Eternal Inflation: The Inside Story

TL;DR

This paper develops a causal patch framework for eternal inflation, emphasizing quantum fluctuations' role and analyzing different terminal vacua outcomes, including crunches, flat regions, and bubble collisions.

Contribution

It introduces a causal patch approach to eternal inflation, incorporating quantum field fluctuations and analyzing the effects of different terminal vacua mechanisms.

Findings

Observers cannot assign semiclassical geometry outside their horizon.

Eternal inflation can end in a crunch, flat region, or bubble collision.

Bubble collisions can preserve asymptotic flatness, as shown by an exact solution.

Abstract

Motivated by the lessons of black hole complementarity, we develop a causal patch description of eternal inflation. We argue that an observer cannot ascribe a semiclassical geometry to regions outside his horizon, because the large-scale metric is governed by the fluctuations of quantum fields. In order to identify what is within the horizon, it is necessary to understand the late time asymptotics. Any given worldline will eventually exit from eternal inflation into a terminal vacuum. If the cosmological constant is negative, the universe crunches. If it is zero, then we find that the observer's fate depends on the mechanism of eternal inflation. Worldlines emerging from an eternal inflation phase driven by thermal fluctuations end in a singularity. By contrast, if eternal inflation ends by bubble nucleation, the observer can emerge into an asymptotic, locally flat region. As evidence that bubble collisions preserve this property, we present an exact solution describing the collision of two bubbles.