Large D gravity and low D strings

TL;DR

The paper analyzes the large-$D$ limit of gravity and shows that the near-horizon region of many non-extremal black holes becomes universal and is described by the two-dimensional string-theory black hole, implying a 2D conformal symmetry that explains massless-scalar behavior. For charged branes, the near-horizon sector can become the three-dimensional black strings, indicating a close link between near-horizon gravity in large $D$ and low-dimensional string geometries. The authors argue that the large-$D$ expansion mirrors the string-theory $\alpha'$ expansion, with $\sqrt{\alpha'}\sim r_0/D$, yielding string-scale near-horizon dynamics while the far region remains effectively flat, and discuss consequences for entropy scaling, evaporation, and critical collapse, including a conjectured exponent $\gamma=\tfrac{1}{2}$. They also propose that an effective string description may emerge in this limit, controlled by the interplay between the near-horizon conformal symmetry and the dimensional reduction, with practical simplifications for studying wide classes of black holes.

Abstract

We point out that in the limit of large number of dimensions a wide class of non-extremal neutral black holes has a universal near horizon limit. The limiting geometry is the two-dimensional black hole of string theory with a two-dimensional target space. Its conformal symmetry explains properties of massless scalars found recently in the large D limit. In analogy to the situation for NS fivebranes, the dynamics near the horizon does not decouple from the asymptotically flat region. We generalize the discussion to charged black p-branes. For black branes with string charges, the near horizon geometry is that of the three-dimensional black strings of Horne and Horowitz. The analogies between the alpha' expansion in string theory and the large D expansion in gravity suggest a possible effective string description of the large D limit of black holes. We comment on applications to several subjects, in particular to the problem of critical collapse.