On `rolling moduli' solutions in string cosmology

TL;DR

This paper explores time-dependent solutions in string cosmology by replacing static moduli with functions of time, presenting new cosmological models and analyzing their properties and dualities.

Contribution

It introduces new 'rolling moduli' solutions in string cosmology, extending known static solutions to time-dependent cases with multiple moduli.

Findings

Reinterpretation of Nappi-Witten solution as time-dependent

Construction of new two-moduli cosmological solutions

Analysis of duality transformations in time-dependent backgrounds

Abstract

Given a static string solution with some free constant parameters (`moduli') it may be possible to construct a time-dependent solution by just replacing the moduli by some functions of time. We present several examples when such `rolling moduli' ansatz is consistent. In particular, the anisotropic D=4 cosmological solution of Nappi and Witten can be reinterpreted as a time-dependent generalisation of the (analytic continuation of) D=3 `charged black string' background with the `charge' changing with time. We find some new D=4 cosmological solutions which are `two rolling moduli' generalisations of the previously known ones. We also comment on interplay between duality transformations and the replacement of moduli by functions of time.