arXiv subjects

Xiangdong Zhang

Publications and source records attributed to Xiangdong Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Thermodynamic Supercriticality and Complex Phase Diagram for Charged AdS Black Holes in Trace Anomaly Gravity

We extend the Lee-Yang phase transition framework to charged anti-de Sitter (AdS) black holes in four-dimensional trace anomaly gravity. By treating the horizon radius as a complex variable, we derive a fully resolved complex phase diagram that uncovers novel supercritical phenomena within this modified gravity setting. Relative to the standard Reissner-Nordström-AdS black hole, the trace anomaly shifts the location of the critical point: for a representative set of anomaly parameters, both the critical pressure and critical temperature are suppressed. The Widom line is rigorously identified as the projection of the complex Lee-Yang zeros onto the real physical phase plane, a trajectory that demarcates the small-black-hole-like and large-black-hole-like phases throughout the supercritical regime. We also independently recover this same Widom line via the thermodynamic response function method, demonstrating that the two definitions are in excellent quantitative agreement in the near-critical region and share identical universal scaling behavior. Furthermore, our analysis reveals a smooth, continuous crossover across the Widom line as probed by thermodynamic response functions--a behavior fundamentally distinct from the discontinuous first-order phase transitions that occur below the critical point. These results offer new, physically concrete insights into the thermodynamics of quantum-corrected black holes.

gr-qc↗

arXiv2026

Evaporation and fate of covariant quantum black holes

A growing number of phenomena and theoretical problems indicate that quantum gravity theory is necessary. In this paper, we investigate the evaporation of covariant quantum BHs for particles with different spins and compare the results with the Schwarzschild case. Our results show that the Hawking radiation and mass loss rate of covariant quantum BHs differ from those of Schwarzschild BHs and they depend on the spins of the emitted particles. Therefore, these results suggest that it may be insufficient to consider only BH evaporation in the massless scalar field case and may provide a possible way to test loop quantum gravity in the future.

gr-qc↗

arXiv2026

xHC: Expanded Hyper-Connections

Hyper-Connections (HC) expand the residual stream of Transformers into $N$ parallel streams, providing a form of memory scaling beyond model width and depth. Manifold-Constrained HC (mHC) stabilizes this formulation at scale. The large gains from $N{=}1$ to $N{=}4$ suggest residual-stream expansion as a promising scaling axis. However, existing HC-family methods typically stop at $N{=}4$. Our experiments reveal why: scaling mHC beyond this point yields diminishing performance gains and rapidly increasing training cost. We attribute this limitation to two bottlenecks: insufficient write-back information for an expanding number of streams and residual-mixing generation whose cost scales cubically with $N$. To address both bottlenecks, we propose xHC (Expanded Hyper-Connections), the first HC-family method to achieve meaningful expansion beyond $N{=}4$. xHC combines temporal feature augmentation for richer write-back with a sparse residual-stream architecture that updates only $k=4$ of the $N=16$ streams while retaining dense access to the full residual state. Across 18B and 28B MoE models, xHC delivers strong and consistent downstream improvements. On an 18B MoE model, xHC improves the average downstream score by 4.0 points over mHC, while adding only modest training FLOPs over the vanilla baseline. Scaling-law experiments show that the vanilla and mHC require $1.50\times$ and $1.19\times$ the compute of xHC, respectively, to reach the same loss. Practical large-$N$ training also requires controlling memory traffic from the expanded residual state. We therefore introduce xHC-Flash, which reduces the per-sublayer memory traffic from $73.5C$ to $40C$, comparable to the $34C$ required by mHC at $N{=}4$, while retaining the gains of full xHC. Together, xHC and xHC-Flash make large-$N$ residual-stream expansion effective and practical for LLM pre-training.

