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Euclidean wormholes, baby universes, and their impact on particle physics and cosmology

Arthur Hebecker, Thomas Mikhail, Pablo Soler

TL;DR

The paper surveys Euclidean wormholes and baby universes in quantum gravity, focusing on how gravitational instantons can renormalize couplings and induce axion potentials. It develops the bilocal-to-local action framework via $\alpha$-parameters, interprets wormhole effects in terms of a baby-universe Hilbert space, and discusses phenomenological implications for axions, inflation, and dark matter. It also highlights deep conceptual issues, including the FKS catastrophe, negative modes in Euclidean gravity, and holographic tensions in AdS/CFT, outlining potential resolutions and the need for a deeper nonperturbative quantum gravity understanding. Overall, the work shows that wormhole physics can both illuminate and complicate effective field theories, with especially strong relevance for axion phenomenology and quantum-gravity constraints like the Weak Gravity Conjecture.

Abstract

The euclidean path integral remains, in spite of its familiar problems, an important approach to quantum gravity. One of its most striking and obscure features is the appearance of gravitational instantons or wormholes. These renormalize all terms in the Lagrangian and cause a number of puzzles or even deep inconsistencies, related to the possibility of nucleation of "baby universes". In this review, we revisit the early controversies surrounding these issues as well as some of the more recent discussions of the phenomenological relevance of gravitational instantons. In particular, wormholes are expected to break the shift symmetries of axions or Goldstone bosons non-perturbatively. This can be relevant to large-field inflation and connects to arguments made on the basis of the Weak Gravity or Swampland conjectures. It can also affect Goldstone bosons which are of physical interest in the context of the strong CP problem or as dark matter.

Euclidean wormholes, baby universes, and their impact on particle physics and cosmology

TL;DR

The paper surveys Euclidean wormholes and baby universes in quantum gravity, focusing on how gravitational instantons can renormalize couplings and induce axion potentials. It develops the bilocal-to-local action framework via -parameters, interprets wormhole effects in terms of a baby-universe Hilbert space, and discusses phenomenological implications for axions, inflation, and dark matter. It also highlights deep conceptual issues, including the FKS catastrophe, negative modes in Euclidean gravity, and holographic tensions in AdS/CFT, outlining potential resolutions and the need for a deeper nonperturbative quantum gravity understanding. Overall, the work shows that wormhole physics can both illuminate and complicate effective field theories, with especially strong relevance for axion phenomenology and quantum-gravity constraints like the Weak Gravity Conjecture.

Abstract

The euclidean path integral remains, in spite of its familiar problems, an important approach to quantum gravity. One of its most striking and obscure features is the appearance of gravitational instantons or wormholes. These renormalize all terms in the Lagrangian and cause a number of puzzles or even deep inconsistencies, related to the possibility of nucleation of "baby universes". In this review, we revisit the early controversies surrounding these issues as well as some of the more recent discussions of the phenomenological relevance of gravitational instantons. In particular, wormholes are expected to break the shift symmetries of axions or Goldstone bosons non-perturbatively. This can be relevant to large-field inflation and connects to arguments made on the basis of the Weak Gravity or Swampland conjectures. It can also affect Goldstone bosons which are of physical interest in the context of the strong CP problem or as dark matter.

Paper Structure

This paper contains 30 sections, 120 equations, 10 figures.

Table of Contents

  1. Introduction
  2. From instantons to wormholes
  3. Instantons
  4. Giddings-Strominger solution
  5. Dilatonic wormholes
  6. The effect of wormholes
  7. The bilocal action
  8. Local action involving parameters
  9. Baby universes
  10. The perspective of -vacua and the wormhole density
  11. Multiple large universes
  12. Phenomenological applications
  13. Random values of couplings and the cosmological constant problem
  14. Axion potentials from wormholes
  15. Superplanckian axions
  16. ...and 15 more sections

Figures (10)

  • Figure 1: Wormhole corresponding to the creation and absorption of a baby universe.
  • Figure 2: Gauge theory instantons as well-separated, localized lumps of field strength.
  • Figure 3: Euclidean brane instanton as particle-antiparticle fluctuation wrapping the compact space.
  • Figure 4: Wormholes: A semiwormhole (left), a wormhole connecting two distinct large asymptotically flat universes (center) and a wormhole on a single universe (right).
  • Figure 5: Extremal (left) and Cored (right) gravitational instanton.
  • ...and 5 more figures