Renormalization group properties of the conformal sector: towards perturbatively renormalizable quantum gravity

TL;DR

This paper explores the renormalization group properties of the conformal sector in quantum gravity, revealing unique flow behaviors and fixed points that suggest a link between universe size and inhomogeneity, aiming for a perturbatively renormalizable theory.

Contribution

It introduces a novel analysis of the conformal sector's RG properties, highlighting non-perturbative eigenoperators and the conditions for RG flow existence in quantum gravity.

Findings

Complete RG flows exist only in the reverse direction for the conformal sector.

Eigenoperators are non-perturbative in ar and orthonormal.

The minimum universe size relates to inhomogeneity levels.

Abstract

The Wilsonian renormalization group (RG) requires Euclidean signature. The conformal factor of the metric then has a wrong-sign kinetic term, which has a profound effect on its RG properties. Generically for the conformal sector, complete flows exist only in the reverse direction (i.e. from the infrared to the ultraviolet). The Gaussian fixed point supports infinite sequences of composite eigenoperators of increasing infrared relevancy (increasingly negative mass dimension), which are orthonormal and complete for bare interactions that are square integrable under the appropriate measure. These eigenoperators are non-perturbative in $\hbar$ and evanescent. For $\mathbb{R}^4$ spacetime, each renormalised physical operator exists but only has support at vanishing field amplitude. In the generic case of infinitely many non-vanishing couplings, if a complete RG flow exists, it is characterised in the infrared by a scale $\Lambda_\mathrm{p}>0$, beyond which the field amplitude is exponentially suppressed. On other spacetimes, of length scale $L$, the flow ceases to exist once a certain universal measure of inhomogeneity exceeds $O(1)+2\pi L^2\Lambda^2_\mathrm{p}$. Importantly for cosmology, the minimum size of the universe is thus tied to the degree of inhomogeneity, with spacetimes of vanishing size being required to be almost homogeneous. We initiate a study of this exotic quantum field theory at the interacting level, and discuss what the full theory of quantum gravity should look like, one which must thus be perturbatively renormalizable in Newton's constant but non-perturbative in $\hbar$.