arXiv subjects

Donald Marolf

Publications and source records attributed to Donald Marolf.

At least 19 recordsLinked to original sources

arXiv2026

The JLMS formula in a large code with approximate error correction

Gauge/gravity duality is often described as a quantum error correcting code. However, as seen in the Jafferis-Lewkowycz-Maldacena-Suh (JLMS) formula, exact quantum error correction with complementary recovery (and thus entanglement wedge reconstruction) emerges only in the limit $G \to 0$. As a result, precise arguments controlling error terms have focused on what we call `small' codes which, as $G \to 0$, describe only perturbative excitations near a given classical solution. Such settings are quite restrictive and, in particular, they prohibit discussion of any modular flow that would change the classical background. As a result, they forbid consideration of modular flows generated by semiclassical bulk states at order-one modular parameters. In contrast, we present a single `large' code for the bulk theory that can accommodate such flows and, in particular, in the $G \to 0$ limit includes superpositions of states associated with distinct classical backgrounds. This large code is assembled from small codes that each satisfy an approximate Faulkner-Lewkowycz-Maldacena formula. In this extended setting we clarify the meaning of the (approximate) JLMS relation between bulk and boundary modular Hamiltonians and quantify its validity in an appropriate class of states.

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arXiv2026

On the Positivity of de Sitter Group Averaging

Perturbative quantum gravity about a global de Sitter (dS$_d$) background involves constraints that restrict the state of the linearized theory but which appear only at higher orders in perturbation theory. In particular, for familiar gravitational theories with $d>2$ these constraints require all charges defined by continuous de Sitter isometries to vanish in the linearized state. While standard Fock spaces have the property that the only normalizable state satisfying these constraints is the vacuum, Higuchi suggested that this lack of allowed states should be remedied by instead using the so-called group-averaging inner product. This inner product satisfies important uniqueness conditions, but its positivity has to date remained an open question. For $d > 2$, we establish this positivity here for all such states with $N$ particles (including particles of arbitrary spin) in standard global dS${}_d$ modes where the particle-weights $Δ_i$ satisfy the condition ${\rm Re} \, \sum_{i=1}^N Δ_i > d-1$, which is the criterion for group averaging to converge absolutely for arbitrary finite linear combinations of such states. We also treat the case of bosonic and fermionic (integer and half-integer spin) particles in dS$_2$ as a toy model for the more general case. However, we can identify certain classes of states for which group averaging continues to converge absolutely for smaller weights. Interestingly, in some cases these smaller-weight states turn out to have {\it negative} group-averaging norms. We thus also discuss the implications of this observation for perturbative de Sitter quantum gravity.

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arXiv2026

Axion Wormholes and the AdS/CFT Factorization Problem

This work investigates the relevance of Euclidean and complex axion wormholes to the AdS/CFT factorization problem. We use a framework that defines bulk gravitational path integrals by integrating over a real Lorentz-signature contour and then, as needed, perhaps further analytically continuing the resulting functions of boundary conditions. For technical reasons we focus on the case of 2+1 bulk dimensions. The AdS boundary conditions (in any dimension) require us to impose Dirichlet boundary conditions on the standard Euclidean axion $χ_E$. Fixing its asymptotic values on two boundary spheres to $\pm χ_{E,\infty}$, we find such wormholes to be subdominant to a UV-sensitive endpoint contribution for $χ_{E, \infty}$ near the real axis, and that (with our conventions) they become dominant only for $χ_{E, \infty}$ near the negative imgainary axis. Furthermore, such wormholes are irrelevant to our computation for ${\rm Im} χ_{E, \infty} >0$ (in the sense that the associated ascent contour fails to intersect the contour of integration). The relevance of the wormhole saddle for real positive $χ_{E, \infty}$ is in fact a matter of choice, as the saddle then lies on a Stokes' line at which the relevant intersection number changes from zero to one.

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arXiv2026

Dirichlet walls and the end of time

We study evolution in Einstein-Hilbert gravity with Dirichlet boundary conditions imposed on a finite surface. We argue that there are open sets of initial data where such evolutions terminate at finite times due to singularities that reach the boundary. In any dimension, the simplest such examples occur in cosmologies. However, in 2+1 dimensions we also show that Dirichlet walls initially outside a BTZ black hole can fall through the horizon, and that this also leads to generic singularities. A similar construction in higher dimensions leads to trapped surfaces that reach the wall, though the end result of such evolutions is more difficult to study.

