Deformed General Relativity and Effective Actions from Loop Quantum Gravity

TL;DR

This paper develops effective actions from loop quantum gravity using canonical methods, revealing deformations in dispersion relations, inverse-triad corrections, and a novel signature-change cosmology that addresses initial conditions and entropy.

Contribution

It systematically extends classical constructions to quantum constraints, demonstrating new effects like signature change and larger-than-expected quantum corrections.

Findings

Deformed dispersion relations ensure causality.

Inverse-triad corrections differ from curvature effects.

High-curvature regimes show universe emergence from Euclidean space.

Abstract

Canonical methods can be used to construct effective actions from deformed covariance algebras, as implied by quantum-geometry corrections of loop quantum gravity. To this end, classical constructions are extended systematically to effective constraints of canonical quantum gravity and applied to model systems as well as general metrics, with the following conclusions: (i) Dispersion relations of matter and gravitational waves are deformed in related ways, ensuring a consistent realization of causality. (ii) Inverse-triad corrections modify the classical action in a way clearly distinguishable from curvature effects. In particular, these corrections can be significantly larger than often expected for standard quantum-gravity phenomena. (iii) Finally, holonomy corrections in high-curvature regimes do not signal the evolution from collapse to expansion in a "bounce," but rather the emergence of the universe from Euclidean space at high density. This new version of signature-change cosmology suggests a natural way of posing initial conditions, and a solution to the entropy problem.