Super-renormalizable Quantum Gravity

TL;DR

This paper introduces a super-renormalizable, ghost-free extension of higher derivative quantum gravity involving infinite derivatives, which improves unitarity and modifies black hole solutions with regular cores.

Contribution

It develops a novel infinite-derivative gravity model that is super-renormalizable and ghost-free, addressing unitarity issues in quantum gravity.

Findings

The theory is one-loop renormalizable and finite from two loops onward.

It exhibits a reduction in spacetime dimension at high energies.

Black hole solutions are regular with de Sitter-like cores.

Abstract

In this paper we study perturbatively an extension of the Stelle higher derivative gravity involving an infinite number of derivative terms. We know that the usual quadratic action is renormalizable but suffers of the unitarity problem because of the presence of a ghost (state of negative norm) in the theory. The new theory is instead ghost-free since the introduction of (in general) two entire functions in the model with the property do not introduce new poles in the propagator. The local high derivative theory is recovered expanding the entire functions to the lowest order in the mass scale of the theory. Any truncation of the entire functions gives rise to the unitarity violation but if we keep all the infinite series we do not fall into these troubles. The theory is renormalizable at one loop and finite from two loops on. Since only a finite number of graphs are divergent then the theory is super-renormalizable. We analyze the fractal properties of the theory at high energy showing a reduction of the spacetime dimension at short scales. Black hole spherical symmetric solutions are also studied omitting the high curvature corrections in the equation of motions. The solutions are regular and the classical singularity is replaced by a "de Sitter-like core" in r=0. Black holes may show a "multi-horizon" structure depending on the value of the mass.