Constructing "hair" for the three charge hole
Samir D. Mathur, Ashish Saxena, Yogesh K. Srivastava
TL;DR
The work argues that black hole entropy arises from a large ensemble of horizonless microstate geometries, with the 2-charge D1-D5 system providing a concrete realization parameterized by a profile $\vec{F}(v)$. By constructing a regular, normalizable perturbation that carries one unit of momentum on the 2-charge background and matching inner AdS$_3\times$S$^3$ and outer asymptotically flat regions, the authors demonstrate a viable 3-charge hair for at least one microstate. They develop a perturbative scheme in $\epsilon$, perform spherical-harmonic expansions, and show consistent inner/outer matching up to $O(\epsilon^3)$, arguing for the existence of an exact solution. The results imply a nontrivial interior structure for 3-charge extremal holes and support broader implications for black hole interiors, coarse-graining, and the role of fractionation in setting the horizon scale, while calling for a complete classification of generic 3-charge bound-state geometries.
Abstract
It has been found that the states of the 2-charge extremal D1-D5 system are given by smooth geometries that have no singularity and no horizon individually, but a `horizon' does arise after `coarse-graining'. To see how this concept extends to the 3-charge extremal system, we construct a perturbation on the D1-D5 geometry that carries one unit of momentum charge $P$. The perturbation is found to be regular everywhere and normalizable, so we conclude that at least this state of the 3-charge system behaves like the 2-charge states. The solution is constructed by matching (to several orders) solutions in the inner and outer regions of the geometry. We conjecture the general form of `hair' expected for the 3-charge system, and the nature of the interior of black holes in general.
