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

TL;DR

This paper reviews the role of Euclidean wormholes and gravitational instantons in quantum gravity, discussing their effects on particle physics, cosmology, and potential phenomenological implications like inflation and dark matter.

Contribution

It provides a comprehensive review of the controversies and recent developments regarding gravitational instantons and wormholes in quantum gravity and their phenomenological consequences.

Findings

Wormholes can break axion shift symmetries non-perturbatively.

Implications for large-field inflation and the Weak Gravity conjecture.

Potential effects on the strong CP problem and dark matter.

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.