A Multi-Boundary AdS Orbifold and DLCQ Holography: A universal holographic description of extremal black hole horizons

TL;DR

<3-5 sentence high-level summary> This paper analyzes a smooth AdS3 orbifold produced by boosts, which has two causally connected null-boundary components. It develops a detailed bulk and holographic dictionary, showing that the dual description naturally involves two DLCQ copies of the D1-D5 CFT (or a conformal orbifold thereof) and that correlators mix across boundaries via analytic structures in the complexified spacetime. Through scalar field analysis, geodesics, and the method of images, the authors obtain a consistent holographic framework and demonstrate universality by connecting to Penrose-like limits of extremal BTZ black holes, AdS2 with flux, and multiple string/M-theory duals that lead to Matrix model descriptions. The work offers a cohesive holographic picture of horizon dynamics for extremal black holes in AdS3 and related 4D/5D stringy black holes, with potential implications for how holography operates behind horizons and in time-dependent backgrounds.</p>

Abstract

We examine a stationary but non-static asymptotically AdS_3 spacetime with two causally connected conformal boundaries, each of which is a ``null cylinder'', namely a cylinder with a null direction identified. This spacetime arises from three different perspectives: (i) as a non-singular, causally regular orbifold of global AdS_3 by boosts, (ii) as a Penrose-like limit focusing on the horizon of extremal BTZ black holes, and (iii) as an S^1 fibration over AdS_2. Each of these perspectives sheds an interesting light on holography. Examination of the conformal boundary of the spacetime shows that the dual to the space should involve DLCQ limits of the D1-D5 conformal field theory. The Penrose-like limit approach leads to a similar conclusion, by isolating a sector of the complete D1-D5 CFT that describes the physics in the vicinity of the horizon of an extremal black hole. As such this is a holographic description of the universal horizon dynamics of the extremal black holes in AdS_3 and also of the four and five dimensional stringy black holes whose states were counted in string theory. The AdS_2 perspective draws a connection to a 0+1d quantum mechanical theory. Various dualities lead to a Matrix model description of the spacetime. Many interesting issues that are related to both de Sitter physics and attempts to ``see behind a horizon'' using AdS/CFT arise from (a) the presence of two disconnected components to the boundary, and (b) the analytic structure of bulk physics in the complex coordinate plane.