26/02/2021 Colóquio (WEBINAR) Recent progress in fighting ghosts in quantum gravity - Ilya L. Shapiro (Universidade Federal de Juiz de Fora)
Recent progress in fighting ghosts in quantum gravity
Ilya L. Shapiro (Universidade Federal de Juiz de Fora)
The main difficulty of perturbative quantum gravity (QG) in D=4 is what is called 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 then we are risking to meet the bunch 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. Another point is that, even if initial frequencies of the tensor gravitational perturbations are transplanckian, 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. Finally, there are strong indications that the first-order stability may be extended to the non-perturbative domain, but only if the initial amplitudes of transplanckian perturbations are sufficiently small.