Recent Progress in Fighting Ghosts in Quantum Gravity

TL;DR

This paper reviews recent advances in understanding the stability and unitarity issues caused by ghost states in higher derivative quantum gravity, exploring conditions under which ghosts may be rendered harmless.

Contribution

It surveys recent progress on conditions that could mitigate ghost problems in higher derivative quantum gravity theories.

Findings

Certain physical conditions can make ghosts non-offensive

Progress in understanding stability of classical solutions

Discussion on properties of fundamental theories that address ghost issues

Abstract

We review some of the recent results which can be useful for better understanding of the problem of stability of vacuum and in general classical solutions in higher derivative quantum gravity. The fourth derivative terms in the purely gravitational vacuum sector are requested by renormalizability already in both semiclassical and complete quantum gravity theories. However, because of these terms the spectrum of the theory has unphysical ghost states which jeopardize the stability of classical solutions. At the quantum level ghosts violate unitarity, and and thus ghosts look incompatible with the consistency of the theory. The `dominating' or `standard' approach is to treat higher derivative terms as small perturbations at low energies. Such an effective theory is supposed to glue with an unknown fundamental theory in the high energy limit. We argue that the perspectives for such a scenario are not clear, to say the least. On the other hand, recently there was certain progress in understanding physical conditions which can make ghosts not offensive. We survey these results and discuss the properties of the unknown fundamental theory which can provide these conditions satisfied.