cs.LG↗

arXiv2026

NITP: Next Implicit Token Prediction for LLM Pre-training

Standard next-token prediction (NTP) supervises language models solely through discrete labels in the output logit space. We argue that this sparse one-hot supervision leaves the latent representation space under-constrained, allowing hidden states to drift into degenerate and anisotropic configurations that can limit generalization. To address this issue, we propose Next Implicit Token Prediction (NITP), which augments discrete prediction with dense continuous supervision directly in the representation space. NITP trains the model to predict the implicit semantic content of the next token, using shallow-layer representations from the same model as stable self-supervised targets. We provide theoretical analysis showing that NITP regularizes the optimization landscape by mitigating under-constrained degrees of freedom and encouraging a compact, structured representation geometry. Empirically, across dense and MoE models ranging from 0.5B to 9B parameters, NITP consistently improves downstream performance with negligible computational overhead. On a 9B MoE model, NITP achieves a 5.7% absolute improvement on MMLU-Pro, along with gains of 6.4% on C3 and 4.3% on CommonsenseQA, with approximately 2% additional training FLOPs and no additional inference cost. Our implementation is available at https://github.com/aHapBean/NITP.

cs.CL↗

arXiv2026

Extended spherically symmetric solutions in revised Deser--Woodard nonlocal gravity

In this work, we extend the static spherically symmetric black hole solutions of revised Deser--Woodard (D-W) nonlocal gravity. Due to the linearity of the field equations, we show that the first-order expansion around the Schwarzschild solution is linear in both the temporal metric component and the reciprocal of the radial metric component. Therefore, a mode-by-mode superposition can be properly defined for different types of correction terms. We then further study two additional asymptotically flat extensions, a logarithmically dressed correction and an exponentially suppressed correction. The logarithmically dressed correction gives a slowly decaying deviation from Schwarzschild as the radius becomes large, making it a more extended nonlocal effect, whereas the exponentially suppressed correction is localized near the horizon and shifts the horizon inward for positive correction amplitude. These correction terms are physically motivated by common mechanisms in modified gravity: Logarithmic terms are possibly related to effects of quantum corrections, and exponential terms can arise from finite-range or screened gravitational effects.

gr-qc↗

arXiv2026

Higher-dimensional quantum-corrected Oppenheimer-Snyder model with a cosmological constant

We extended the higher-dimensional quantum Oppenheimer-Snyder model to the case with a cosmological constant. For AdS case, we discuss its thermodynamic properties in extend phase space formalism and make comparison with classical black holes. For quantum-corrected small black holes in AdS spacetime, the temperature no longer diverges but tends to zero. Additionally, the heat capacity exhibits characteristic behavior indicative of an extra phase transition induced by quantum corrections, highlighting the profound impact of quantum effects on black hole thermodynamics.

gr-qc↗

arXiv2026

Critical collapse of a self-interacting scalar field in asymptotically anti-de Sitter spacetime

We study the critical gravitational collapse of a spherically symmetric massless scalar field in asymptotically anti-de Sitter (AdS) spacetime. The scalar field potential adopted here is inversely proportional to the square of the AdS curvature radius $\ell$, and the system admits a well-known exact static solution. Working in polar coordinates, we first confirm that type II critical collapse occurs for a range of distinct initial configurations when $\ell=8$, where the measured echoing period and critical exponent are in excellent agreement with Choptuik's classic results. We then fine-tune the initial amplitude of the scalar field for a series of AdS radii $\ell$, performing calculations in both polar coordinates and double null coordinates to cross-validate our results. We find that the form of the potential does not alter the critical behavior of gravitational collapse in any meaningful way: in particular, both the echoing period ($Δ\approx 3.4$) and critical exponent ($γ\approx 0.37$) remain essentially unchanged across all tested values of $\ell$.

gr-qc↗

arXiv2026

Stress-energy tensor of quantized scalar fields in thermal states on a zero-tidal wormhole

The construction of a static traversable wormhole requires exotic matter that satisfies the Morris-Thorne conditions. Quantum energy-momentum tensors have long been considered the most promising candidate for such exotic matter. In this paper, we present the first calculation of the stress-energy tensor for a quantum massive scalar field in thermal states localized on the throat of a zero-tidal-force wormhole. By varying the dimensionless temperature and dimensionless mass of the scalar field, we find that the Morris-Thorne conditions can only be satisfied when the scalar field mass falls within a specific bounded interval. Furthermore, for any scalar field mass within this interval, there always exists a mass-dependent dimensionless critical temperature: the Morris-Thorne conditions are fulfilled only if the temperature remains below this critical threshold.

