Infrared limit of quantum gravity

TL;DR

This paper investigates the infrared behavior of quantum gravity with a cosmological constant, showing how functional renormalization can address instabilities and potentially solve the cosmological constant problem through a runaway solution.

Contribution

It introduces a functional renormalization approach to quantum gravity's infrared limit, revealing a graviton barrier and a mechanism for natural smallness of the cosmological constant.

Findings

Functional renormalization avoids instabilities in quantum gravity.

A graviton barrier limits the growth of the effective potential.

Variable gravity can address the cosmological constant problem.

Abstract

We explore the infrared limit of quantum gravity in presence of a cosmological constant or effective potential for scalar fields. For a positive effective scalar potential, one-loop perturbation theory around flat space is divergent due to an instability of the graviton propagator. Functional renormalization solves this problem by a flow of couplings avoiding instabilities. This leads to a graviton barrier limiting the maximal growth of the effective potential for large values of scalar fields. In the presence of this barrier, variable gravity with a field dependent Planck mass can solve the cosmological constant problem by a cosmological runaway solution. We discuss the naturalness of tiny values of the cosmological constant and cosmon mass due to a strong attraction towards an infrared fixed point.