arXiv2019
We show that a critical condition exists in four dimensional scale invariant gravity given by the pure quadratic action $β\,C_{μνσρ} C^{μνσρ} + α\,R^2$ where $C^μ_{\,\,νσρ}$ is the Weyl tensor, $R$ is the Ricci scalar and $β$ and $α$ are dimensionless parameters. The critical condition in a dS or AdS background is $β=6 α$. This leads to critical gravity where the massive spin two physical ghost becomes a massless spin two graviton. In contrast to the original work on critical gravity, no Einstein gravity with a cosmological constant is added explicitly to the higher-derivative action. The critical condition is obtained in two independent ways. In the first case, we show the equivalence between the initial action and an action containing Einstein gravity, a cosmological constant, a massless scalar field plus Weyl squared gravity. The scale invariance is spontaneously broken. The linearized Einstein-Weyl equations about a dS or AdS background yield the critical condition $β=6α$. In the second case, we work directly with the original quadratic action. After a suitable field redefinition, where the metric perturbation is traceless and transverse, we obtain linearized equations about a dS or AdS background that yield the critical condition $β= 6α$. As in the first case, we also obtain a propagating massless scalar field. Substituting $β=6α$ into the energy and entropy formula for the Schwarzschild and Kerr AdS or dS black hole in higher-derivative gravity yields zero, the same value obtained in the original work on critical gravity. We discuss the role of boundary conditions in relaxing the $β=6α$ condition.