gr-qc↗

arXiv2026

Research progress on quantum neural networks and quantum machine learning

Machine learning holds fundamental computational significance due to the increasing demand for efficient solutions to complex tasks in data analysis, pattern recognition, and optimization, which are essential for addressing the multifaceted challenges of modern society. As the volume of data proliferates at an unprecedented rate, the need for more powerful machine learning strategies becomes increasingly evident. Quantum neural networks (QNNs) represent an emerging and transformative research field that seeks to harness the unique principles of quantum mechanics to enhance the capabilities of machine learning algorithms. This survey examines various QNN approaches, including fully connected QNNs, quantum convolutional neural networks, equivariant QNNs, quantum Hopfield networks, quantum Boltzmann machines, quantum reservoir computing, and composite networks for quantum reinforcement learning, quantum generative learning, and quantum transfer learning. We summarize the relevant investigations on their performance, including learning accuracy, training time, and resource requirements, etc. Each QNN type has unique strengths and weaknesses, offering diverse solutions for different applications.

quant-ph↗

arXiv2026

Loop Quantum Kaluza-Klein Cosmology and Inflation

We present the detailed analyses of five-dimensional loop quantum Kaluza-Klein cosmology based on the symmetric reduction of the connection formulation of the full theory. The previous results in a particular scenario are extended to more general cases. The effective scalar constraint for the geometric sector of the model is derived by the systematic semi-classical analysis in both the canonical and path-integral formulations, incorporating the quantum fluctuations as a subleading-order correction. The resulting effective scalar constraint not only exhibits the correct classical limit of the quantum system, but also serves as the basis for investigating the following three distinct effective scenarios through the incorporation of matter contributions: (i) vacuum, (ii) minimally coupling with a scalar field, and (iii) coupling with the dust. In all the three effective scenarios, the big bang and potential past big rip singularities in the classical model are naturally resolved by including the leading-order quantum correction of holonomies. Moreover, the visible universe undergoes a super-inflationary phase after overcoming the classical big bang singularity, during which the phenomenologically desired 55 e-folds can be achieved by appropriate initial conditions. In the case where the subleading-order quantum fluctuation term is included as a constant, the evolutions of the five-dimensional universe in all the three effective scenarios not only achieve sufficient inflation in the visible dimensions, but also exhibit re-collapse behaviors at certain large scales. Hence the cosmic inflation may originate from the interplay between compact extra dimensions and quantum geometric effects.

gr-qc↗

arXiv2026

Non-singular Inflation-Dark Energy Unification Model Based on Loop Quantum Cosmology and Mass-Varying Neutrinos

Unifying the early-universe inflationary paradigm with late-time cosmic acceleration, while resolving the initial Big Bang singularity, remains one of the most profound challenges in modern cosmology. In this paper, we propose a non-singular quintessential inflation model embedded within the effective dynamics of Loop Quantum Cosmology (LQC) based on a Generalized Regularization Scheme. The quantum geometry effects naturally replace the initial singularity with a quantum bounce, followed by a phase of superinflation that sets robust initial conditions for the subsequent slow-roll inflation. To achieve a viable late-time dark energy epoch and address the coincidence problem, we introduce a coupling between the scalar field and massive neutrinos, known as Mass-Varying Neutrinos (MaVaNs). As neutrinos become non-relativistic in the post-inflationary evolution, their backreaction effectively freezes the scalar field, triggering the late-time accelerated expansion. We numerically trace the full background dynamics from the quantum bounce to the present day. Furthermore, we tightly constrain the model parameters utilizing the observational data, including the Type Ia supernovae sample, the Dark Energy Spectroscopic Instrument (DESI) Baryon Acoustic Oscillations (BAO) and Cosmic Microwave Background (CMB) distance priors. Our results demonstrate that this unified LQC-MaVaNs quintessential framework is highly consistent with current precision cosmological observations.

