Massless Islands in Wedge Holography

Abstract

Entanglement islands are most easily realized in doubly holographic models with massive gravitons or non-gravitating baths. In wedge holography, however, Neumann boundary conditions on both branes give a normalizable massless graviton, while the island saddle of the purely geometric Ryu--Takayanagi (RT) problem collapses to the horizon. Negative Dvali--Gabadadze--Porrati (DGP) terms can restore nontrivial islands by modifying the endpoint condition, but this branch contains a massive ghost. We propose a different semiclassical mechanism. We keep the wedge gravitational action free of DGP terms and couple the healthy Neumann wedge sector to a unitary defect CFT localized at the codimension-two corner. This sector is distinct from the standard corner CFT dual to the undeformed wedge, so its entropy enters as the ordinary matter-entropy term in the quantum extremal surface (QES) prescription without double counting the wedge RT area. If the defect theory is holographic, this entropy can be evaluated by an auxiliary RT surface. We show that when the wedge endpoint determines the defect entangling region, the auxiliary area can vary in the opposite way to the wedge area while all couplings and central charges remain positive. A local endpoint model gives an isolated stable saddle that dominates over the Hartman--Maldacena (HM) surface at late times. The QES condition then replaces the pure orthogonality condition and permits a non-horizon island saddle in a long-range, massless, ghost-free gravitational theory. Thus, the obstruction to massless islands in minimal wedge holography is not masslessness itself, but the absence of a healthy matter-entropy contribution capable of balancing the horizon-minimizing area variation.

9 pages, two-column format; substantially revised to clarify the semiclassical defect-assisted QES construction, its relation to causal-wedge inclusion, and the ghost-free Kaluza-Klein sector. Accepted for publication in Physics Letters B