Exotic branes and non-geometric backgrounds

TL;DR

This work argues that exotic branes, predicted by enhanced $U$-duality in lower dimensions, originate from non-geometric backgrounds or $U$-folds in higher dimensions. It shows how the supertube effect can create exotic dipole charges even from configurations with only standard charges, avoiding problematic divergences and affecting black hole microstate physics. The authors provide a concrete supergravity realization of the $5^2_2$ brane via T-duality on a KK monopole, revealing a non-geometric $T$-fold monodromy and giving explicit mass and metric expressions that depend on radii and the string coupling. They further discuss how a network of puffed-up exotic charges could enrich black hole entropy counting and motivate the search for non-geometric solutions in the 3-charge regime to fully capture the microstate structure.

Abstract

When string/M-theory is compactified to lower dimensions, the U-duality symmetry predicts so-called exotic branes whose higher dimensional origin cannot be explained by the standard string/M-theory branes. We argue that exotic branes can be understood in higher dimensions as non-geometric backgrounds or U-folds, and that they are important for the physics of systems which originally contain no exotic charges, since the supertube effect generically produces such exotic charges. We discuss the implications of exotic backgrounds for black hole microstate (non-)geometries.