Recent Progress in Fighting Ghosts in Quantum Gravity

Understand

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.