Conformal sector of Quantum Einstein Gravity in the local potential approximation: non-Gaussian fixed point and a phase of unbroken diffeomorphism invariance

TL;DR

This paper investigates the nonperturbative renormalization group flow of conformally reduced Quantum Einstein Gravity using the Local Potential Approximation, identifying fixed points and phase transitions relevant for asymptotic safety and diffeomorphism invariance.

Contribution

It introduces a novel analysis of the conformal sector in quantum gravity with the Local Potential Approximation, revealing fixed points and phase structure supporting asymptotic safety.

Findings

Identified Gaussian and non-Gaussian fixed points in the conformal sector.

Found evidence for a phase transition to unbroken diffeomorphism invariance.

Numerically constructed RG trajectories within the UV critical surface.

Abstract

We explore the nonperturbative renormalization group flow of Quantum Einstein Gravity (QEG) on an infinite dimensional theory space. We consider "conformally reduced" gravity where only fluctuations of the conformal factor are quantized and employ the Local Potential Approximation for its effective average action. The requirement of "background independence" in quantum gravity entails a partial differential equation governing the scale dependence of the potential for the conformal factor which differs significantly from that of a scalar matter field. In the infinite dimensional space of potential functions we find a Gaussian as well as a non-Gaussian fixed point which provides further evidence for the viability of the asymptotic safety scenario. The analog of the invariant cubic in the curvature which spoils perturbative renormalizability is seen to be unproblematic for the asymptotic safety of the conformally reduced theory. The scaling fields and dimensions of both fixed points are obtained explicitly and possible implications for the predictivity of the theory are discussed. Spacetime manifolds with $R^d$ as well as $S^d$ topology are considered. Solving the flow equation for the potential numerically we obtain examples of renormalization group trajectories inside the ultraviolet critical surface of the non-Gaussian fixed point. The quantum theories based upon some of them show a phase transition from the familiar (low energy) phase of gravity with spontaneously broken diffeomorphism invariance to a new phase of unbroken diffeomorphism invariance; the latter phase is characterized by a vanishing expectation value of the metric.