Quadratic gravity

Quantum quadratic gravity (QQG), also known as quadratic gravity (QG) is a specific realization of quantum Einstein gravity. This a quantum extension of general relativity obtained by adding all local quadratic-in-curvature terms to the Einstein-Hilbert Lagrangian.^[1][2] Doing this makes quantum gravity renormalizable.^[1]

QQG, besides being renormalizable, has also been shown to feature an ultraviolet fixed point^[3] (even in the presence of realistic matter sectors).^[4] It can, therefore, be regarded as a concrete realisation of asymptotic safety.

The theory was originally developed by Kellogg Stelle, but had difficulty being accepted as a viable theory because of its introduction of 'ghost particles'.^[5] As of 2025^[update], QQG is making a resurgence in the theoretical physics community, with proponents including Luca Buoninfante.^[6][5]

Work has been directed towards using the Event Horizon Telescope to test for the possibility of QG being a valid theory.^[7]