arXiv subjects

Shinji Hirano

Publications and source records attributed to Shinji Hirano.

At least 19 recordsLinked to original sources

arXiv2026

A Third-Quantized Description of Spacetime Wormholes in AdS/CFT

We propose an extension of AdS/CFT in which quantum gravitational wavefunctions of connected bulk geometries are assembled into a Fock space of universes. Splitting and joining interactions in a third-quantized Hamiltonian then provide a quantum description of changes in bulk connectivity and topology. Reduced phase-space quantization supplies the physical one-universe Hilbert space and a radial Schr\"odinger evolution with respect to which the topology-changing interactions are ordered. We develop the construction concretely in AdS$_3$ gravity with a torus boundary, where these ingredients can be described explicitly. The light spectrum is treated as fixed, non-normalizable background data, whereas the normalizable states above the black-hole threshold are quantized as dynamical modes. A conventional boundary theory is selected by choosing a coherent state whose universe-field expectation value, together with the fixed background contribution, yields consistent CFT data at the asymptotic boundary. Generic one-universe wavefunctions need not admit such an interpretation. This raises the possibility that CFT-realizable wavefunctions are nongeneric in the gravitational state space and are not closed under arbitrary superpositions of gravitational states. The known two-torus spacetime-wormhole amplitude, which lies at the heart of the factorization problem, provides concrete input for the topology-changing interactions of the third-quantized theory. A complementary cutting-and-gluing description yields an effective ensemble interpretation when the resulting gravitational sectors admit consistent boundary-theory interpretations, without assuming a fundamental ensemble of boundary theories. We also discuss replica wormholes and baby-universe processes within the same framework.

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arXiv2026

Wavefunctions of AdS$_3$ Universes and $T\bar{T}$-deformed Torus Partition Functions

We study wavefunctions of quantum gravity in asymptotically AdS$_3$ spacetimes and their relation to $T\bar{T}$-deformed torus partition functions. We show that the deformed partition function is given by an invertible integral transform of bulk wavefunctions, with a kernel encoding the relation between the $T\bar{T}$ coupling and the radial or temporal coordinate in the bulk. The invertibility of the transform allows the bulk wavefunction to be reconstructed from the family of $T\bar{T}$-deformed partition functions. In the CFT limit, the kernel effectively localizes at the asymptotic boundary, recovering the standard holographic relation in which the partition function on the torus is determined by the boundary value of the bulk wavefunction. For finite deformation, the kernel shifts the effective holographic screen away from the asymptotic boundary and broadens it into a finite bulk region. A $T\bar{T}$-deformed partition function at fixed coupling corresponds to a finite-width bulk wavepacket centered around a radial or temporal location set by the deformation scale. In Euclidean signature, the construction arises naturally from wavefunctions of Rindler AdS$_3$, while after a double Wick rotation it admits a Lorentzian interpretation in terms of closed AdS$_3$ torus universes without asymptotic boundaries. Finally, we discuss the extension of the present construction to de Sitter torus universes and its implications for de Sitter holography, and comment on possible generalizations to more general spatial topologies and higher-dimensional spacetimes.

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arXiv2025

$T\bar{T}$ braneworld holography

We study a constructive gravitational dual of two-dimensional $T\bar{T}$-deformed conformal field theories (CFTs) grounded in their two-dimensional gravity description. This framework can be viewed as a Randall-Sundrum-type braneworld, where two-dimensional gravity localized on the $AdS_3$ boundary is dynamical. Assuming the AdS/CFT correspondence, the holographic dictionary provides a straightforward translation between the $T\bar{T}$-deformed CFT and its gravitational dual. In particular, this $T\bar{T}$ braneworld holography remains valid in the presence of matter. To clarify how our framework relates to -- and differs from -- the cutoff AdS proposal, we examine, in the absence of matter, the effect of dynamical boundary gravity on the bulk geometry. In general, semiclassical integration of the boundary two-dimensional gravity -- weighted by the massive gravity action -- leads to a deformation of the two-dimensional metric on constant-radial surfaces. However, we find that the surface commonly interpreted as the cutoff emerges dynamically as a characteristic bulk surface on which the deformation is neutralized. This perspective opens the door to possible extensions of our framework to incorporate matter effects on the bulk surface in future work.