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arXiv2026

Holographic codes and bulk RG flows

We consider the coarse-graining of holographic quantum error correcting codes under a generalized notion of bulk renormalization-group flow. In particular, we study the renormalization under this flow of the $A/4G$ term in the Faulkner-Lewkowycz-Maldacena formula and in its Rényi generalization. This provides a general quantum code perspective on the arguments of Susskind and Uglum. Specifically, given a 'UV' code with two-sided recovery and appropriately flat entanglement spectrum together with a set of 'seed' states in the UV code, we explicitly construct an 'IR' code with corresponding properties which contains the given seed states and is of minimal size in a sense we describe.

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arXiv2026

Modular flow and the area operator

In the context of the AdS/CFT correspondence, the Jafferis-Lewkowycz-Maldacena-Suh (JLMS) formula relates the boundary modular Hamiltonian to the HRT area operator and the corresponding bulk modular Hamiltonian. In standard discussions of a holographic code built from perturbative excitations on a fixed classical background, the $A/4G$ term dominates over the bulk modular Hamiltonian in the limit $G \to 0$. However, this setup is insufficient to describe the action of boundary modular flows that change the classical background. In contrast, our recent discussion of `large' codes provides a useful framework to obtain a JLMS formula that relates bulk and boundary modular flows acting on appropriate states in the code. Here, one sees that the bulk modular Hamiltonian is not in fact subdominant when the modular flow defined by one state acts on another state peaked around a sufficiently different classical background. Instead, the bulk modular Hamiltonian can be of order $1/G$ and can dominate over the $A/4G$ term. Nevertheless, we show that if the two states have the same classical background except for data conjugate to the HRT area then, under mild assumptions and over a large range of flow parameters, the modular-flowed state is described by a classical background given by the HRT-area flow alone.

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arXiv2026

Superconformal indices and black hole saddles

The AdS/CFT correspondence implies that the superconformal index ${\mathcal I}$ in ${\mathcal N=4}$ SU(N) supersymmetric Yang-Mills theory can be computed using the dual bulk theory. In particular, in the limit of large $N$, the index should be given by a sum over appropriate saddles. However, ${\mathcal I}$ depends on potentials $σ, τ, \vec Δ$ and, at large ${\rm Im}\, τ= {\rm Im}\, σ$, the CFT index ${\mathcal I}$ rapidly approaches $1$ at all values of $N$. As a result, black hole saddles associated with exponentially large contributions in $N$ cannot contribute in this limit. This in particular excludes saddles that were previously suggested to be relevant in such regimes. We thus consider an approach to the bulk path integral motivated by taking it to be defined as an integral over real Lorentz-signature spacetimes with codimension-2 singularities. This approach leads only to saddles that satisfy the above bound, and to the enforcement of this bound via Stokes phenomena. We also find similar results for bulk AdS$_4$ calculations of the ABJM superconformal index.

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arXiv2026

How to tame your (black hole) saddles: Lessons from the Lorentzian Gravitational Path Integral

We resolve a puzzle associated with the spherically-symmetric sector of the AdS$_4$ Einstein-Maxwell partition function with inverse temperature $β$. Since charge is quantized, the semiclassical limit of the partition function is expected to be given by a sum over complex black hole solutions obtained by shifting the associated chemical potential $μ$ by $\frac{2πi n}{e β}$ in terms of the relevant charge quantum $e$. However, the sum over all such saddles turns out to diverge at any finite value of $β$. We therefore consider a definition of this partition function as an integral over a space of metrics that are real and of Lorentz-signature up to the presence of certain conical singularities. A Picard-Lefshetz analysis shows that only a finite subset of the above saddles contribute to our integral at finite $β$, and thus that the sum over such saddles converges. The low temperature limit is nonetheless associated with a convergent sum over all saddles that (as $β\rightarrow \infty$) approach the usual large real Euclidean black holes. We also analyze the analogous partition function for the (uncharged) BTZ black hole in the ensemble defined by fixing an angular velocity $Ω$ up to shifts by $\frac{2πi m}{s β}$, where $s=\frac{1}{2}$ or $s=1$ depending on the presence of absence of fermionic states. In this case, at all $β$ we find that all saddles contribute and that the sum over saddles converges. We also comment briefly on the apparent lack of utility of the so-called KSW condition in our context.

