"Recent progress in fighing ghosts in quantum gravity"

Ilya L. Shapiro

(Universidad de Juiz de Fora, Brasil)

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 then we risk 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. 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.

Facultad de Ciencias

Aula 4, primera planta, Edificio A
Universidad de Zaragoza