Parafermions, Brane Distributions and FRW Hierarchies

TL;DR

This paper reviews exact string backgrounds organized in hierarchies, where boundary target spaces relate to other models, revealing connections to cosmological solutions and raising questions about the nature of time in string theory.

Contribution

It introduces a class of hierarchical exact string backgrounds involving parafermions and explores their relation to cosmological models and the concept of time in string theory.

Findings

Hierarchical structure of exact string backgrounds identified.

Connection between boundary target spaces and cosmological solutions established.

Implications for the nature of time in string theory discussed.

Abstract

I review a class of exact string backgrounds, which appear in hierarchies, where the boundary of the target space of an exact sigma model is itself the target space of another exact model. From the worldsheet viewpoint this is due to the existence of (1,1) operators based on parafermions. From the target space side, it is reminiscent of the structure of maximally symmetric Friedmann-Robertson-Walker cosmological solutions, with broken homogeneity though. Cosmological evolution in this framework raises again the question of the nature of time in string theory.