A rough end for smooth microstate geometries

TL;DR

This paper investigates the instability of supersymmetric microstate geometries, proposing that large angular momentum microstates transition to more typical, stable states due to stringy corrections, with implications for understanding black hole microstates.

Contribution

It introduces a new interpretation of microstate instability as a transition driven by angular momentum reduction, incorporating explicit solutions with backreaction and a stabilization mechanism.

Findings

Instability linked to growth of excitations at an ergosurface.

Transition from high to lower angular momentum microstates.

Stringy corrections potentially stabilize microstates.

Abstract

Supersymmetric microstate geometries with five non-compact dimensions have recently been shown by Eperon, Reall, and Santos (ERS) to exhibit a non-linear instability featuring the growth of excitations at an "evanescent ergosurface" of infinite redshift. We argue that this growth may be treated as adiabatic evolution along a family of exactly supersymmetric solutions in the limit where the excitations are Aichelburg-Sexl-like shockwaves. In the 2-charge system such solutions may be constructed explicitly, incorporating full backreaction, and are in fact special cases of known microstate geometries. In a near-horizon limit, they reduce to Aichelburg-Sexl shockwaves in $AdS_3 \times S^3$ propagating along one of the angular directions of the sphere. Noting that the ERS analysis is valid in the limit of large microstate angular momentum $j$, we use the above identification to interpret their instability as a transition from rare smooth microstates with large angular momentum to more typical microstates with smaller angular momentum. This entropic driving terminates when the angular momentum decreases to $j \sim \sqrt{n_1n_5}$ where the density of microstates is maximal. We argue that, at this point, the large stringy corrections to such microstates will render them non-linearly stable. We identify a possible mechanism for this stabilization and detail an illustrative toy model.