflationary models with a pre-inflationary stage, in whi ch the Einstein equations are obeyed, the null energy condition is satisfie d, and spacetime topology is trivial, we argue that homogeneity on super-Hu bble scales must be assumed as an initial condition, Models in which inflat ion arises from field dynamics in a Friedmann-Robertson-Walker background f all into this class but models in which inflation originates at the Planck epoch may evade this conclusion. Our arguments rest on causality and genera l relativistic constraints on the structure of spacetime. We discuss modifi cations to existing scenarios that may avoid the need for initial large-sca le homogeneity.