gr-qc↗

arXiv2026

Distinguishability of magnetic massive black holes from environmental mimics with inspiral gravitational waves

In this work, we investigate the ppE waveform imprints induced by the external magnetic fields of Bertotti-Robinson and Bonnor-Melvin black holes, with the aim of distinguishing such magnetic effects from environmental influences. We first compute the ppE frequency-domain waveform for a small black hole inspiraling into a massive KBR black hole, which corresponds to a Kerr black hole embedded in an external magnetic field. We find that the leading-order correction arising from the magnetic field is at the $-2$ PN order relative to the quadrupole term, while the next-leading-order correction is at $-1.5$ PN, originating from the spin of the black hole. We further examine the effects of a spinning KBM black hole, whose leading-order magnetic correction is at $-3$ PN, whereas its spin-induced correction is also at $-1.5$ PN. The leading-order ppE corrections for both KBR and KBM black holes do not appear degenerate with any modified theory of gravity effects; nonetheless, we demonstrate that they resemble the gravitational pull contributions from additional matter with power-law distributions of index $γ=1$ and 0, respectively. To break the degeneracy with a single event, we adopt the statistic F in former research to discriminate between these two classes of beyond-vacuum GR effects using multiple gravitational wave events. We show even with multiple event statistic, it is not always efficient to distinguish real magnetic field effect from corresponding gravitational pull effect, especially for Bertotti-Robinson magnetic effect. For Bonnor-Melvin black hole, there is a transition value of $ρ_0$ estimated around $10^{-4}\text{kg}/\text{m}^3$ and corresponding $B\sim 10^{4}\text{T}$ above which real magnetic effect can be efficiently distinguished from gravitational pull and below the transition value it cannot.

gr-qc↗

arXiv2026

Novel five-dimensional rotating Lifshitz black holes with electric and axionic charges

In this work, we construct a new family of exact five-dimensional charged and rotating asymptotically Lifshitz black holes. The spacetime solves Einstein equations coupled to a dilaton, two Abelian gauge fields, and axionic scalars supplemented by two generalized Chern-Simons terms. This configuration is characterized by a range of the free dynamical exponent $z$ and possesses nontrivial thermodynamical parameters, where we verify the first law of black hole thermodynamics and derive the corresponding Smarr relation. As an application of this new gravitational background, we then investigate a holographic superconductor in the rotating Lifshitz background. We study the condensation of the scalar operator and the AC conductivity of the dual system. These results show that increasing the rotation parameter suppresses the condensate and weakens the superconducting phase, while increasing the dynamical critical exponent enhances the superconducting order. To the best of our knowledge, these solutions provide the first explicit family of five-dimensional rotating Lifshitz black holes supported simultaneously by electric and axionic charges.

hep-th↗

arXiv2026

Quantum Oppenheimer-Snyder primordial black holes as all the dark matter

Primordial black holes (PBHs) are widely considered as candidates for dark matter in many recent studies, and they are often modeled as Schwarzschild or Kerr black holes (BHs), which have curvature singularities. Nevertheless, resolving the classical singularity may require quantum gravity motivated corrections, thereby yielding an effective quantum corrected BH spacetime geometry different from the Schwarzschild or Kerr cases. Therefore, it is well motivated to consider BHs beyond the Schwarzschild or Kerr as viable PBH candidates. Based on these considerations, we investigate quantum Oppenheimer Snyder BHs as PBHs which could account for all the dark matter. Our results show that these BHs have temperatures and greybody factors markedly different from the Schwarzschild case, suppressing Hawking emission and thereby relaxing the $γ$-ray constraints from HEAO-1, COMPTEL, and EGRET, which, relative to the Schwarzschild case, broadens the allowed PBH mass window in the asteroid-mass range where PBHs can constitute all of the dark matter.

