Phases of Brans-Dicke cosmology(II):Matter from NS-NS sector

TL;DR

This paper explores the cosmological solutions of Brans-Dicke theory coupled with matter from the NS-NS sector, revealing non-singular solutions in string frame but singular ones in Einstein frame, depending on parameters.

Contribution

It provides exact solutions for Brans-Dicke cosmology with NS-NS matter and classifies their singularity properties based on key parameters.

Findings

Non-singular solutions in string frame for certain parameters

All Einstein frame solutions are singular

Classification of solutions by equation of state parameter and Brans-Dicke parameter

Abstract

We study the cosmology of the Brans-Dicke(BD) theory coupled to perfect fluid type matter. In our previous works, the case where matter is coming from the Ramond-Ramond sector of the string theory was studied. Here, we study the case where matter is coming from the NS-NS sector. Exact solutions are found and the cosmology is classified according to the values of $\gamma$, the parameter of the equation of state and $\omega$, Brans-Dicke parameter. We find taht, in string frame, there are solutions without singularity for some ranges of $\gamma$ and $\omega$. In Einstein frame, however, all solutions are singular.