Exorcising ghosts in quantum gravity

TL;DR

This paper demonstrates that apparent ghosts in higher-derivative quantum gravity are artifacts of truncation and can be eliminated by boundary condition redefinition, preserving the theory's renormalizability and unitarity.

Contribution

It shows that Ostrogradsky ghosts in quadratic gravity are fictitious and can be removed without losing renormalizability by redefining boundary conditions.

Findings

Ghosts are due to truncation, not fundamental.

Redefining boundary conditions removes ghosts.

Quadratic gravity remains renormalizable and unitary.

Abstract

We remark that Ostrogradsky ghosts in higher-derivative gravity, with a finite number of derivatives, are fictitious as they result from an unjustified truncation performed in a complete theory containing infinitely many curvature invariants. The apparent ghosts can then be projected out of the quadratic gravity spectrum by redefining the boundary conditions of the theory in terms of an integration contour that does not enclose the ghost poles. This procedure does not alter the renormalizability of the theory. One can thus use quadratic gravity as a quantum field theory of gravity that is both renormalizable and unitary.