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arXiv2025

$T\bar{T}$-deformed correlators from a 2D gravity description

We study correlators in two-dimensional $T\bar{T}$-deformed conformal field theories by interpreting the $T\bar{T}$ deformation as a coupling to two-dimensional gravity. To demonstrate the utility of the massive gravity framework as a particular realization of the gravitational interpretation, we show how the $T\bar{T}$-deformed correlators at finite coupling can be computed by adopting a judicious parametrization of the 2D metric and a preferred choice of zweibeins. To illustrate how this method works in practice, we compute the leading logarithmic contributions to two- and three-point functions to all orders in the $T\bar{T}$ coupling, reproducing a known result while producing new findings. This framework generalizes the random geometry approach to finite coupling.

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arXiv2025

Nonperturbative effects in $T\bar{T}$-deformed conformal field theories: A toy model for Planckian physics

We propose a nonperturbative completion of two-point correlators in $T\bar{T}$-deformed conformal field theories (CFTs), and analyze their behavior at distance scales shorter than the fundamental length scale set by the $T\bar{T}$ deformation. Building on the interpretation of the $T\bar{T}$ deformation as a coupling to two-dimensional quantum gravity with a unique built-in length scale, we advance the study of $T\bar{T}$-deformed CFTs as a toy model for Planckian physics. As we probe shorter distances, trans-Planckian oscillations are followed by a super-Planckian regime in which correlations are typically suppressed by geometric randomness, in contrast to the power-law growth characteristic of CFTs. Moreover, their dependence on distance becomes exponentially weaker, suggesting that the underlying geometric structure has been largely erased -- a behavior broadly consistent with expectations for quantum spacetime in this regime.

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arXiv2024

Conformal field theory on $T\bar{T}$-deformed space and correlators from dynamical coordinate transformations

We study the map between two descriptions of the $T\bar{T}$ deformation of conformal field theory (CFT): One is the defining description as a deformation of CFT by the $T\bar{T}$-operator. The other is an alternative description as the undeformed CFT on the dynamical $T\bar{T}$-deformed space that backreacts to the state or operator insertions, reminiscent of the theory of gravity. Instead of adopting the topological gravity description, we develop a more literal CFT-based operator formalism that facilitates systematic and straightforward computations of the $T\bar{T}$-deformation of the stress tensor, operators, and their correlators, while rederiving known results in the literature. Along the way, we discuss the backreaction to the $T\bar{T}$-deformed space in response to local operators and exhibit the hard-disk and free-space structures in the UV-cutoff and Hagedorn phases, respectively, suggested by Cardy-Doyon and Jiang. To capitalize on the alternative description of the $T\bar{T}$-deformed CFT, we focus on the correlators of semi-heavy operators, i.e., the operators of large conformal dimension $\Delta\gg\sqrt{c}$, and show an intuitive and simple way to obtain the $T\bar{T}$-deformed correlators from those of the undeformed CFT on the $T\bar{T}$-deformed space via dynamical coordinate transformations. This may have implications in the holographic dual description, pointing towards a working dictionary for a class of matter correlators in the cutoff AdS picture.

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arXiv2023

Island Formula from Wald-like Entropy with Backreaction

We propose a Lorentzian derivation of the generalized entropy associated with the island formula for black holes as a Wald-like entropy without reference to the exterior non-gravitating region or field-theoretic von Neumann entropy of Hawking radiation in a fixed curved spacetime background. We illustrate this idea by studying two-dimensional black holes in the Jackiw-Teitelboim gravity and the Russo-Susskind-Thorlacius model in which Hawking radiation is represented by conformal scalars. With some prescriptions assumed, we show that the generalized entropy for the island formula can be reproduced as the Wald-like entropy of the two-dimensional dilaton-gravity theories upon the inclusion of the backreaction from Hawking radiation described by conformal anomaly. We give a discussion on how a similar idea can be applied to higher-dimensional black holes. It is emphasized that the generalized entropy is obtained in a fully gravitational fashion, yet it yields the same Page curve as that of the half-gravitational set-up. We argue that the results in this paper exacerbate the issues raised in the work of massive islands and inconsistency of islands in theories of long-range gravity.

