The Swampland: Introduction and Review (1903.06239v3)

Abstract: The Swampland program aims to distinguish effective theories which can be completed into quantum gravity in the ultraviolet from those which cannot. This article forms an introduction to the field, assuming only a knowledge of quantum field theory and general relativity. It also forms a comprehensive review, covering the range of ideas that are part of the field, from the Weak Gravity Conjecture, through compactifications of String Theory, to the de Sitter conjecture.

• The paper introduces the Swampland framework, identifying effective field theories that cannot be completed into quantum gravity.
• It explains key conjectures like the Weak Gravity Conjecture and the Swampland Distance Conjecture using string-theoretic and holographic methods.
• The review highlights implications for black hole physics and global symmetry constraints, guiding future directions in quantum gravity research.

Overview of "The Swampland: Introduction and Review"

Introduction

This paper, authored by Eran Palti, provides a detailed introduction and comprehensive review of the Swampland program, which aims to distinguish effective field theories that can be completed into quantum gravity in the ultraviolet from those that cannot. The Swampland encompasses those theories appearing self-consistent at low energies but inconsistent upon attempts for ultraviolet completion, essentially forming constraints beyond empirical testing realms. A central thesis is the differentiation between the 'Landscape,' i.e., theories deriving from string theory vacua, and the 'Swampland,' where such derivation is not possible.

Criteria and Conjectures

Several conjectural criteria underpin the Swampland program:

1. No Global Symmetries: Quantum gravity forbids exact global symmetries to ensure a gauge description of fundamental interactions.
2. Completeness: All possible charges (consistent with Dirac quantization) should appear in the spectra of theories consistent with quantum gravity, ensuring no observational differences between symmetry gauge and global transformations.
3. Weak Gravity Conjecture (WGC): It postulates that for any gauge theory coupled to gravity, a state exists where gravity is the weakest force. This conjecture leads to the stronger requirement that black holes must be capable of shedding all charge through particle emission, with the particle masses satisfying $m \leq g q M_p$.
4. Swampland Distance Conjecture (SDC): In moduli spaces describing scalar fields, any infinite geodesic distance ensures the occurrence of massive states becoming light and forming a tower, invalidating predictions of effective field theories in vast field space excursions.

Implications and Evidence

The theoretical grounding spans across general arguments using semi-classical gravity, holography, and heap specific attention to string-theoretic scenarios offering corroborative case studies. For instance, dimensional reduction exemplifies these principles, further extending to specifics like the landscape's robustly populated weak gravity-friendly structures.

Black Hole Physics

Certain conjectural principles directly map to black hole characteristics, where extremal black holes, charged via gauge groups, inherently necessitate specific particle physics properties to prevent etheriacal remnants during evaporation, further reinforcing the proposals like WGC.

Future Directions and Connections

The paper emphasizes uncovering ultraviolet principles possibly missed within string theory and suggests that Swampland's criteria interconnect with emergent low-energy dynamics. As speculative as some of these connections might be due to underlying subtleties within higher-dimensional geometries, they posit a critical viewpoint on hypothesizing physics' completeness from scalar-tied expectations to detailed holographic propositions under assumptions of large gauge symmetry or gravitational constraints.

Through this lens, researchers may use this foundational framework to probe new developments, envisioning future theories' implications in global symmetry avoidance, anomaly consistency, and vast field space avoidance. As such, the Swampland program posits broader insight avenues upon quantum gravity, inviting challenges akin to proving possible theoretical constructs wrong, fostering a rigorous pursuit resembling established empirical sciences within conducted dimensions' high/gravitational layer interplay.