arXiv subjects

Chengxun Yuan

Publications and source records attributed to Chengxun Yuan.

At least 37 records · Page 2Linked to original sources

arXiv2026

Dynamics, Ringdown, and Accretion-Driven Multiple Quasi-Periodic Oscillations of Kerr-Bertotti-Robinson Black Holes

We study the motion of test particles around the Kerr--Bertotti--Robinson (KBR) black hole (BH) and explore how the three defining parameters, the mass $M$, rotation parameter $a$, and magnetic parameter $B$ influence their dynamics. We derive analytical expressions for the energy and angular momentum of stable equatorial circular orbits, along with the corresponding radial and latitudinal oscillation frequencies, as functions of $M$, $a$, and $B$. We also examine the key features of the quasi-periodic oscillations (QPOs) of test particles near stable circular orbits, including the precession effects such as periastron precession and the Lense-Thirring effect. We compare our results with those corresponding to the Kerr BH. We find that the particle motion is strongly shaped by the BH parameters. Using a WKB approach, we also study scalar quasinormal modes of rotating KBR BH in an external magnetic field and show that the magnetic field increases damping, while rotation and angular momentum mainly set the oscillation frequencies. Alternatively, general relativistic modeling of Bondi-Hoyle-Lyttleton (BHL) accretion onto rapidly rotating KBR BH shows that two distinct physical structures emerge and cyclically transform into one another over time. These processes produce either a strongly oscillating flip-flop shock cone or a nearly stationary toroidal structure, with their formation governed by the BH spin and magnetic curvature. Power spectral analysis shows that these configurations give rise to low- and high-frequency QPO, providing a unified theoretical framework to understand how multiple QPO-like features can arise in rapidly spinning accreting systems.

gr-qc↗

arXiv2026

Probing Quantum Gravity through Chaotic Orbits and Strong-Field Effects in Kerr Black Holes Embedded in Perfect Fluid Dark Matter

We study the nonlinear photon dynamics in quantum improved rotating black hole surrounded by perfect fluid dark matter (PFDM) using several methods of analysis, including Poincaré sections, Lyapunov exponents, Kolmogorov-Sinai (KS) entropy and weighted Birkhoff averages (WBA). In particular, we examine the effect of quantum-improved parameter $(\tildeω)$ and PFDM parameter $(ζ)$ on the null geodesic motion hence stability of circular orbit. Poincaré sections illustrate the transition from regular to chaotic motion as these parameters increase, characterized by the deformation and fragmentation of invariant tori and the emergence of scattered chaotic regions in phase space. The stability properties of null circular orbits are quantified through the Lyapunov indicators, revealing that both the quantum improvement parameter and the PFDM parameter enhance the sensitivity of photon trajectories to initial conditions. The KS entropy provides an independent measure of dynamical complexity and confirms the growth of chaotic behavior with increasing quantum and PFDM corrections. Additionally, the WBA method offers a robust quantitative criterion for distinguishing regular and chaotic orbits and allows a detailed mapping of the phase-space structure. The results demonstrate that the combined effects of quantum gravity corrections and PFDM significantly modify the effective potential governing photon motion, leading to a rich mixed phase-space structure with coexisting regular and chaotic regions. These findings underscore the crucial role of quantum and dark matter contributions in shaping photon dynamics near rotating black holes and suggest possible observational implications on black hole shadows and gravitational lensing in strong-field regimes.

gr-qc↗

arXiv2026

Scalar and Electromagnetic Perturbations around a Black Hole with a Topological Defect: Quasinormal Modes and Quasi-bound States in a Plasma Medium

We investigated the influence of a plasma environment on the optical and perturbative properties of a black hole with a topological defect, characterized by the parameter $k$. We first established a straightforward correspondence between the real part of the quasinormal-mode (QNM) frequencies in the eikonal limit and the black-hole shadow radius. We then demonstrated that the Lyapunov exponent associated with the photon sphere exhibits only a weak dependence on the plasma frequency, while it monotonically decreases as the topological-defect parameter $k$ increases. Subsequently, we analyzed massive scalar-field perturbations by deriving the associated effective potential and computing the QNM spectrum using the third- and sixth-order WKB approximations for both homogeneous and radially inhomogeneous plasma configurations, including the singular isothermal sphere (SIS) and non-singular isothermal sphere (NSIS) density profiles. Our results show that the presence of plasma induces shifts in both the oscillation frequencies and the damping rates of the modes, and that larger values of $k$ systematically suppress the real part of the QNM frequencies. Among the plasma models considered, the NSIS profile generally yields slightly higher oscillation frequencies than both the SIS and homogeneous cases. Finally, we derived the dynamical equations governing electromagnetic perturbations in a cold, unmagnetized plasma and demonstrated that the axial and polar sectors decouple. In the axial sector, the plasma frequency enters as an effective mass term, thereby permitting the existence of quasi-bound states only in the case of a homogeneous plasma and only when the plasma frequency lies below a critical threshold that depends on the topological-defect parameter $k$ and the multipole index $l$.

