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Abstract

Recent progress in our understanding of the black hole information paradox
has lead to a new prescription for calculating entanglement entropies, which
involves special subsystems in regions where gravity is dynamical, called
\textit{quantum extremal islands}. We present a simple holographic framework
where the emergence of quantum extremal islands can be understood in terms of
the standard Ryu-Takayanagi prescription, used for calculating entanglement
entropies in the boundary theory. Our setup describes a $d$-dimensional
boundary CFT coupled to a ($d$-1)-dimensional defect, which are dual to global
AdS${}_{d+1}$ containing a codimension-one brane. Through the Randall-Sundrum
mechanism, graviton modes become localized at the brane, and in a certain
parameter regime, an effective description of the brane is given by Einstein
gravity on an AdS${}_d$ background coupled to two copies of the boundary CFT.
Within this effective description, the standard RT formula implies the
existence of quantum extremal islands in the gravitating region, whenever the
RT surface crosses the brane. This indicates that islands are a universal
feature of effective theories of gravity and need not be tied to the presence
of black holes.