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arXiv2026

De Sitter quantum gravity and the emergence of local algebras

Quantum theories of gravity are generally expected to have some degree of non-locality, with familiar local physics emerging only in a particular limit. Perturbative quantum gravity around backgrounds with isometries and compact Cauchy slices provides an interesting laboratory in which this emergence can be explored. In this context, the remaining isometries are gauge symmetries and, as a result, gauge-invariant observables cannot be localized. Instead, local physics can arise only through certain relational constructions. We explore such issues below for perturbative quantum gravity around de Sitter space. In particular, we describe a class of gauge-invariant observables which, under appropriate conditions, provide good approximations to certain algebras of local fields. Our results suggest that, near any minimal $S^d$ in dS$_{d+1}$, this approximation can be accurate only over regions in which the corresponding global time coordinate $t$ spans an interval $Δt \lesssim O(\ln G^{-1})$. In contrast, however, we find that the approximation can be accurate over arbitrarily large regions of global dS$_{d+1}$ so long as those regions are located far to the future or past of such a minimal $S^d$. This in particular includes arbitrarily large parts of any static patch.

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arXiv2026

Lorentzian Path Integrals and Jackiw-Teitelboim wormholes with imaginary scalars

The Lorentzian path integral was recently used to argue that standard Euclidean axion wormholes do not dominate computations of connected AdS/CFT partition functions. We now apply similar methods to study the seemingly-analogous Jackiw-Teitelboim wormholes constructed by Garcia-Garcia and Godet using Jackiw-Teitelboim gravity with an imaginary-valued minimally-coupled massless scalar field. However, this time we find that these wormholes do dominate our path integral for the relevant connected partition function. This supports the suggestion by Garcia-Garcia and Godet that contributions from such wormholes parallel the physics of the Sachdev-Ye-Kitaev model at complex couplings. The result also illustrates the sensitivity of wormhole contributions to details of the relevant physics.

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arXiv2025

Looking at extremal black holes from very far away

Near-extremal black holes are subject to large quantum effects, which modify their low-temperature thermodynamic behavior. Hitherto, these quantum effects were analyzed by separating the geometry into the near-horizon region and its exterior. It is desirable to understand and reproduce such corrections from the full higher-dimensional asymptotically flat or AdS geometry's perspective. We address this question in this article and fill this gap. Specifically, we find off-shell eigenmodes of the quadratic fluctuation operator of the Euclidean gravitational dynamics, with eigenvalues that vanish linearly with temperature. We illustrate this for BTZ and neutral black holes with hyperbolic horizons in AdS in Einstein-Hilbert theory, and for the charged black holes in Einstein-Maxwell theory. The linear scaling with Matsubara frequency, which is a distinctive feature of the modes, together with the fact that their wavefunctions localize close to the horizon as we approach extremality, identifies them as responsible for the aforementioned quantum effects. We provide a contour prescription to deal with the sign indefiniteness of the Euclidean Einstein-Maxwell action, which we derive to aid our analysis. We also resolve a technical puzzle regarding modes associated with rotational isometries in stationary black hole spacetimes.

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arXiv2025

Are $S^1\times S^2$ wormholes generic with large sources?

