Sp-brane accelerating cosmologies

TL;DR

This paper explores S-brane solutions in cosmology, analyzing their potential to produce late-time acceleration of the universe within different theoretical frames and conditions, considering scalar fields and internal space dynamics.

Contribution

It introduces specific S-brane cosmological models that demonstrate accelerated expansion under certain conditions and frames, with detailed analysis of their stability and observational constraints.

Findings

Accelerating expansion occurs in Brans-Dicke frame for all external space types.

In Einstein frame, only flat external space with hyperbolic internal space accelerates.

Hyperbolic external space in Brans-Dicke frame satisfies experimental bounds.

Abstract

We investigate time dependent solutions (S-brane solutions) for product manifolds consisting of factor spaces where only one of them is non-Ricci-flat. Our model contains minimally coupled free scalar field as a matter source. We discuss a possibility of generating late time acceleration of the Universe. The analysis is performed in conformally related Brans-Dicke and Einstein frames. Dynamical behavior of our Universe is described by its scale factor. Since the scale factors of our Universe are described by different variables in both frames, they can have different dynamics. Indeed, we show that with our S-brane ansatz in the Brans-Dicke frame the stages of accelerating expansion exist for all types of the external space (flat, spherical and hyperbolic). However, applying the same ansatz for the metric in the Einstein frame, we find that a model with flat external space and hyperbolic compactification of the internal space is the only one with the stage of the accelerating expansion. Scalar field can prevent this acceleration. It is shown that the case of hyperbolic external space in Brans-Dicke frame is the only model which can satisfy experimental bounds for the fine structure constant variations. We obtain a class of models where a pare of dynamical internal spaces have fixed total volume. It results in fixed fine structure constant. However, these models are unstable and external space is non-accelerating.