Criticality and Scaling in 4D Quantum Gravity

TL;DR

This paper identifies the critical anomaly coefficient in 4D conformal factor quantum gravity where a phase transition occurs, using singular configurations, and relates it to known transitions in statistical models.

Contribution

It provides a simple argument to determine the critical value of the anomaly coefficient in 4D quantum gravity, linking phase transitions to spike configurations.

Findings

Critical anomaly coefficient value determined

Phase transition analogous to BKT transition identified

Scaling relations in the smooth phase rederived

Abstract

We present a simple argument which determines the critical value of the anomaly coefficient in four dimensional conformal factor quantum gravity, at which a phase transition between a smooth and elongated phase should occur. The argument is based on the contribution of singular configurations ("spikes") which dominate the partition function in the infrared. The critical value is the analog of c=1 in the theory of random surfaces, and the phase transition is similar to the Berezenskii-Kosterlitz-Thouless transition. The critical value we obtain is in agreement with the previous canonical analysis of physical states of the conformal factor and may explain why a smooth phase of quantum gravity has not yet been observed in simplicial simulations. We also rederive the scaling relations in the smooth phase in light of this determination of the critical coupling.