Causality and Cosmic Inflation

TL;DR

This paper examines the initial conditions necessary for cosmic inflation, emphasizing the role of causality and general relativity, and discusses potential modifications to inflationary models to relax the homogeneity requirement.

Contribution

It provides a causality-based argument that homogeneity on large scales must be assumed initially in certain inflationary models, contrasting with models originating at the Planck epoch.

Findings

Homogeneity on super-Hubble scales is an initial condition in some inflation models.

Models from the Planck epoch may avoid the homogeneity assumption.

Causality constrains the structure of spacetime in inflationary scenarios.

Abstract

In the context of inflationary models with a pre-inflationary stage, in which the Einstein equations are obeyed, the weak energy condition is satisfied, and spacetime topology is trivial, we argue that homogeneity on super-Hubble scales must be assumed as an initial condition. Models in which inflation arises from field dynamics in a Friedman-Robertson-Walker background fall into this class but models in which inflation originates at the Planck epoch, {\it eg.} chaotic inflation, may evade this conclusion. Our arguments rest on causality and general relativistic constraints on the structure of spacetime. We discuss modifications to existing scenarios that may avoid the need for initial large-scale homogeneity.