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Inflation and Eternal Inflation

Alan H. Guth

TL;DR

This paper surveys inflationary cosmology and argues that the universe is most plausibly the product of inflation. It then analyzes eternal inflation in both new and chaotic models, showing that it generates an infinite mosaic of pocket universes and has consequences for how predictions are extracted from theory. A central focus is the problem of defining probabilities in eternally inflating spacetimes, including the youngness paradox that arises with synchronous gauge regularization. Vilenkin's alternative probability prescription is discussed as a way to avoid some paradoxes and potentially restore predictive power in theories with many vacua. The paper concludes that while eternal inflation offers deep insights, a fully satisfactory treatment requires quantum gravity to address initial conditions and measure ambiguities.

Abstract

The basic workings of inflationary models are summarized, along with 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. Vilenkin's proposal for avoiding these problems is also discussed.

Inflation and Eternal Inflation

TL;DR

This paper surveys inflationary cosmology and argues that the universe is most plausibly the product of inflation. It then analyzes eternal inflation in both new and chaotic models, showing that it generates an infinite mosaic of pocket universes and has consequences for how predictions are extracted from theory. A central focus is the problem of defining probabilities in eternally inflating spacetimes, including the youngness paradox that arises with synchronous gauge regularization. Vilenkin's alternative probability prescription is discussed as a way to avoid some paradoxes and potentially restore predictive power in theories with many vacua. The paper concludes that while eternal inflation offers deep insights, a fully satisfactory treatment requires quantum gravity to address initial conditions and measure ambiguities.

Abstract

The basic workings of inflationary models are summarized, along with 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. Vilenkin's proposal for avoiding these problems is also discussed.

Paper Structure

This paper contains 13 sections, 30 equations, 5 figures.

Table of Contents

  1. Introduction
  2. How Does Inflation Work?
  3. The New Inflationary Scenario:
  4. Chaotic Inflation:
  5. Evidence for Inflation
  6. Eternal Inflation: Mechanisms
  7. Eternal New Inflation:
  8. Eternal Chaotic Inflation:
  9. Eternal Inflation: Implications
  10. Difficulties in Calculating Probabilities
  11. The Youngness Paradox
  12. An Alternative Probability Prescription
  13. Conclusion

Figures (5)

  • Figure 1: Generic form of the potential for the new inflationary scenario.
  • Figure 2: Generic form of the potential for the chaotic inflationary scenario.
  • Figure 3: A schematic illustration of eternal inflation.
  • Figure 4: Evolution of the inflaton field during eternal chaotic inflation.
  • Figure 5: A schematic picture of a pocket universe, illustrating Vilenkin's proposal for the calculation of probabilities.