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arXiv2022

The holography of duality in ${\cal N}=4$ Super-Yang-Mills theory

The space of ${\cal N}=4$ supersymmetric Yang-Mills theories exhibits an intricate structure of global one-form symmetries and $SL(2,\mathbb{Z})$ duality orbits. In this paper we study this structure from the point of view of the holographic dual Type IIB string theory. Generalizing work by Witten, we map the different theories based on the gauge algebras $su(N)$, $so(N)$, and $sp(N)$ to a choice of boundary conditions on bulk gauge fields. We show how the one-form symmetries and their anomalies, as well as the duality properties of the gauge theories, arise in the holographic picture. Along the way we prove that the number of disjoint $SL(2,\mathbb{Z})$ duality orbits for the $su(N)$ theories is given by the number of square divisors of $N$.

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arXiv2022

Replica Wormholes from Liouville Theory

The replica wormholes are a key to the existence of the islands that play a central role in a recent proposal for the resolution of the black hole information paradox. In this paper, we study the replica wormholes in the JT gravity, a model of two-dimensional quantum gravity coupled to a non-dynamical dilaton, by making use of the 2$d$ conformal field theory (CFT) description, namely, the Liouville theory coupled to the $(2,p)$ minimal matter in the $p\to\infty$ limit. In the Liouville CFT description, the replica wormholes are created by the twist operators and the gravitational part of the bulk entanglement entropy can be reproduced from the twist operator correlators. We propose the precise dictionary and show how this correspondence works in detail.

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arXiv2020

$T\bar{T}$ Deformation of Stress-Tensor Correlators from Random Geometry

We study stress-tensor correlators in the $T\bar{T}$-deformed conformal field theories in two dimensions. Using the random geometry approach to the $T\bar{T}$ deformation, we develop a geometrical method to compute stress-tensor correlators. More specifically, we derive the $T\bar{T}$ deformation to the Polyakov-Liouville conformal anomaly action and calculate three and four-point correlators to the first-order in the $T\bar{T}$ deformation from the deformed Polyakov-Liouville action. The results are checked against the standard conformal perturbation theory computation and we further check consistency with the $T\bar{T}$-deformed operator product expansions of the stress tensor. A salient feature of the $T\bar{T}$-deformed stress-tensor correlators is a logarithmic correction that is absent in two and three-point functions but starts appearing in a four-point function.

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arXiv2020

Random Boundary Geometry and Gravity Dual of $T\bar{T}$ Deformation

We study the random geometry approach to the $T\bar{T}$ deformation of 2d conformal field theory developed by Cardy and discuss its realization in a gravity dual. In this representation, the gravity dual of the $T\bar{T}$ deformation becomes a straightforward translation of the field theory language. Namely, the dual geometry is an ensemble of AdS$_3$ spaces or BTZ black holes, without a finite cutoff, but instead with randomly fluctuating boundary diffeomorphisms. This reflects an increase in degrees of freedom in the renormalization group flow to the UV by the irrelevant $T\bar{T}$ operator. We streamline the method of computation and calculate the energy spectrum and the thermal free energy in a manner that can be directly translated into the gravity dual language. We further generalize this approach to correlation functions and reproduce the all-order result with universal logarithmic corrections computed by Cardy in a different method. In contrast to earlier proposals, this version of the gravity dual of the $T\bar{T}$ deformation works not only for the energy spectrum and the thermal free energy but also for correlation functions.

