Hiding the extra dimensions: A review on scale separation in string theory

TL;DR

This review discusses the current understanding, methods, and limitations of achieving scale separation in string theory flux compactifications, including classical constructions, higher-dimensional perspectives, and theoretical constraints.

Contribution

It provides a comprehensive overview of classical and recent developments in scale separation, highlighting challenges and theoretical insights from Swampland and holography.

Findings

Classical flux compactifications can achieve parametric scale separation.

Limitations exist in current constructions due to theoretical constraints.

Swampland and holographic arguments question the feasibility of scale separation.

Abstract

We present an overview of both older and recent developments concerning scale separation in string theory. We focus on parametric scale separation obtained at the classical level in flux compactifications down to AdS vacua. We review the scenarios that have been proposed to achieve a hierarchy of scales between spacetime and the internal space, built from a low-dimensional perspective. We then discuss how they have been understood to arise from proper higher-dimensional descriptions. Eventually, limitations of these constructions as well as Swampland and holographic arguments addressing the question of scale separation in string theory are discussed. The purpose of the review is to draw an accurate picture of the state of the art of the subject at the moment.