gr-qc↗

arXiv2026

Dilaton-Induced Resonant Production of Ultralight Vector Dark Matter

A dilatonic half-mass resonance can produce ultralight vector dark matter only if the Floquet instability becomes efficient before the oscillating spectator scalar dominates the cosmic expansion. We formulate this requirement in terms of the microscopic modulation parameter $ε_i=Φ_i/M$ and the gravitational onset fraction $r_i=Φ_i^2/(6\Mpl^2)$. For a background with constant equation-of-state parameter $w_b$, the narrow-band Floquet exponent obeys $μ/H\propto a^{3w_b/2}$; during radiation domination this ratio grows as $a^{1/2}$, while it remains constant for matter-like expansion. Imposing that the delayed instability occurs before spectator domination yields the amplitude-independent bound $M/\Mpl\lesssim\sqrt6,c_1\simeq0.31$, with $c_1\simeq1/8$ determined by the linear half-mass branch. An explicit expanding-background analysis confirms that $a_\star<a_{\rm dom}$ for sub-Planckian $M$, whereas $M\simeq\Mpl$ postpones efficient growth until after domination. Combining this embedding condition with the efficient-transfer normalization gives $m_{γ'}\propto r_i^{-2}$, implying that the ultralight range $m_{γ'}\sim10^{-20}$--$10^{-18},{\rm eV}$ corresponds to $r_i\sim10^{-5}$--$10^{-4}$ rather than to early spectator domination. The polarization-resolved canonical analysis shows that longitudinal production is more strongly concentrated in the infrared than transverse production, while derivative terms from canonical normalization modify the leading Floquet exponents at order unity. Stückelberg and Higgsed completions impose distinct ultraviolet consistency conditions, including radial decoupling and symmetry-restoration constraints. The viable branch is therefore radiation-era, perturbative, infrared-dominated, and associated with a sub-Planckian kinetic scale.

hep-ph↗

arXiv2026

Long Inflation Screens Euclidean-Wormhole Initial States

Euclidean wormholes can prepare inflation in non--Bunch--Davies initial states, but long Lorentzian expansion screens this memory from the CMB. We derive a visibility bound for Euclidean-matched Bogoliubov data: the pivot excitation satisfies $|β_*| \lesssim e^{-2N_{\rm pre}}$, and smooth Euclidean filters confine residual signatures to a comoving edge $k_w=a_iM$. Only near-minimal inflation, or an edge inside the observable window, leaves detectable scalar, tensor, and higher-point imprints. For longer inflation, wormhole-prepared perturbations are driven to the Bunch--Davies prediction. Euclidean memory therefore, becomes a quantitative bound on inflationary duration, with direct targets in CMB polarization and large-scale structure: the longer inflation lasts, the less of the wormhole remains on the sky.

hep-ph↗

arXiv2026

Quantitative symmetry-breaking and nonlinear harmonic generation in plasmonics

We develop a quantitative mathematical theory that offers new perspectives on nonlinear harmonic generation in plasmonic structures arising from symmetry breaking. Focusing on second harmonic generation--the most fundamental process and the most extensively studied owing to its practical significance--we establish a theoretical framework that can be readily extended to higher-order harmonics. We investigate the plasmonic system in the static regime using a columnar nanowire with $n$-fold rotational symmetry ($n \in \mathbb{N}$) and construct a phenomenological model in which the second harmonic response originates from nonlinear sources confined to a selvedge region near the surface. By introducing a notion of symmetry degree grounded in group theory, we precisely quantify the second harmonic generation in terms of multipolar contributions. Our theory complements existing physical descriptions of this practically important phenomenon and provides a rigorous account of how nonlinear optical efficiency depends on shape, size, symmetry, and defects in plasmonic structures.

