Spatially Homogeneous String Cosmologies

TL;DR

This paper classifies the most general antisymmetric H-field configurations in spatially homogeneous string cosmologies, revealing differences in their degrees of freedom compared to vacuum and perfect-fluid models.

Contribution

It determines the maximum arbitrary components of the H-field for all Bianchi-type cosmologies and compares their generality to inhomogeneous solutions.

Findings

Maximum components of H-field vary by Bianchi type.

Homogeneous string cosmologies have different degrees of freedom than vacuum models.

Hierarchy of generality is significantly different from perfect-fluid cosmologies.

Abstract

We determine the most general form of the antisymmetric $H$-field tensor derived from a purely time-dependent potential that is admitted by all possible spatially homogeneous cosmological models in 3+1-dimensional low-energy bosonic string theory. The maximum number of components of the $H$ field that are left arbitrary is found for each homogeneous cosmology defined by the Bianchi group classification. The relative generality of these string cosmologies is found by counting the number of independent pieces of Cauchy data needed to specify the general solution of Einstein's equations. The hierarchy of generality differs significantly from that characteristic of vacuum and perfect-fluid cosmologies. The degree of generality of homogeneous string cosmologies is compared to that of the generic inhomogenous solutions of the string field equations.