Exponential cosmological solutions in Einstein-Gauss-Bonnet gravity with two subspaces: general approach

TL;DR

This paper systematically investigates exponential solutions in Einstein-Gauss-Bonnet gravity with two subspaces, providing a general scheme for finding solutions, analyzing their stability, and exploring physically relevant compactification scenarios.

Contribution

It introduces a comprehensive method to find and analyze exponential solutions in Einstein-Gauss-Bonnet gravity with two subspaces, including stability conditions and specific compactification cases.

Findings

Up to four solutions for given parameters and subspace dimensions.

Stable solutions can always be achieved by choosing the sign of expansion rates.

Bounds on parameters for physically relevant compactification scenarios.

Abstract

In this paper we perform systematic investigation of all possible exponential solutions in Einstein-Gauss-Bonnet gravity with the spatial section being a product of two subspaces. We describe a scheme which always allow to find solution for a given $\{p, q\} > 2$ (number of dimensions of two subspaces) and $\zeta$ (ratio of the expansion rates of these two subspaces). Depending on the parameters, for given $\{\alpha, \Lambda\}$ (Gauss-Bonnet coupling and cosmological constant) there could be up to four distinct solutions (with different $\zeta$'s). Stability requirement introduces relation between $\zeta$, $\{p, q\}$ and sign of the expansion rate. Nevertheless, for any $\{p, q\} > 2$ we can always choose sign for expansion rates so that the resulting solution would be stable. The scheme for finding solutions is described and the bounds on the parameters are drawn. Specific cases with $\{p, q\} = \{1, 2\}$ are also considered. Finally, we separately described physically sensible case with one of the subspaces being three-dimensional and expanding (resembling our Universe) while another to be contracting (resembling extra dimensions), describing successful compactification; for this case we also drawn bounds on the parameters where such regime occurs.