Speaker
Description
The main difficulty of perturbative quantum gravity (QG) in D=4 is the conflict between renormalizability and unitarity of the theory. The simplest version of QG is based on General Relativity and is non-renormalizable. One can construct renormalizable and even superrenormalizable versions of QG by introducing higher derivatives, but this leads to emergence of unphysical higher-derivative massive ghosts. The non-polynomial models of QG have no ghosts at the tree
level, but taking loop corrections into account one meets infinite amount of ghost-like complex states. The same is true for the string-induced gravitational action, which requires an infinite amount of fine-tuning to remain free of ghosts. We discuss the recent proposal of dealing with ghosts at the energies much lower than their masses, when they can not be presumably generated from vacuum. The very recent result is that even if initial frequencies of the tensor gravitational perturbations are transplanckian, due to the fast expansion of the universe in the period when such waves could be generated, these frequencies rapidly decrease and the explosion caused by ghosts does not have destructive effect on the classical cosmological solutions.
