The R^2 phase-diagram of QEG and its spectral dimension

TL;DR

This paper investigates the phase diagram of Quantum Einstein Gravity (QEG) with R^2 terms, analyzing RG flows and spectral dimensions to understand the fractal nature of spacetime in asymptotic safety.

Contribution

It extends the analysis of QEG by including R^2 truncations, constructing RG trajectories with classical regimes, and examining the robustness of the multi-fractal spacetime picture.

Findings

Spectral dimension exhibits regimes of constancy linked to fixed points or singularities.

Multi-fractal spacetime structure remains stable under R^2 extension.

Universal features of RG fixed points influence spectral dimension behavior.

Abstract

Within the gravitational asymptotic safety program, the RG flow of the R^2 truncation in three and four spacetime dimensions is analyzed in detail. In particular, we construct RG trajectories which emanate from the non-Gaussian UV fixed point and possess long classical regimes where the effective average action is well approximated by the classical Einstein-Hilbert action. As an application we study the spectral dimension of the effective QEG spacetimes resulting from these trajectories, establishing that the picture of a multi-fractal spacetime is robust under the extension of the truncated theory space. We demonstrate that regimes of constant spectral dimensions can either be attributed to universal features of RG fixed points or singular loci in the \beta functions.