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Euclidean wormholes, baby universes, and their impact on particle physics and cosmology
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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.
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Cited by 4 Pith papers
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Positivity of the gravitational path integral implies the axionic weak gravity conjecture
Positivity of gravitational path integral inner products requires non-perturbative instabilities in axion wormholes that break shift symmetry, implying a sharp axion weak gravity conjecture with precise constants.
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The EDM inverse problem: Identifying the sources of CP violation and PQ breaking with electric dipole moments
Six classes of CP-violating operators near the QCD scale produce distinct EDM patterns that enable discrimination of their origins and distinguish high-scale versus low-energy sources of the axion vacuum expectation value.
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Axion Quality in Warped Extra-Dimension
Warped extra-dimensional axion models achieve high quality when nonlocal U(1)-charged field effects are sufficiently suppressed by the warp factor and orbifold structure.
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Before the Bang: Wormholes at the Dawn of the Universe
Euclidean wormholes broaden the landscape of regular semiclassical initial conditions for inflating universes, offering conceptual and practical advantages over the no-boundary proposal in quantum gravity.
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