On the Ghost Problem of Conformal Gravity

Abstract

We study the metric perturbations around the de Sitter and Minkowski backgrounds in Conformal Gravity. We confirm the presence of ghosts in both cases. In the de Sitter case, by applying the Maldacena boundary conditions - the Neumann boundary condition and the positive-frequency mode condition - to the metric, we show that one cannot recover a general solution for the perturbations. In turn, alongside the Neumann boundary condition, we derive an additional condition with which the perturbations of conformal gravity and dS perturbations of Einstein gravity with cosmological constant coincide. We further show that the Neumann boundary condition does not lead to a general solution in Minkowski space. Conversely, we derive the alternative boundary conditions, with which we attain an agreement between the perturbations of conformal and Einstein gravity in full generality, thus removing the ghost of conformal gravity.

24 pages

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• Ultraviolet Completion of the Big Bang in Quadratic Gravity
• Emergent Lorentzian dispersion relations from a Euclidean scalar-tensor theory
• Unification of Conformal and Fuzzy Gravities with Internal Interactions resulting in SO(10) and a Possible Probe through Stochastic Gravitational Wave Background
• Nonconformally Ricci-flat instantons in Conformal Gravity with and without nonlinear matter fields