arXiv subjects

Hongbao Zhang

Publications and source records attributed to Hongbao Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Tidal Love Numbers of Thin-Shell Wormholes, de Sitter Gravastars, and Anti-de Sitter Gravastars via Full Perturbed Junction Conditions

In this work, we revisit the tidal Love numbers (TLNs) of several classes of exotic compact objects (ECOs) using the recently derived full perturbed junction conditions. We investigate four representative thin-shell models: wormholes, de Sitter gravastars with zero surface energy density, general de Sitter gravastars, and anti-de Sitter gravastars. Previous studies of wormholes and de Sitter gravastars with zero surface energy density, based on simplified matching conditions, suggested that the TLNs of ultracompact ECOs vanish logarithmically in the black-hole (BH) limit, leading to the conjecture that this logarithmic dependence is a generic feature of ultracompact exotic objects. We show that this logarithmic behavior is not universal once the full perturbed junction conditions are imposed. In particular, the odd-parity TLNs exhibit polynomial scaling in the BH limit for all the models considered, while the even-parity sector displays both logarithmic and polynomial behaviors depending on the model. We also find that the signs of the TLNs can differ qualitatively from previous results, especially in the odd-parity sector. Our results demonstrate that the tidal response of ECOs is strongly affected by the perturbed matching conditions and that the previously conjectured universal logarithmic scaling near the BH limit is not generic.

gr-qc↗

arXiv2026

Modeling QNMs-Filtered Gravitational Waveforms: Approximation Limits and Filtering Effects

The waveform left after quasinormal modes filtering of binary gravitational waveforms, often called the direct wave, has recently attracted considerable debate. Analytic models of this residual waveform involve several levels of approximation, while the effects of filtering on the modeling remain unclear. These issues require further investigation. In this paper, we examine the approximation errors, limits of applicability, and role of filtering in these models. We find that the point-particle models examined here show notable discrepancies with the filtered Binary-black-hole waveforms. Analytic particle templates based on the stationary-phase approximation perform reasonably well for the $(\ell,m)=(2,2)$ multipole, but exhibit substantial limitations for the $(4,4)$. Although the filter is all-pass, it substantially affects the modeling results. Our analytic calculations on a Schwarzschild background further show that vacuum perturbations with Gaussian initial data can also exhibit direct-wave-like waveforms after filtering, with their morphology depending on the location of the initial data relative to the light ring.

gr-qc↗

arXiv2026

Thermodynamic Volumes of Extended Black Hole Thermodynamics with Auxiliary Gauge Fields

By applying the Iyer-Wald formalism to the modified Lagrangian form for the gravitational system, where the coupling constants appearing in the original Lagrangian form, such as the cosmological constant, are promoted to the dynamical scalar fields by introducing the corresponding auxiliary gauge fields, we derive the first law of the extended black hole thermodynamics, where the resulting thermodynamic volumes as the conjugate thermodynamic variables to the coupling constants are nevertheless independent of the auxiliary gauge fields and exactly the same as those obtained before via varying the coupling constants by brute force. In addition, it is further shown that such volumes can also be expressed in terms of the auxiliary gauge fields and evaluated on the black hole horizon, validating the proposal made in the previous literature.

gr-qc↗

arXiv2026

Characteristic evolution of conformal scattering: I. Scalar Waves in Minkowski Spacetime

We study the conformal scattering of massless scalar waves in Minkowski spacetime. The conformal scattering problem is formulated as a Goursat (characteristic initial-value) problem of the physical wave equation in compactified double-null coordinates, including the neighborhood of spatial infinity $i^0$. As null infinities $\mathcal{I}^\pm$ lie on the domain boundary by construction, asymptotic radiation is directly accessible. We consider three physical scenarios: free wave propagation, scattering off a Pöschl--Teller (PT) potential, and the semi-linear $|ϕ|^{n-1}ϕ$ wave equation. For multipole numbers $\ell =0,1$, an explicit stencil, averaging along the spatial direction, yields globally second-order convergent results. For $\ell \ge 2$, an implicit stencil averaging along the temporal direction is required for numerical stability. Although the singular $i^0$ reduces the convergence of the radiation data on $\mathcal{I}^+$ to first order, Richardson extrapolation enhances the effective convergence rate to approximately $1.5$. For PT scattering, our method accurately computes scattering quantities, notably the phase shifts induced by the potential. In the semi-linear case, our method captures the physical signatures of a self-defocusing Kerr nonlinearity, including self-phase modulation and spectral broadening. The compactified double-null framework proves to be simple and efficient, suggesting a promising approach to the global evolution of conformal scattering.

