Quadratic gravity: from weak to strong

TL;DR

This paper explores quadratic gravity, suggesting that nonperturbative effects at strong coupling could eliminate the spin-2 ghost problem and recover general relativity in the infrared regime.

Contribution

It proposes a new perspective on quadratic gravity by analyzing the strongly interacting regime and drawing analogies with QCD to address ghost issues.

Findings

Nonperturbative effects may remove the spin-2 ghost.

Strong coupling regime could lead to emergent general relativity.

Analogy with QCD provides insights into nonperturbative dynamics.

Abstract

More than three decades ago quadratic gravity was found to present a perturbative, renormalizable and asymptotically free theory of quantum gravity. Unfortunately the theory appeared to have problems with a spin-2 ghost. In this essay we revisit quadratic gravity in a different light by considering the case that the asymptotically free interaction flows to a strongly interacting regime. This occurs when the coefficient of the Einstein-Hilbert term is smaller than the scale $\Lambda_{\mathrm{QG}}$ where the quadratic couplings grow strong. Here QCD provides some useful insights. By pushing the analogy with QCD, we conjecture that the nonperturbative effects can remove the naive spin-2 ghost and lead to the emergence of general relativity in the IR.