Time-dependent Orbifolds and String Cosmology

TL;DR

This paper reviews the physics of time-dependent orbifolds in string theory, focusing on their stability, interactions, and cosmological applications, including dualities with orientifold planes.

Contribution

It provides a detailed analysis of the stability and perturbative consistency of time-dependent orbifolds and explores their dualities and cosmological implications.

Findings

Resumming perturbative expansions yields finite amplitudes.

Certain orbifold models are dual to orientifold plane physics.

Cosmological models based on orientifold dynamics are discussed.

Abstract

In these lectures, we review the physics of time-dependent orbifolds of string theory, with particular attention to orbifolds of three-dimensional Minkowski space. We discuss the propagation of free particles in the orbifold geometries, together with their interactions. We address the issue of stability of these string vacua and the difficulties in defining a consistent perturbation theory, pointing to possible solutions. In particular, it is shown that resumming part of the perturbative expansion gives finite amplitudes. Finally we discuss the duality of some orbifold models with the physics of orientifold planes, and we describe cosmological models based on the dynamics of these orientifolds.