math.AP↗

arXiv2026

Spatiotemporal Co-reflection with Spacetime Discontinuities at Moving Interfaces

The control of reflection and refraction at interfaces using engineered media is central to numerous optical technologies, with negative refraction and the suppression of backscattering representing two prominent research frontiers. In this work, we demonstrate that an effective negative refraction accompanied by an absence of backscattering can be realized at a moving spatiotemporal interface when temporal and spatial reflections occur concurrently. While such spatiotemporal co-reflection is prohibited in one-dimensional linear dispersive media, we show that it becomes permissible under oblique incidence within a specific range of traveling-wave modulation velocities. Leveraging this mechanism, we propose a spatiotemporal flat lens capable of nonreciprocal electromagnetic wave focusing. These findings provide a framework for developing advanced spatiotemporal metamaterials and time-varying metasurfaces.

physics.optics↗

arXiv2026

Repetitive Penrose Process in Kerr-Taub-NUT black hole spacetime

In this article, we study the repetitive Penrose process for the Kerr-Taub-NUT black hole (BH). First of all, we briefly review the spacetime of the Kerr-Taub-NUT BH, including horizon and ergosphere structures. The results indicate that the event horizon and ergosphere radii increase under the influence of the gravitomagnetic charge $l$. Subsequently, we find by using the irreducible mass of the BH that the extractable energy decreases with the rise of the gravitomagnetic charge. We then turn to the repetitive Penrose process by writing the conservation laws and setting the corresponding iterative stopping conditions. Furthermore, we numerically calculate the change in the BH's parameters, along with the corresponding quantities of the repetitive Penrose process, for each iteration.

gr-qc↗

arXiv2026

Breaking the Limitations of Temporal Modulation via Mixed Continuity Conditions

The conventional description of time-varying media assumes that electromagnetic fields evolve according to fixed continuity conditions during parameter jumps. Here we reveal that these conditions are not physical constraints but tunable design degrees of freedom. By developing a unified framework that treats continuity rules as engineerable parameters, we expand the scope of time-varying metamaterials and enable wave phenomena previously considered impossible. For instance, non-resonant, reflectionless wave amplification without momentum bandgaps, and reversible conversion between propagating waves and static fields for optical memory, etc. This work opens a new dimension for controlling light-matter interactions.

physics.optics↗

arXiv2026

Chaotic motion and power spectral density in Schwarzschild Bertotti-Robinson black hole spacetime

In this paper, we show that in weak field limit Schwarzschild Bertotti-Robinson black hole (Schwarzschild-BR BH) turns into Schwarzschild black hole immersed in external uniform magnetic field which is given in 1. The dynamics of both magnetized and electrically charged particles in the vicinity of a Schwarzschild-BR black hole are investigated. The innermost stable circular orbits (ISCOs) for both magnetized and electrically charged particles are examined in detail, revealing that the magnetic field parameter B exerts a considerable influence, leading to an increase in the ISCO radius. The orbital and epicyclic motion of test particles in Schwarzschild-BR black hole spacetime was analyzed, including both circular orbits and their oscillatory perturbations. Additionally, the trajectories of both magnetized and electrically charged particles are analyzed for various configurations of the magnetic parameter B. We also demonstrate how the magnetic field B, electric charge q, and magnetic moment μ influence the dynamics of charged particles, specifically affecting the chaotic behavior, Poincare' sections, oscillatory frequencies and power spectral density.

gr-qc↗

arXiv2026

Plasma impact on black hole shadow and gravitational weak lensing for Schwarzschild-like black hole

This article delves into the observational properties of a Schwarzschild-like black hole (BH). Initially, the research provides a succinct examination of the spacetime geometry and the configuration of its horizon. Furthermore, we study the photon dynamics around the Schwarzschild-like BH in the presence of the plasma using the Hamiltonian formalism. It was found that the photon sphere radii increase under the influence of the plasma frequency and vice versa for the spacetime parameters. Further exploration is dedicated to understanding how the plasma affects the shadow of the BH, and we find that the radius of the BH shadow shrinks with the rise of the $ξ$ parameter and plasma frequency. We then turn to the getting constraint of the spacetime parameters and the plasma frequency by using the observational data released by the Event Horizon Telescope (EHT) collaboration for the M87* and Sgr A*. Additionally, the research scrutinises the phenomenon of gravitational weak lensing in the vicinity of a Schwarzschild-like BH, considering both uniform and non-uniform plasma scenarios. The outcomes demonstrate that the angle of deflection increases under the influence of a uniform plasma frequency, whereas the opposite is true for non-uniform plasma. In both scenarios, a rise in the spacetime parameters results in a decrease in the deflection angle. Finally, we investigate the magnification of the gravitationally lensed image. The effect of the spacetime parameters and plasma frequencies on the total magnification are same as in the deflection angles.