gr-qc↗

arXiv2026

Splitting Dynamics of Multiply Quantized Vortices in Holographic Superfluid of Finite Temperature

We study the splitting dynamics of multiply quantized vortices with winding numbers $n=5,6,7$ and $8$ in a two-dimensional holographic superfluid at finite temperature, by combining linear perturbation analysis of quasinormal modes with fully nonlinear real-time numerical simulations. Three new physical phenomena are revealed. First, the number of unstable modes no longer strictly follows the $2n-3$ formula as $n$ increases. For the vortex with $n=8$, the unstable mode with $p=2(n-1)$ is absent throughout the entire temperature range, so that only $2n-4$ unstable modes exist. Second, the transition of the dominant unstable mode with increasing temperature exhibits new characteristics. For vortices with $n\le 6$, the dominant mode changes sequentially as $p=2,3,\dots,n$, whereas for $n\ge 7$ jump-like transitions occur-for instance, for $n=7$ the dominant mode jumps from $p=2$ to $p=4$ at $T=0.325T_c$ and then directly to $p=7$ at $T=0.359T_c$, and for $n=8$ it jumps directly from $p=2$ to $p=8$ at $T=0.302T_c$. Third, a single splitting pattern of high-winding-number vortices can contain multiple sub-splitting patterns with distinct topological structures, as exemplified by the $l=4$ pattern of the $n=8$ vortex, which exhibits three sub-patterns at low, intermediate and high temperatures. The nonlinear simulations confirm the predictions of the linear stability analysis, and the implications of our results for cold-atom experiments are discussed.

hep-th↗

arXiv2026

MemForest: An Efficient Agent Memory System with Hierarchical Temporal Indexing

Memory is a fundamental component for long-context LLM agents, supporting persistent state across interactions through a continuous serve-and-update lifecycle. Despite substantial prior work, many stateful systems retain sequential autoregressive extraction or state-dependent maintenance on the write path, delaying when new evidence becomes queryable. To address these challenges, we present MemForest, a memory framework that reformulates agent memory as a write-efficient temporal data-management problem. MemForest breaks the sequential bottleneck via parallel extraction, decoupling memory construction into concurrent, independent operations. We further introduce MemTree, a hierarchical temporal index that organizes memory as time-ordered trees and replaces global rewrites with localized dirty-path refresh. Dirty summaries can be refreshed in parallel across nodes and trees. End-to-end work remains proportional to incoming content; the logarithmic bound applies only to structural insertion and level-dependent refresh depth in balanced trees. We evaluate MemForest on two long-context benchmarks, LongMemEval-S and LoCoMo. Experiments use Qwen3-4B, Qwen3-30B, and Gemma-4-12B-IT. With Qwen3-30B, MemForest reaches 81.8 percent pass at 1 on LongMemEval-S, while its input-normalized build rate is 6.0 times that of EverMemOS. On LoCoMo categories 1 to 4, it reaches 84.09 percent, within 0.13 percentage points of EverMemOS; on a matched conversation, its build rate is 9.5 times higher. These results show that MemForest reduces memory-freshness latency while retaining strong answer quality.

cs.DB↗

arXiv2026

Extremal non-rotating black holes have no fermionic Love

The static tidal Love numbers (TLNs) of $4$-dimensional black holes vanish for bosonic perturbations but are generically nonzero for fermions, with rare exceptions. In this paper, we show that for static, spherically symmetric black holes, fermionic TLNs vanish if and only if the black hole is extremal, in the sense that its horizon is degenerate. This follows from a closed formula for the static fermionic TLN of any asymptotically flat black hole, obtained by solving the static massless Dirac equation exactly on an arbitrary such spacetime and imposing regularity at the horizon. As applications, we analyze the Culetu--Simpson--Visser regular black hole and the loop-quantum-gravity remnant black holes, whose extremal configurations lead to vanishing fermionic yet nonvanishing bosonic TLNs.

gr-qc↗

arXiv2026

Formation of holographic vortex in a rotating shell-shaped superfluid

We investigate the holographic superfluid dynamics subjected to external rotation on a spherical geometry. Through a linear perturbation analysis, we identify several dynamically unstable phases in the phase diagram, each characterized by distinct unstable modes. Employing fully nonlinear numerical simulations, we further demonstrate that these unstable modes generically drive the system into vortex-antivortex configurations with definite winding numbers, determined by the symmetry of the corresponding unstable modes.

