A new class of spatially homogeneous 4D string backgrounds

Abstract

A new class of spatially homogeneous 4D string backgrounds, the $X(d\to )$ 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 $X(d\to )$ 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 $VI{I}_0$ in a rather appealing physical context. Other aspects of the solutions found for types $X=I,II,III,V{I}_{-1}$, and of the $VI{I}_0$ case are briefly discussed.

LaTeX, 13 pages, no figures.

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