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Euclidean wormholes, baby universes, and their impact on particle physics and cosmology
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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.
Forward citations
Cited by 20 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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Ensemble averaging over wormhole α-parameters is dominated by the symmetric point α=0 for spontaneously broken U(1) p-form symmetries, so standard PQ models evade the wormhole-induced axion quality problem.
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Domain Walls From Confining Bubbles: $SU(N_{c})$ Yang Mills at Finite $\theta$
A nonzero theta angle weakens supercooling in SU(Nc) Yang-Mills confinement and makes any resulting domain-wall gravitational-wave signal invisible except under severe fine-tuning.
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Non-invertible symmetries in the axiverse, and the imaginary wormholes
Imaginary wormholes and the IDB imply that towers of BPS EFT instantons generate infinitely many superpotential terms that break non-invertible axion shift symmetries in N=1 axiverse models.
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Supergravity flows, wormholes and their pseudo-Hermitian holographic duals
Supergravity truncations admit real-metric wormhole and flow solutions via imaginary scalar extensions, interpreted as dual to pseudo-Hermitian PT-symmetric theories and entangled brane-antibrane states.
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A Menagerie of Wormholes and Cosmologies in the Gravitational Path Integral
The paper identifies a variety of Euclidean saddle solutions including wormholes and oscillatory configurations in Einstein-Scalar-Maxwell models, demonstrates how oscillations are controlled by lifting flat potential...
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A Menagerie of Wormholes and Cosmologies in the Gravitational Path Integral
Euclidean AdS wormholes, including new oscillatory 'wineglass' saddles, are classified and their on-shell action competition shows when inflation beats a big crunch.
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AdS3 axion wormholes as stable contributions to the Euclidean gravitational path integral
The paper shows that Euclidean axion wormholes remain regular and stable in AdS3 for any mouth radius to AdS radius ratio and across topologies, with computable actions that can be included in the 3D gravitational pat...
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The Minimal High-Quality QCD Axion
A flat-interval 5D U(1) Wilson-line axion with 4D KSVZ/DFSZ anomaly sector supplies exponential quality protection while remaining minimal.
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Sharpening the Supersymmetric Axion Weak Gravity Conjecture
The paper verifies the bound fS/|n| ≤ (π/(2 κ_d)) sqrt((d-1)/(d-2)) for axion instantons and sharpens it to fS/|n| ≤ (1/κ_4) sqrt(7/2) for supersymmetric 4d instantons using three approaches in the string landscape.
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Supergravity flows, wormholes and their pseudo-Hermitian holographic duals
Supergravity truncations admit real-metric wormhole solutions via imaginary scalar extensions whose holographic duals are pseudo-Hermitian and PT-symmetric, interpretable as entangled brane states.
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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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Axion Quality in Warped Extra-Dimension
Warped extra-dimensional Wilson-line axions can achieve high quality when non-QCD potential contributions from charged bulk fields are parametrically suppressed by geometry and brane effects.
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Non-invertible symmetries in the axiverse, and the imaginary wormholes
Identifies invertible and non-invertible generalized symmetries in axiverse EFTs and argues that wormholes break non-invertible axion symmetries via the Imaginary Distance Bound, implying a distinguished role for towe...
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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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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 can distinguish high-scale versus low-scale sources of axion VEV.
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Type IIB Axion--Dilaton Wormholes and the BPS Limit Hessian
In the axion charge sector of Type IIB, the E=0 BPS instanton has a physical Hessian that factorizes as Q dagger Q, interpreted as an endpoint theorem beyond stability for E>0 wormholes, with separation of throat from...
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Before the Bang: Wormholes at the Dawn of the Universe
Euclidean wormholes enlarge the semiclassical initial-condition landscape for the universe in a physically rich manner that aligns with holographic expectations and supports early-universe model building.
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Lectures on Naturalness, String Landscape and Multiverse
Lecture notes providing a technical introduction to naturalness problems and the string theory landscape for graduate students.
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