gr-qc↗

arXiv2026

Spherically-symmetrical vacuum solution in Freund-Nambu scalar-tensor gravity

Scalar--tensor theories of gravity provide a natural extension of general relativity and may predict naked singularities as alternative compact objects. In this work, we investigate a novel exact solution within the Freud--Nambu scalar--tensor gravity framework, generalizing the Janis--Newman--Winicour (JNW) naked singularity spacetime through the introduction of a parameter $q$ coupled to a real scalar field $φ$ with mass $μ$. Although the metric remains identical to the JNW solution, the scalar field profile is modified, providing a parametrized deformation of this class of spacetimes. We analyze particle dynamics in this background, including a direct linear coupling between the test particle and the scalar field characterized by the parameter $g_s$. The influence of these parameters on astrophysical observables is studied through the specific angular momentum, the innermost stable circular orbit (ISCO), and the radiative efficiency of accretion. We also derive the epicyclic frequencies governing oscillatory motion and explore their implications for quasi-periodic oscillations (QPOs) in black hole binaries. Within the epicyclic resonance model, the upper and lower QPO frequencies depend sensitively on the parameters $n$, $g_s$, and $q$. To constrain the model, we perform a Markov Chain Monte Carlo analysis using twin-peak QPO data from the microquasars XTE~J1550--564 and GRS~1915+105. The resulting black hole masses agree with previous estimates and provide the first observational constraints on the parameters $q$ and $g_s$, indicating that modified gravity effects may leave detectable imprints on strong-field astrophysical phenomena.

gr-qc↗

arXiv2026

Effect of gravitational lensing around black hole in dark matter halo in the presence of plasma

This article is devoted to the investigation of the observational properties of the Schwarzschild black hole (BH) surrounded by a dark matter (DM) halo. Our study commences with a brief review of spacetime, including the horizon structure and curvature invariants, which are the Ricci scalar, the square of the Ricci tensor, and the Kretschmann scalar. Subsequently, we explore the massive and massless particle dynamics around the Schwarzschild BH surrounded by a dark matter halo, including the innermost stable circular orbit (ISCO) and photon sphere radii. It was found that the radius of the ISCO increases under the influence of the spacetime parameters. Additionally, we investigate the weak gravitational lensing with the assumption that the BH is surrounded by a uniform and non-uniform plasma. Finally, we examine the impact of a plasma on the BH shadow and employ Event Horizon Telescope (EHT) observational data to constrain the BH's parameters.

gr-qc↗

arXiv2026

Can a Dehnen-type dark matter halo affect the neutrino flavor oscillations?

The gravitational weak lensing of neutrinos in the presence of a Schwarzschild black hole surrounded by a Dehnen-type dark matter halo is investigated. The event horizon structure is explored, and the existence of the BH is studied in two different slices of the parameter space. Additionally, we derive analytic expressions for the oscillation phase and transition probabilities for both radial and non-radial neutrino propagation. Finally, we use a two-flavor toy model, and numerically analyze how the dark matter halo parameters as density and scale radius, affect oscillation probabilities and examine the role of decoherence. The results show that the presence of a dark matter halo modifies the oscillation phase and damping factor, leading to measurable deviations from the standard Schwarzschild BH case. These findings suggest that neutrino oscillations could, in principle, serve as a probe for dark matter distributions around compact astrophysical objects.

gr-qc↗

arXiv2026

New analytical model of static black hole with a dark matter halo and parametric constraints through quasiperiodic oscillations