Euclidean path integrals can be used to prepare states of a Lorentzian QFT. So long as any sources are turned off on the $t=0$ surface, the resulting Lorentzian states all belong to the same Hilbert space. Constructing more states than allowed by the Lorentzian density of states means that the resulting states must be linearly dependent. For large amplitude sources and a fixed cutoff on energy, the AdS bulk dual of this effect has been conjectured to be captured by spacetime wormholes. Wormholes should then be generic in the presence of large such Euclidean sources. This hypothesis can be studied in a context with asymptotically locally AdS$_4$ boundaries of topology $S^1 \times S^2$ in which the wormhole is supported by a source for minimally-coupled massless bulk scalars. In preparation for a later more complete study, we consider here a preliminary toy version of the model in which the spacetimes are cohomogeneity-1, but with the consequence that the sources do not vanish at $t=0$. We then find that generic sources at large masses do {\it not} lead to wormholes. Along the way we map out the phase diagram for wormhole, thermal AdS, and black hole phases of our cohomogeneity-1 ansatz. We also numerically evaluate their stability by identifying negative modes. In parallel with the previously-studied case of $S^3$ boundaries, the results are analogous to those associated with the familiar Hawking-Page transition.

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arXiv2025

Euclidean wormholes stability analysis revisited

Previous studies of linearized stability of asymptotically flat Euclidean axion wormholes found that symmetric modes suffered from divergences. We show that such divergences were an artifact of a particular way of solving the constraints, and that a full treatment leads to finite actions for such modes. The modes must thus be included in a stability analysis. However, since the action for these modes turns out to be positive, this turns out not to affect previous statements about stability of axion wormholes. We also introduce a technique that allows us to show this positivity at a pseudo-analytic level that avoids heavy numerics. Our techniques should be useful to future studies of stabilities of other wormholes as well.

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arXiv2025

Constraints are not enough

The Euclidean Einstein-Hilbert action is well-known to be unbounded below and thus to raise many questions regarding the definition of the gravitational path integral. A variety of works since the late 1980's have suggested that this problem disappears when one fixes a foliation of the spacetime and imposes the corresponding gravitational constraints. However, we show here that this approach fails with various classes of boundary conditions imposed on the foliation: compact slices without boundary, asymptotically flat, or asymptotically locally anti-de Sitter slices. We also discuss the idea of fixing the scalar curvature and Wick-rotating the conformal factor, and show that it also fails to produce an action bounded from below.

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arXiv2025

Geometric Entropies and their Hamiltonian Flows

In holographic theories, the Hubeny-Rangamani-Takayanagi (HRT) area operator plays a key role in our understanding of the emergence of semiclassical Einstein-Hilbert gravity. When higher derivative corrections are included, the role of the area is instead played by a more general functional known as the geometric entropy. It is thus of interest to understand the flow generated by the geometric entropy on the classical phase space. In particular, the fact that the associated flow in Einstein-Hilbert or Jackiw-Teitelboim (JT) gravity induces a relative boost between the left and right entanglement wedges is deeply related to the fact that gravitational dressing promotes the von Neumann algebra of local fields in each wedge to type II. This relative boost is known as a boundary-condition-preserving (BCP) kink-transformation. In a general theory of gravity (with arbitrary higher-derivative terms), it is straightforward to show that the flow continues to take the above geometric form when acting on a spacetime where the HRT surface is the bifurcation surface of a Killing horizon. However, the form of the flow on other spacetimes is less clear. In this paper, we use the manifestly-covariant Peierls bracket to explore such flows in two-dimensional theories of JT gravity coupled to matter fields with higher derivative interactions. The results no longer take a purely geometric form and, instead, demonstrate new features that should be expected of such flows in general higher derivative theories. We also show how to obtain the above flows using Poisson brackets.

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arXiv2025

Euclidean and complex geometries from real-time computations of gravitational Rényi entropies

Gravitational Rényi computations have traditionally been described in the language of Euclidean path integrals. In the semiclassical limit, such calculations are governed by Euclidean (or, more generally, complex) saddle-point geometries. We emphasize here that, at least in simple contexts, the Euclidean approach suggests an alternative formulation in terms of the bulk quantum wavefunction. Since this alternate formulation can be directly applied to the real-time quantum theory, it is insensitive to subtleties involved in defining the Euclidean path integral. In particular, it can be consistent with many different choices of integration contour. Despite the fact that self-adjoint operators in the associated real-time quantum theory have real eigenvalues, we note that the bulk wavefunction encodes the Euclidean (or complex) Rényi geometries that would arise in any Euclidean path integral. As a result, for any given quantum state, the appropriate real-time path integral yields both Rényi entropies and associated complex saddle-point geometries that agree with Euclidean methods. After brief explanations of these general points, we use JT gravity to illustrate the associated real-time computations in detail.

