Is the horizon of an eternal black hole really smooth?

TL;DR

This paper investigates the nature of the horizon in eternal black holes, proposing that instant folded strings and D-branes create singularities outside the horizon, challenging the notion of a smooth horizon.

Contribution

It introduces the concept that instant folded strings and D-branes can form outside the horizon, affecting the classical understanding of black hole horizons.

Findings

Instant folded strings are created in the past wedge of eternal black holes.

These strings cause singularities just outside the horizon.

Bulk modes responsible for Poincare recurrence are linked to these instant folded objects.

Abstract

We point out that in many eternal black holes, including a Schwarzschild eternal black hole and an eternal black hole in $AdS_5$, instant folded strings are created in the past wedge and render the region just outside the horizon singular. We also make a conjecture regarding instant folded D-branes and discuss their possible implications for eternal black holes. In particular, we argue that the bulk modes responsible for Poincare recurrence, when it occurs in the dual quantum field theory, are either instant folded strings or instant folded D-branes.