Single-Sided Black Holes in Double-Scaled SYK Model and No Man's Island

TL;DR

This paper explores a single-sided black hole in the double-scaled SYK model, revealing a non-trivial boundary algebra with a no man's island behind the horizon, and constructing matter-brane states that solve the spectrum.

Contribution

It introduces a deformation of the DSSYK Hamiltonian with an exponential wormhole operator, leading to new algebraic structures and insights into the no man's island phenomenon.

Findings

Boundary algebra has a non-trivial commutant and is a type II₁ von Neumann factor.

The no man's island appears as a consequence of the algebraic structure in the semiclassical limit.

Constructed matter-brane states exactly solve the full spectrum of DSSYK.

Abstract

We study a single-sided black hole with an end-of-the-world (EoW) brane behind the horizon in the double-scaled SYK (DSSYK). The new Hamiltonian is a deformation of the original DSSYK Hamiltonian with an extra exponential wormhole length operator, which leads to a new chord diagram rule. The boundary algebra is defined as generated by the new Hamiltonian and boundary matter. There is an alternative but equivalent definition with a $q$-coherent state due to a nontrivial isomorphism of the vN algebra of DSSYK. This isomorphism induces a unitary equivalence, which yields a surprising result that the boundary algebra of a single-sided black hole in DSSYK has a non-trivial commutant and is a type II$_1$ vN factor. It follows that the full bulk reconstruction from the boundary is impossible, and there is a ``no man's island" behind the horizon in the semiclassical JT limit. Inspired by the EoW brane, we construct a family of matter-brane states with an arbitrary number of matter chords and behaving like an EoW brane. They exactly solve the full spectrum of DSSYK. We take different ways to understand the nontrivial commutant. We show that the commutant is complex on chord number basis and thus non-geometric. In the semiclassical JT limit, the commutant becomes the canonical purification of the boundary algebra and claims the no man's island. In the context of Hawking radiation after Page time, the unitary equivalence is interpreted as encoding the canonical purification into the old Hawking radiation, and the no man's island has the same essence as the island. Including the exponential wormhole length operator independently, the boundary algebra is extended to all bounded operators and reconstructs the no man's island. This can be regarded as a different choice for the definition of boundary algebra. This type I$_\infty$ algebra is closely related to the EoW brane in Kourkoulou-Maldacena.