Massless black holes and black rings as effective geometries of the D1-D5 system

TL;DR

This paper investigates how typical microstates of the D1-D5 system in string theory can be effectively described by geometries like black holes and black rings, based on correlation functions in the AdS/CFT framework.

Contribution

It demonstrates that typical microstates correspond to effective geometries such as massless black holes and black rings, providing a bridge between microstate structure and emergent spacetime geometries.

Findings

Typical ground states with zero R-charge are described by the M=0 BTZ black hole.

Effective geometries break down at large time separations, revealing microstate details.

States with nonzero R-charge correspond to black ring geometries.

Abstract

We compute correlation functions in the AdS/CFT correspondence to study the emergence of effective spacetime geometries describing complex underlying microstates. The basic argument is that almost all microstates of fixed charges lie close to certain "typical" configurations. These give a universal response to generic probes, which is captured by an emergent geometry. The details of the microstates can only be observed by atypical probes. We compute two point functions in typical ground states of the Ramond sector of the D1-D5 CFT, and compare with bulk two-point functions computed in asymptotically AdS_3 geometries. For large central charge (which leads to a good semiclassical limit), and sufficiently small time separation, a typical Ramond ground state of vanishing R-charge has the M=0 BTZ black hole as its effective description. At large time separation this effective description breaks down. The CFT correlators we compute take over, and give a response whose details depend on the microstate. We also discuss typical states with nonzero R-charge, and argue that the effective geometry should be a singular black ring. Our results support the argument that a black hole geometry should be understood as an effective coarse-grained description that accurately describes the results of certain typical measurements, but breaks down in general.