gr-qc↗

arXiv2026

A Non-Abelian Route to Z2 Non-Hermitian Skin Effects

The non-Hermitian skin effect (NHSE), characterized by extensive boundary accumulation of eigenstates under open boundary conditions, has emerged as a central phenomenon in non-Hermitian physics. Conventionally, the NHSE arises from either non-reciprocal couplings or onsite gain and loss combined with synthetic gauge fields. Existing studies, however, have been largely confined to frameworks with Abelian-coupling, leaving the role of non-Abelian couplings essentially unexplored. Here, we demonstrate that non-Abelian-couplings can generate the NHSE, giving rise to a time-reversal-symmetry-protected Z2 skin effect with pseudospin-dependent boundary localization and dynamical pseudospin separation. Experimentally, we implement a representative four-level model using a programmable topolectrical circuit and directly observe both the predicted NHSE and the boundary-induced pseudospin-inversion reflection. Our work establishes a fundamental link between non-Abelian coupling and non-Hermitian topology, opening new avenues for realizing non-reciprocity-free topological materials and devices.

physics.optics↗

arXiv2026

Periodic orbits and gravitational waveforms of black holes in bumblebee gravity

In this paper, we investigate the dynamics of massive particles and the associated gravitational waveforms in the spacetime of a black hole within the framework of Einstein-Bumblebee gravity. Our analysis encompasses both charged and uncharged black hole configurations, with a particular focus on the spontaneous Lorentz symmetry breaking mechanism inherent to this model, which is governed by a dimensionless coupling parameter $l$. We analyze the geodesic equations and the effective potential to determine the allowed parameter space for bound orbits, demonstrating that in the charged case, both the Lorentz-violating parameter $l$ and the electric charge $Q$ significantly enhance the confinement capacity of the potential, thereby broadening the energy and angular momentum windows for bound states. A key focus is placed on the classification and properties of periodic orbits, characterized by rational frequency ratios using the whirl, zoom, and vertex taxonomy. We demonstrate that in the uncharged case ($Q=0$), the radial effective potential and standard innermost stable circular orbit (ISCO) properties are degenerate with those of a Schwarzschild black hole. However, despite this degeneracy in static potential properties, the structure of periodic orbits exhibits qualitative differences, providing a possible observational signature that can break this degeneracy. Finally, we compute the corresponding gravitational waveforms extracted from these periodic orbits using the quadrupole formula. The results reveal that $l$ and $Q$ introduce contrasting phase-shifting effects on the waveforms. This suggests that bumblebee gravity leaves measurable imprints on gravitational-wave signals that could be detected by future space-based gravitational-wave observatories.

gr-qc↗

arXiv2026

Information paradox and island of covariant black holes in LQG

We study information paradox of four dimensional covariant black holes inspired by loop quantum gravity (LQG) with two well motivated solutions. We first prepare the spacetime in the Hartle-Hawking state, compute the radiation entropy and recover a linear growth at late time. When considering the mass loss and incorporating greybody factors, we show that for Solution~1 the LQG parameter $ζ$ leaves temperature and Planckian factor of the spectrum unchanged but enhances the near-horizon barrier, leading to a faster evaporation rate as $M$ decreases. This behavior contrasts sharply with Solution~2, which has slow evaporation rate at small $M$ and admits a non-singular continuation suggestive of a remnant or a black-to-white-hole transition. We then apply the island prescription on the eternal background and find that quantum extremal surfaces exist in solution 1 geometries; $ζ$ primarily shifts the island boundary and suppresses the late time entropy growth, preserving unitarity. Our results highlight that covariance-respecting LQG black hole do not exhibit a universal late time behavior.

gr-qc↗

arXiv2026

Critical collapse of a massive scalar field in semi-classical loop quantum gravity

We investigate critical phenomena during the gravitational collapse of a massive scalar field under two distinct semi-classical loop quantum gravity (LQG) approaches within spherical symmetry. Numerical simulations reveal that the massive scalar field in both semi-classical frameworks exhibits two distinct types of critical behavior, consistent with the classical scenario. When the scalar field's mass parameter is small, type II critical phenomena emerge, with the resulting echoing periods and critical exponents precisely matching those obtained in general relativity. In contrast, a large mass parameter triggers type I critical phenomena, where the resulting black holes possess a finite minimum mass. These findings suggest that semi-classical corrections from LQG have a negligible impact on the dynamics of critical collapse.

gr-qc↗