Regular cosmological solutions in low energy effective action from string theories

TL;DR

This paper explores singularity-free bouncing cosmological solutions derived from string theory effective actions, incorporating dilaton, axion, and Ramond-Ramond fields, which connect smoothly to standard cosmology.

Contribution

It demonstrates the existence of nonsingular bouncing solutions with finite dilaton in string-inspired models, extending previous cosmological solutions to include additional string-theoretic fields.

Findings

Bouncing solutions are found for models with flat or negative curvature.

Solutions connect smoothly to standard radiation-dominated cosmology.

Dilaton remains finite and positive throughout the evolution.

Abstract

The possibility of obtaining singularity free cosmological solutions in four dimensional effective actions motivated by string theory is investigated. In these effective actions, in addition to the Einstein-Hilbert term, the dilatonic and the axionic fields are also considered as well as terms coming from the Ramond-Ramond sector. A radiation fluid is coupled to the field equations, which appears as a consequence of the Maxwellian terms in the Ramond-Ramond sector. Singularity free bouncing solutions in which the dilaton is finite and strictly positive are obtained for models with flat or negative curvature spatial sections when the dilatonic coupling constant is such that $\omega < - 3/2$, which may appear in the so called $F$ theory in 12 dimensions. These bouncing phases are smoothly connected to the radiation dominated expansion phase of the standard cosmological model, and the asymptotic pasts correspond to very large flat spacetimes.