hep-th↗

arXiv2026

Emergence of critical phenomena from the black hole interior

The emergence of the $r=0$ singularity inside a spherically symmetric charged black hole, is studied numerically within the Einstein-Maxwell-real scalar model. When the scalar field reaches a critical strength, the $r=0$ singularity emerges inside the black hole at the tip of the causal diamond. By varying the parameter $p$ of the initial profile for the scalar field towards the critical value ${p_*}$, we observe the areal radius at the tip follows a power-law scaling, ${r_S } \propto {| {p - {p_*}}|^γ}$, with a universal critical exponent $γ\approx 0.5$. This remarkable discovery, analogous to Choptuik's critical phenomena for the black hole formation, provides the first evidence of the universality and scaling for the emergence of the $r=0$ singularity inside spherically symmetric charged black holes, offering new insights into the nonlinear dynamics of strong gravitational field.

gr-qc↗

arXiv2026

Holographic Turbulence and Numerical Estimate of the Fractal Dimension of the Turbulent Horizon

We numerically study two-dimensional turbulence driven by a scalar operator within the framework of the AdS/CFT correspondence, where the external driving source is used to sustain a quasi-steady turbulent state. We propose a simple and efficient evolution scheme within the Bondi-Sachs formalism. Applying this scheme to numerically solve the full nonlinear equations of motion, we obtain a turbulent black hole in asymptotically $\mathrm{AdS}_4$ spacetime. The inverse energy cascade and the corresponding energy spectrum of both decaying and driven dual turbulence are analyzed. The scalar driving leads to a compressible-energy-dominated flow, and the corresponding power law scaling, $E(k)\propto k^{-1.79}$, agrees well with previous simulations of two-dimensional turbulence in weakly coupled compressible fluids in fluid dynamics. This differs from the Kolmogorov $-5/3$ scaling law. Furthermore, we perform a direct numerical estimate of the fractal structure of the turbulent black hole, obtaining a fractal dimension $D\approx 2.65$, which suggests an interesting universality in the fractal dimension.

hep-th↗

arXiv2026

Fermionic Love number of higher-dimensional Reissner-Nordström black holes

In this paper, we generalize our previous work on the fermionic tidal Love numbers (TLNs) to higher-dimensional Reissner-Nordström black holes. The massless Dirac equation is solved in $D$-dimensional spacetime using ingoing Eddington coordinates and regular tetrads. After identifying the regular solution branch, we extract the fermionic TLNs from its asymptotic behavior at infinity. The resulting TLNs exhibit a rich dimension-dependent structure that generalizes the four-dimensional case. Unlike bosonic TLNs, which vanish for certain values of the total angular momentum $l$ in dimensions $D>4$, fermionic TLNs remain non-zero for all $l$ and $D \geq 4$, except for extremal black holes. Moreover, the $l$-dependence weakens as $D$ increases, disappearing entirely in the infinite-dimensional limit. These results provide new insights into black hole responses to fermionic perturbations in higher-dimensional spacetimes.

gr-qc↗

arXiv2026

Internal structure of Hayward black holes

Regular black holes, free of central singularities, provide an ideal laboratory for probing the geometric structure of spacetime. The global structure of some regular black holes, e.g. Hayward black hole, features an event horizon and a Cauchy horizon, raising fundamental questions about the latter's stability. In this work, we investigate collapse of a scalar field in Hayward spacetime. Under weak scalar perturbations, the inner horizon maintains a stable finite radius. In the circumstance of a strong scalar field, the inner horizon shrinks to zero volume, accompanied by the formation of a spacelike singularity. The Hayward geometry is effectively converted into a Schwarzschild-like geometry. Furthermore, the strength of the scalar field governs the contraction dynamics of the inner horizon. As the parameter $p$ of the initial profile for the scalar field approaches the critical threshold ${p_*}$, the radius of the inner horizon ${r_{-}}$ exhibits a universal scaling behavior: ${r_{-}}\propto{|p - {p_*}|^γ}$, with a critical exponent $γ\approx 0.5$.

