Eternal inflation and its implications

TL;DR

This paper discusses the concept of eternal inflation, its mechanisms, implications for universe predictions, and the challenges in defining probabilities within eternally inflating models, including the past-incompleteness of inflation.

Contribution

It provides a comprehensive review of eternal inflation mechanisms, its implications for cosmological predictions, and the issues related to probability measures and past-incompleteness.

Findings

Eternal inflation leads to a multiverse of unobservable pocket universes.

Probabilistic predictions in eternal inflation are ambiguous due to measure issues.

Inflation cannot be past-eternal; it must have a beginning.

Abstract

I summarize the arguments that strongly suggest that our universe is the product of inflation. The mechanisms that lead to eternal inflation in both new and chaotic models are described. Although the infinity of pocket universes produced by eternal inflation are unobservable, it is argued that eternal inflation has real consequences in terms of the way that predictions are extracted from theoretical models. The ambiguities in defining probabilities in eternally inflating spacetimes are reviewed, with emphasis on the youngness paradox that results from a synchronous gauge regularization technique. Although inflation is generically eternal into the future, it is not eternal into the past: it can be proven under reasonable assumptions that the inflating region must be incomplete in past directions, so some physics other than inflation is needed to describe the past boundary of the inflating region.