Long-wavelength approximation for string cosmology with barotropic perfect fluid

TL;DR

This paper develops a long-wavelength approximation method to solve string cosmology equations with a barotropic perfect fluid, analyzing quasi-isotropic solutions and their implications for pre-big bang models.

Contribution

It introduces an iterative solution approach for string cosmology equations with matter, including second-order solutions and their physical interpretation.

Findings

Zero-order quasi-isotropic solutions compared with pure dilaton gravity.

Second-order solutions with growing and decaying modes identified.

Implications for pre-big bang cosmological scenarios analyzed.

Abstract

The field equations derived from the low energy string effective action with a matter tensor describing a perfect fluid with a barotropic equation of state are solved iteratively using the long-wavelength approximation, i.e. the field equations are expanded by the number of spatial gradients. In the zero order, a quasi-isotropic solution is presented and compared with the general solution of the pure dilaton gravity. Possible cosmological models are analyzed from the point of view of the pre-big bang scenario. The second order solutions are found and their growing and decaying parts are studied.