A novel analytical Schwarzschild-like black hole (BH) solution is derived. It exhibits a static BH with a dark matter (DM) halo characterized by a Dehnen-type density profile. This solution could represent an alternative perspective on the interaction of black hole-dark matter systems, providing new insights into the fundamental properties of DM halos. We study the properties of the newly derived BH solution by examining its spacetime curvature characteristics and energy conditions, providing insights into how the DM halo influences these fundamental characteristics. Additionally, we analyze the timelike geodesics of test particles in the obtained BH-DM spacetime, highlighting how the presence of the novel Dehnen-type DM halo alters the gravitational dynamics and modifies particle trajectories. Increase of the DM halo's density $ρ_s$ and characteristic scale $r_s$ leads to an outward shift of both stable and unstable circular orbits. Finally, we test our model by fitting it to real data from the microquasars GRO J1655-40, GRS 1915+105, and XTE J1550-564 using a statistical Markov Chain Monte Carlo (MCMC) method. This allows us to find the best estimates for the properties of the DM halo surrounding these systems.

gr-qc↗

arXiv2026

New analytical model of rotating black hole with dark matter halo: constraints from EHT observations and accretion disk

In this paper, we start from a static black hole (BH) immersed in a Deanne-type dark matter (DM) halo and employ the Newman-Janis algorithm (NJA) to generate the rotating black hole solution with a dark matter halo. Also, we have checked the validity of the obtained space-time. Then we study optical properties of newly obtained rotating BH in DM halo, including the shadow's geometrical shape, deflection angle of light based Ono, Ishihara and Asada (OID) method, photon sphere and the dependence of the shadow radius on DM parameters. Additionally, assuming that spacetime of a supermassive black hole (SMBH) is described by the newly obtained rotating BH solution, we analyze the parameters of the model with shadow size estimates based on the Event Horizon Telescope (EHT) and Gravity collaboration observations of M87* and Sgr A* SMBHs. Then we have used Markov Chain Monte Carlo (MCMC) analysis to constrain DM parameters $ρ_s$, $r_s$ and BH mass M, BH spin a, also we show that best-fit values for the parameters $ρ_s$, $r_s$ are well agreement with previous results which indicate physically reasonability of the our model. Finally, we have analyzed the electromagnetic radiation flux of the rotating BH in the DM halo employing a ray tracing code.

gr-qc↗

arXiv2026

Relativistic Dynamics and Bondi-Hoyle-Lyttleton Accretion onto Rotating Embedded Black Hole Models

In this paper, we examine the motion of test particles and relativistic accretion mechanisms within the spacetime of a rotating and embedded BH. In this case, the geometric properties of the metric and their dynamical consequences for particle trajectories are systematically studied, with a specific focus on circular orbits together with their existence criteria and stability constraints. Also, the effective potential and the corresponding effective force are constructed to quantify the influence of rotation and embedding parameters on the attractive and repulsive sectors of the gravitational interaction. Closed-form expressions for orbital frequencies as measured by a distant observer are derived, enabling a quantitative analysis of relativistic precession phenomena, including periastron advance and Lense-Thirring precession. Furthermore, we conduct general-relativistic hydrodynamic simulations of BHL accretion onto rotating embedded BHs. In addition, within the framework of the BHL accretion mechanism, the numerical solution of the GRH equations shows that the embedding parameter αsystematically modifies the morphology of the shock cone formed around embedded BHs compared to the Kerr model. In particular, a wider opening angle of the cone is produced, the compression of matter in the post-shock region is weakened, and the dynamical variability of the flow is enhanced. The time-dependent mass accretion rate exhibits increasing oscillation amplitudes and long-term variations with increasing α, while these amplitudes are found to be suppressed by the frame-dragging effect associated with the BH spin parameter. At the same time, increasing values of $α$ lead to a strengthening of the QPO frequencies formed around embedded BHs in the LFQPO regime, enhancing their observability and increasing the likelihood of detecting commensurate frequency ratios such as 3:2.

astro-ph.HE↗

arXiv2025

Probing the gravity of a Schwarzschild black hole in the presence of a cloud of strings with EMRIs

Here, we explore the effect of the cloud of strings (CoS) on the gravitational waveforms of extreme mass ratio inspirals (EMRIs). The EMRI system consists of a supermassive black hole (BH) and a compact stellar mass object moving around it. We begin with studying the test particle motion around the Schwarzschild BH surrounded by a CoS by using the Lagrangian formalism. Moreover, we investigated the effect of the CoS parameter on the evolution of the semi-latus rectum and eccentricity. We then turn to the exploration of the impact of the CoS parameter on the gravitational waveforms of the EMRI system. The analysis performed shows that Laser Interferometer Space Antenna (LISA) could detect the CoS imprint in gravitational waveforms when the values of the string cloud parameter $α\gtrsim 2 \times 10^{-6}$.

gr-qc↗