A new class of spatially homogeneous 4D string backgrounds

TL;DR

This paper introduces a new class of spatially homogeneous 4D string backgrounds with five generic types, highlighting their anisotropic properties and conditions under which they approach FRW symmetry.

Contribution

It presents a novel classification of 4D string backgrounds, analyzing their symmetry properties and asymptotic behavior, expanding understanding of string cosmological models.

Findings

Contains five generic types of backgrounds.

Exact isotropy is broken due to the H-field.

FRW symmetry can be approached asymptotically in certain cases.

Abstract

A new class of spatially homogeneous 4D string backgrounds, the $X(d\rightarrow)$ according to a recent classification, is presented and shown to contain only five generic types. In contrast to the case of $X(d\uparrow)$ (which contains as a subclass all possible FRW backgrounds), exact $SO(3)$ isotropy is always broken in the $X(d\rightarrow)$ class. This is due to the $H$-field, whose dual is necessarily along a principal direction of anisotropy. Nevertheless, FRW symmetry can be attained asymptotically for Bianchi-types $I$ and $VII_0$ in a rather appealing physical context. Other aspects of the solutions found for types $X=I,II,III,VI_{-1}$, and of the $VII_0$ case are briefly discussed.