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arXiv2019

Nearly AdS$_2$ holography in quantum CGHS model

In light of recent developments in nearly AdS$_2$ holography, we revisit the semi-classical version of two-dimensional dilaton gravity proposed by Callan, Giddings, Harvey, and Strominger (CGHS) in the early 90's. In distinction to the classical model, the quantum corrected CGHS model has an AdS$_2$ vacuum with a constant dilaton. By turning on a non-normalizable mode of the Liouville field, i.e. the conformal mode of the $2d$ gravity, the explicit breaking of the scale invariance renders the AdS$_2$ vacuum nearly AdS$_2$. As a consequence, there emerges an effective one-dimensional Schwarzian-type theory of pseudo Nambu-Goldstone mode - the boundary graviton - on the boundary of the nearly AdS$_2$ space. We go beyond the linear order perturbation in non-normalizable fluctuations of the Liouville field and work up to the second order. As a main result of our analysis, we clarify the role of the boundary graviton in the holographic framework and show the Virasoro/Schwarzian correspondence, namely that the $2d$ bulk Virasoro constraints are equivalent to the graviton equation of motion of the $1d$ boundary theory, at least, on the $SL(2,R)$ invariant vacuum.

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arXiv2019

Quantum Holographic Entanglement Entropy to All Orders in $1/N$ Expansion

We study holographic entanglement entropy in four-dimensional quantum gravity with negative cosmological constant. By using the replica trick and evaluating path integrals in the minisuperspace approximation, in conjunction with the Wheeler-DeWitt equation, we compute quantum corrections to the holographic entanglement entropy for a circular entangling surface on the boundary three sphere. Similarly to our previous work on the sphere partition function, the path integrals are dominated by a replica version of asymptotically AdS conic geometries at saddle points. As expected from a general CFT argument, the final result is minus the free energy on the three sphere which agrees with the logarithm of the Airy partition function for the ABJM theory that sums up all perturbative $1/N$ corrections despite the absence of supersymmetries. The all-order holographic entanglement entropy cleanly splits into two parts, (1) the $1/N$-corrected Ryu-Takayanagi minimal surface area and (2) the bulk entanglement entropy across the minimal surface, as suggested in the earlier literature. It is explicitly shown that the former comes from the localized conical singularity of the replica geometries and the latter from the replication of the bulk volume.

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arXiv2019

Information Transfer and Black Hole Evaporation via Traversable BTZ Wormholes

We study traversable wormholes by considering the duality between BTZ black holes and two-dimensional conformal field theory on the thermofield double state. The BTZ black holes can be rendered traversable by a negative energy shock wave. Following Gao, Jafferis and Wall [1], we show that the negative energy shock wave is dual to the infinite boost limit of a specific double trace deformation which couples the left and right CFTs. We spell out the mechanism of information transfer through traversable BTZ wormholes, treating the backreaction of the message as a positive energy shockwave. The corresponding spacetime is that of colliding spherical shells in the BTZ black hole, which we explicitly construct. This construction allows us to obtain a bound on the amount of information that can be sent through the wormhole, which is consistent with previous work in the context of nearly $AdS_2$ gravity [2]. Consequently, we define a notion of traversibility of the wormhole and study it in the context of a multiple shock geometry. We argue that the time-dependence of traversibility in this geometry can be connected to certain aspects of the black hole evaporation process, such as the second half of the Page curve. Finally, we examine the claim that traversable wormholes are fast decoders. We find evidence for this by computing the scrambling time in the shockwave background and showing that it is delayed by the presence of the negative energy shock wave.

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arXiv2018

Brown-Teitelboim Instantons and the First Law of Thermodynamics of (Anti) de Sitter Space