gr-qc↗

arXiv2026

Tidal Love numbers and the dynamical instability of AdS bubbles

In this work, we study non-radial perturbations of AdS bubbles and their tidal Love numbers (TLNs). The odd- and even-parity TLNs are computed up to $l=6$ in the limit $k \to \infty$. The odd-parity TLNs are found to be negative, while the even-parity TLNs are positive for $\upsilon^2_s=-1$. As $l$ increases, the tidal Love numbers approach zero. The TLNs of the even-parity sector up to order $l=41$ are also calculated over the entire parameter space of $k$, from $0$ to $\infty$. We find that in the region where $p/σ>0$, an increasing number of TLNs become negative as $l$ increases. For $l = 41$, the highest order we have examined, the TLNs are negative everywhere except in a narrow region very close to the zero of $p/σ$, which agrees well with the instability criterion in the eikonal limit for self-gravitating membranes proposed by Yang {\it et al.}\ [P. R. L. {\bf 130}, 011402 (2023)].

gr-qc↗

arXiv2026

Can Oscillatory and Persistent Nonlinearities Be Bridged in Black Hole Ringdown?

Quadratic quasinormal modes (QQNMs) and Christodoulou memory effect are key nonlinear phenomena in gravitational wave physics. QQNMs characterize the near zone nonlinear response of a perturbed black hole, whereas the memory effect is a nonlinear remnant imprinted at null infinity by outgoing radiation. This naturally raises the question of whether and in what sense the two can be bridged. We show that they are related through bridge coefficients which depend primarily on remnant black hole parameters during ringdown. Future space-based gravitational wave detectors can probe this relation. These results provide a new avenue for testing gravity and a fresh perspective on the nonlinear regime of general relativity.

gr-qc↗

arXiv2026

Transition of vortex dipole dynamics in holographic superfluids

Using holographic duality, we reveal a transition in vortex dipole dynamics below a critical dipole size in strongly interacting superfluids, characterized by a significant suppression of mutual friction. In the bulk, this transition is triggered by a topological reconnection of vortex tubes, which disconnects the boundary vortices from the black hole horizon and forms a \textit{U-pipe}. Consequently, the post-transition evolution is governed by the contraction of the bulk \textit{U-pipe} rather than the mutual friction associated with the horizon, revealing a scale-dependent dissipation mechanism. We further show that this reconnection persists over a broad temperature range, even when the transition becomes unobservable at high temperatures. Our results provide a dissipation-based interpretation for the anomalous critical dipole scale observed in strongly interacting cold-atom experiments, and suggest the existence of distinct dissipative regimes in strongly interacting superfluids.

hep-th↗

arXiv2026

Explicit and covariant formula for thermodynamic volume in extended black hole thermodynamics

In extended black hole thermodynamics, the cosmological constant and other couplings are treated as thermodynamic variables, yielding the first law $\tildeδM = T\tildeδS+Ω\tildeδJ +\mathcal{V} \tildeδP+\cdots$, where $P\equiv -\fracΛ{8π}$. A long-standing conceptual gap in this framework is that, unlike $M$, $T$, $S$, $Ω$, and $J$, the thermodynamic volume $\mathcal{V} $ lacks a first-principles definition and can only be deduced from other thermodynamic quantities. This deficiency indicates that the underlying origin of $\mathcal{V} $ remains poorly understood. In this paper, we resolve this issue and provide an explicit, covariant formula for $\mathcal{V} $. We demonstrate that $\mathcal{V} $ (and the conjugate quantities of other couplings) universally decomposes into two contributions: one arising from the explicit coupling dependence of the Lagrangian, and the other from the response of the fundamental dynamical fields. This clarifies the physical meaning of the thermodynamic volume and places it on the same footing as other intrinsic thermodynamic quantities.

gr-qc↗

arXiv2026

Fermionic Love number of Reissner-Nordström black holes

The tidal deformation of compact objects, characterised by their Love numbers, provides insights into the internal structure of neutron stars and black holes. While static bosonic tidal Love numbers vanish for black holes in general relativity, it has been recently revealed that static fermionic tidal perturbations can induce non-zero Love numbers for Kerr black holes. In this paper, we investigate the response of the Reissner-Nordström black hole to the fermionic Weyl field. As a result, we find that the corresponding fermionic tidal Love numbers are also non-vanishing for the Reissner-Nordström black holes except for the extremal ones, which highlights the universal distinct behavior of the static fermionic tidal Love numbers compared to the bosonic counterparts.

gr-qc↗

arXiv2026

The identification between the bulk and boundary conserved quantities

By using Wald formalism, we show that the identification between the bulk and boundary conserved quantities induced by the perturbation of generic non-electromagnetic matter field holds not only on top of the asymptotically flat stationary spacetimes but also on top of the asymptotically AdS stationary ones. We further show that such an identification reduces to the familiar form for the test point particle by viewing it as the limiting case of general matter.

hep-th↗