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arXiv2024

Algebras and Hilbert spaces from gravitational path integrals: Understanding Ryu-Takayanagi/HRT as entropy without AdS/CFT

Recent works by Chandrasekaran, Penington, and Witten have shown in various special contexts that the quantum-corrected Ryu-Takayanagi (RT) entropy (or its covariant Hubeny-Rangamani-Takayanagi (HRT) generalization) can be understood as computing an entropy on an algebra of bulk observables. These arguments do not rely on the existence of a local holographic dual field theory. We show that analogous-but-stronger results hold in any UV-completion of asymptotically AdS quantum gravity with a Euclidean path integral satisfying a simple and (largely) familiar set of axioms. We consider a quantum context in which a standard Lorentz-signature classical bulk limit would have Cauchy slices with asymptotic boundaries $B_L \sqcup B_R$ where both $B_L$ and $B_R$ are compact manifolds without boundary. Our main result is that (the UV-completion of) the quantum gravity path integral defines type I von Neumann algebras ${\cal A}^{B_L}_L$, ${\cal A}^{B_R}_{R}$ of observables acting respectively at $B_L$, $B_R$ such that ${\cal A}^{B_L}_L$, ${\cal A}^{B_R}_{R}$ are commutants. The path integral also defines entropies on ${\cal A}^{B_L}_L, {\cal A}^{B_R}_R$. Positivity of the Hilbert space inner product then turns out to require the entropy of any projection operator to be quantized in the form $\ln N$ for some $N \in {\mathbb Z}^+$ (unless it is infinite). As a result, our entropies can be written in terms of standard density matrices and standard Hilbert space traces. Furthermore, in appropriate semiclassical limits our entropies are computed by the RT-formula with quantum corrections. Our work thus provides a Hilbert space interpretation of the RT entropy. Since our axioms do not severely constrain UV bulk structures, it is plausible that they hold equally well for successful formulations of string field theory, spin-foam models, or any other approach to constructing a UV-complete theory of gravity.

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arXiv2024

When left and right disagree: Entropy and von Neumann algebras in quantum gravity with general AlAdS boundary conditions

Euclidean path integrals for UV-completions of $d$-dimensional bulk quantum gravity were studied in [1] by assuming that they satisfy axioms of finiteness, reality, continuity, reflection-positivity, and factorization. Sectors ${\cal H}_{\cal B}$ of the resulting Hilbert space were defined for any $(d-2)$-dimensional surface ${\cal B}$, where ${\cal B}$ may be thought of as the boundary $\partialΣ$ of a bulk Cauchy surface in a corresponding Lorentzian description, and where ${\cal B}$ includes the specification of boundary conditions for bulk fields. Cases where ${\cal B}$ was the disjoint union $B\sqcup B$ of two identical $(d-2)$-dimensional surfaces were studied in detail and, after the inclusion of finite-dimensional `hidden sectors,' were shown to provide a Hilbert space interpretation of the associated Ryu-Takayanagi entropy. The analysis was performed by constructing type-I von Neumann algebras $\mathcal A_L^B,\mathcal A_R^B$ that act respectively at the left and right copy of $B$ in $B\sqcup B$. Below, we consider the case of general ${\cal B} = B_L\sqcup B_R$ with $B_L,B_R$ distinct. For any $B_R$, we find that the von Neumann algebra at $B_L$ acting on ${\cal H}_{B_L\sqcup B_R}$ is a central projection of the corresponding type-I von Neumann algebra on the `diagonal' Hilbert space ${\cal H}_{B_L\sqcup B_L}$. As a result, the von Neumann algebras $\mathcal A_L^{B_L},\mathcal A_R^{B_L}$ defined in [1] using the diagonal Hilbert space coincide precisely with those defined using the full Hilbert space of the theory. A second implication is that, for any ${\cal H}_{B_L\sqcup B_R}$, including the same hidden sectors as in the diagonal case again provides a Hilbert space interpretation of the Ryu-Takayanagi entropy. We also show the above central projections to satisfy consistency conditions that lead to a universal central algebra relevant to all choices of $B_L,B_R$.

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