We study the instantons (or bounces) in the Brown-Teitelboim (BT) mechanism of relaxation of cosmological constant which is a cosmological version of the Schwinger mechanism. The BT mechanism is a false vacuum decay of (A)dS$_{d+1}$ (and $R^{1, d}$) spaces via spontaneous nucleations of spherical $(d-1)$-branes and thus ostensibly has bearings on (A)dS$_{d+1}$/CFT$_d$ holography. In this paper we focus on the four-dimensional case, although the higher or lower-dimensional generalization is straightforward. As is the case with pair productions near black hole and de Sitter horizons, we show that the BT instanton action for a membrane nucleation encodes the first law of thermodynamics of (Anti) de Sitter space. In particular, the membrane instanton precisely accounts for the change of entropy of (A)dS space before and after nucleation, in good accordance with AdS$_{d+1}$/CFT$_d$ in which the $(d-1)$-branes make up all degrees of freedom of AdS$_{d+1}$ space. In light of this lesser-known perspective presented here we also make remarks on (1) (A)dS/CFT and (2) complexity. For the complexity we observe that the Lorentzian bounce action may have close connection to complexity.

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arXiv2018

Airy Function and 4d Quantum Gravity

We study four-dimensional quantum gravity with negative cosmological constant in the minisuperspace approximation and compute the partition function for the $S^3$ boundary geometry. In this approximation scheme the path integrals become dominated by a class of asymptotically AdS "microstate geometries." Despite the fact that the theory is pure Einstein gravity without supersymmetry, the result precisely reproduces, up to higher curvature corrections, the Airy function in the $S^3$ partition function of the maximally supersymmetric Chern-Simons-matter (CSM) theory which sums up all perturbative $1/N$ corrections. We also show that this can be interpreted as a concrete realization of the idea that the CFT partition function is a solution to the Wheeler-DeWitt equation as advocated in the holographic renormalization group. Furthermore, the agreement persists upon the inclusion of a string probe and it reproduces the Airy function in the vev of half-BPS Wilson loops in the CSM theory. These results may suggest that the supergravity path integrals localize to the minisuperspace in certain cases and the use of the minisuperspace approximation in AdS/CFT may be a viable approach to study $1/N$ corrections to large $N$ CFTs.

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arXiv2018

Giant graviton interactions and M2-branes ending on multiple M5-branes

We study splitting and joining interactions of giant gravitons with angular momenta $N^{1/2}\ll J\ll N$ in the type IIB string theory on $AdS_5 \times S^5$ by describing them as instantons in the tiny graviton matrix model introduced by Sheikh-Jabbari. At large $J$ the instanton equation can be mapped to the four-dimensional Laplace equation and the Coulomb potential for $m$ point charges in an $n$-sheeted Riemann space corresponds to the $m$-to-$n$ interaction process of giant gravitons. These instantons provide the holographic dual of correlators of all semi-heavy operators and the instanton amplitudes exactly agree with the pp-wave limit of Schur polynomial correlators in ${\cal N}=4$ SYM computed by Corley, Jevicki and Ramgoolam. By making a slight change of variables the same instanton equation is mathematically transformed into the Basu-Harvey equation which describes the system of M$2$-branes ending on M$5$-branes. As it turns out, the solutions to the sourceless Laplace equation on an $n$-sheeted Riemann space correspond to $n$ M5-branes connected by M2-branes and we find general solutions representing M2-branes ending on multiple M5-branes. Among other solutions, the $n=3$ case describes an M2-branes junction ending on three M5-branes. The effective theory on the moduli space of our solutions might shed light on the low energy effective theory of multiple M5-branes.

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arXiv2017

Matching Renormalisation Schemes in Holography

In holography there is a one-to-one correspondence between physical observables in the bulk and boundary theories. To define physical observables, however, regularisation needs to be implemented in both sides of the correspondence. It is arguable whether the correspondence should extend to regularisation and renormalisation scheme which are not physical in the conventional sense. However, if we are to take the renormalisation group (RG) interpretation of holography seriously, its precise understanding appears to require the matching of regularisations and renormalisation schemes in the bulk and boundary theories. We address this question in the AdS_5/CFT_4 correspondence by considering a simplest physical quantity, the Casimir energy of the N=4 super Yang-Mills (SYM) theory on R x S^3, in a ζ-function regularisation and show that there are choices of scheme which match the bulk AdS result including the radial cutoff dependent corrections when the cutoff is kept finite. We further discuss the implication of this result to the RG interpretation of holography.

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