Seminario de Ilya Shapiro (Universidade Federal de Juiz de Fora, Minas Gerais - Brazil)

Title: Confinement of ghosts and stability in higher derivative quantum gravity

Ilya L. Shapiro
Universidade Federal de Juiz de Fora, Minas Gerais - Brazil

Abstract:
The presence of higher derivatives in the gravitational action is required for
renormalizability in semiclassical and quantum gravity.  On the other hand,
higher derivatives produce unphysical massive ghosts, making classical
solutions unstable and quantum theory non-unitary. Trying to resolve this
contradiction, we explore classical cosmological solutions in the presence
of ghosts and find out that instability takes place only if the seeds of initial
perturbations are about the Planck order of magnitude, such that the ghost
can be generated from the vacuum. In the superrenormalizable versions of
quantum gravity, there may be a pair of complex conjugate massive poles.
In this case, the complex ghost-like states may form normal bound states.
For a while, the demonstration of this mechanism was achieved only for a
simplified toy model that reproduces the general structure of the ghost-like
poles. This model is much simpler than quantum gravity. However, suppose
the same situation takes place in realistic higher derivative models. In that
case, such a mechanism may explain the existence of the Planck scale
cut-off for the frequencies of gravitational perturbations and resolve the
problem of ghosts. At low energies, we show, using another toy model,
how the IR limit of the fundamental theory fits the UV limit of the effective
low-energy theory.

Fecha: jueves, 2 de octubre de 2025
Hora:   12:10 horas
Lugar: Seminario de Física Teórica