Eternal inflation and its implications
Alan H. Guth
TL;DR
The article surveys how inflation explains key features of our universe and extends these ideas to eternal inflation, which posits a multiverse of pocket universes. It reviews mechanisms for eternal inflation in both new and chaotic models, discusses implications for the string theory landscape and anthropic reasoning, and highlights fundamental problems in defining probabilities within an infinite multiverse, including the youngness paradox and past-incompleteness. The work emphasizes that while observations support inflation, crucial questions about measures and initial conditions remain unresolved, motivating further theoretical development of the multiverse framework. Overall, the paper argues for the robustness of inflation but acknowledges deep conceptual challenges in making precise predictions in an eternally inflating cosmos.
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.