Quadratic gravity: From weak to strong

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(QG) where the quadratic couplings grow strong. Here quantum chromodynamics (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 (GR) in the IR.