arXiv subjects

Feiyu Deng

Publications and source records attributed to Feiyu Deng.

8 recordsLinked to original sources

arXiv2026

Boundary flow and geometric realization in holographic $T\bar T$-deformed BCFT

We study the $T\bar T$ deformation of boundary conformal field theories (BCFTs) from an intrinsic field-theoretic perspective. Formulating the deformation as a modification of the asymptotic variational principle in AdS$_3$, we obtain the exact quadratic trace relation for the stress tensor without introducing a finite radial cutoff, which we take as the fundamental definition of the deformed theory. When restricted to a BCFT without independent boundary degrees of freedom, the intrinsic $T\bar T$ deformation becomes genuinely boundary-localized. Imposing reflective boundary conditions collapses the bulk composite operator to a universal one-dimensional irrelevant flow governed entirely by the displacement operator. We integrate this flow in closed form and derive an induced boundary action, showing that the deformation reorganizes existing boundary data without introducing new boundary degrees of freedom. We further establish a precise equivalence between a fixed-boundary description and a moving-boundary description, interpreted as a reparametrization of the variational problem rather than physical boundary dynamics. On the holographic side, we analyze two inequivalent realizations in AdS$_3$/BCFT$_2$, referred to as Type~A and Type~B. In Type~A, a rigid cutoff surface intersects the end-of-the-world brane at finite position, leading to an apparent boundary displacement. In Type~B, the cutoff surface is asymptotically AdS$_2$, so that the BCFT boundary is geometrically pinned and the displacement operator vanishes identically. Using entanglement entropy at zero and finite temperature, we disentangle universal consequences of the intrinsic boundary-localized flow from features that depend on the holographic implementation.

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arXiv2026

Causality Criteria for Island Models

Island models offer a compelling resolution of the black hole information paradox, but they also raise persistent questions about causal consistency in effective descriptions. In particular, effective theories arising in double holography can exhibit apparent violations of micro-causality, despite the underlying bulk dynamics being local and causal. The aim of this work is to clarify the physical origin of this phenomenon and to identify the structural features that control causal consistency in island models. We argue that the apparent non-causality in double holography is neither intrinsic to island physics nor a consequence of nonlocal operator reconstruction. Rather, it reflects a mismatch between effective spacetime separation and bulk causal accessibility, a feature already implicit in earlier analyses. Nonlocal reconstruction instead encodes quantum error correction within a restricted code subspace and does not introduce independent propagation channels. Motivated by this perspective, we formulate a structural criterion for micro-causality in effective island descriptions. The criterion consists of three conditions: the absence of independent propagation channels beyond those of the bulk theory, a local bulk-supported operator dictionary, and a consistent matching between effective spacelike separation and dynamically accessible bulk causal curves. When these conditions are satisfied, effective micro-causality follows directly from bulk micro-causality. We apply the criterion to brane world realizations of island models, including the defect-extremal-surface construction, and show that they preserve causal consistency, in contrast to double holography. We further demonstrate that the criterion remains robust in time-dependent processes such as island formation and evaporation.

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arXiv2025

An Interpretation for the Equivalence of Two Holographic Computations of the Butterfly Velocity with the Canonical Formalism of Gravity

In this paper, we revisit the equivalence of two holographic computations of the butterfly velocity: the computation with the shock wave solution and the computation with the entanglement wedge reconstruction. We provide an interpretation for the equivalence of the two computations with the canonical formalism of gravity. Specifically, by taking use of the canonical formalism, we reformulate both computations into the ones with a similar form. Here, in both reformulated computations, the butterfly velocity is computed from applying a given set of initial data into the constraint equations. And the sets of initial data of both computations have a similar structure. We then interpret the equivalence of the two computations as from the similar form of the reformulated computations.

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arXiv2024

Holographic boundary conformal field theory with $T\bar T$ deformation

We propose a holographic dual of boundary conformal field theory (BCFT) with $T\bar T$ deformation, i.e. of $T\bar T$ BCFT. Our holographic proposal distinguishes two types of $T\bar T$ BCFTs, depending on whether the $T\bar T$ deformation deforms the boundary. For the boundary-deformed case, we find that boundary entropy serves as an effective measure to quantify the impact of boundary deformation. In this scenario, we calculate the energy spectrum for the $T\bar T$ BCFT within a finite interval to support the proposed dual. For the boundary-undeformed case, we calculate the entanglement entropy and R\'enyi entropy from both the field theory side and the gravity side, and find that they match.

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arXiv2023

End of the World Brane meets $T\bar{T}$

End of the world branes in AdS have been recently used to study problems deeply connected to quantum gravity, such as black hole evaporation and holographic cosmology. With non-critical tension and Neumann boundary condition, the end of the world brane often represents part of the degrees of freedom in AdS gravity and geometrically it is only part of the entire boundary. On the other hand, holographic $T\bar{T}$ deformation can also give a boundary as a cutoff surface for AdS gravity. In this paper we consider AdS gravity with both the end of the world boundary and the cutoff boundary. Using partial reduction we obtain a brane world gravity glued to a $T\bar{T}$ deformed bath. We compute both entanglement entropy and Page curve, and find agreement between the holographic results and island formula results.

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arXiv2022

JT Gravity from Partial Reduction and Defect Extremal Surface

We propose the three-dimensional bulk dual for Jackiw-Teitelboim gravity coupled with CFT$_2$ bath based on partial reduction. The bulk dual is classical AdS gravity with a defect brane which has small fluctuation in transverse direction. We derive full Jackiw-Teitelboim gravity action by considering the transverse fluctuation as a dilaton field. We demonstrate that the fine grained entropy computed from island formula precisely agrees with that computed from defect extremal surface. Our construction provides a Lorentzian higher dimensional dual for Jackiw-Teitelboim gravity and therefore offers a framework to study problems such as black hole information paradox as well as gravity/ensemble duality.

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arXiv2021

Page Curve from Defect Extremal Surface and Island in Higher Dimensions

Defect extremal surface is defined by minimizing the Ryu-Takayanagi surface corrected by the defect theory, which is useful when the RT surface crosses or terminates on the defect. Based on the decomposition procedure of a AdS bulk with a defect brane, proposed in arXiv:2012.07612, we derive Page curve in a time dependent set up of AdS$_3$/BCFT$_2$, and find that the result from island formula agrees with defect extremal surface formula precisely. We then extend the study to higher dimensions and find that the entropy computed from bulk defect extremal surface is generally less than that from island formula in boundary low energy effective theory, which implies that the UV completion of island formula gives a smaller entropy in higher dimensions.

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arXiv2020

Defect extremal surface as the holographic counterpart of Island formula

We propose defect extremal surface as the holographic counterpart of boundary quantum extremal surface. The defect extremal surface is defined by minimizing the Ryu-Takayanagi surface corrected by the defect theory. This is particularly interesting when the RT surface crosses or terminates on the defect. In a simple set up of AdS/BCFT, we find that the defect extremal surface formula gives precisely the same results of the boundary quantum extremal surface. We provide a decomposition procedure of an AdS bulk with a defect brane to see clearly how Island formula emerges from a brane world system with gravity glued to a flat space quantum field theory.

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