Gauss-Bonnet dynamical compactification scenarios and their ghosts in the tensor sectors

TL;DR

This paper investigates the stability of dynamical compactification scenarios in Einstein-Gauss-Bonnet theory, revealing ghost instabilities in tensor sectors and exploring alternative solutions that violate gravitational wave speed constraints.

Contribution

It demonstrates the presence of instabilities in standard compactification models and proposes new stable solutions that are ultimately incompatible with gravitational wave observations.

Findings

Standard scenarios exhibit ghostly tensor perturbations.

New solutions are stable but violate gravitational wave speed bounds.

The study highlights the need for further exploration of stable compactification models.

Abstract

In a cosmological context, the Einstein-Gauss-Bonnet theory contains, in $d+4$ dimensions, a dynamical compactification scenario in which the additional dimensions settle down to a configuration with a constant radion/scale factor. Sadly however this work demonstrates that such a quite appealing framework is plagued by instabilities, either from the background configuration's unsteadiness or the ghostly behaviors of the tensorial perturbations. New and stable solutions are found by relaxing one of the hypotheses defining the original compactification scenario. However, such configurations do not respect the current bounds on the speed of propagation of gravitational waves, and thus have to be discarded. Those results thus advocate for a comprehensive study of compactification scenarios in the Gauss-Bonnet framework, their stability, and the effects of matter inclusion.