arXiv subjects

Mohammad Ali S. Afshar

Publications and source records attributed to Mohammad Ali S. Afshar.

At least 19 recordsLinked to original sources

arXiv2026

Can Static Black Holes in Massive Gravity Serve as Candidates for Aschenbach-Like Phenomena?

The Aschenbach effect is widely regarded as a manifestation of two quintessential relativistic features: frame dragging and extreme spacetime curvature. Traditionally associated with rotating geometries, this non-monotonic behavior in orbital angular velocity challenges Newtonian intuition. In our previous work, however, we demonstrated that this velocity irregularity is not exclusive to spinning spacetimes. Specifically, we showed that the presence of a stable minimum in the gravitational potential, corresponding to a stable photon sphere, can reproduce Aschenbach-like behavior in static black holes as well. This observation suggests that, even in the absence of rotational frame dragging, curvature alone (if encoded through appropriate geometric extrema) may be sufficient to induce non-monotonic velocity profiles. In this study, we build upon that foundation to investigate whether black hole architectures in theories of Massive Gravity can inherently support the emergence of Aschenbach-like phenomena. Furthermore, can this Aschenbach-like phenomenon in static configurations be considered as an observable signature in the dynamics of general relativity, similar to the original Aschenbach effect in rotating spacetimes?

gr-qc↗

arXiv2026

Striving to find a bridge between noncommutative and generalized entropy parameters in the harmonic oscillator dynamics

In this study, we use the harmonic oscillator s energy spectrum to connect its dynamical behavior to the formalism of statistical mechanics through entropy. We perform a thermal analysis of the entropy generated by this spectrum in both commutative and noncommutative two-dimensional configuration spaces, employing the Tsallis and Renyi entropy measures. The primary objective is to examine the possibility of an intrinsic link and mutual influence between the noncommutative parameter(θ) and the parameters arising from generalized entropies. By analyzing both Tsallis and Renyi frameworks, we investigate whether a fundamental relationship exists between θ and the entropy index q. Establishing such a connection could provide new insights into the interplay between spacetime geometry and generalized statistical mechanics.

physics.gen-ph↗

arXiv2026

Gravitational Wave Signatures of Periodic Orbits around a Schwarzschild-like Black Holes Submerged in an Exponential Density Dark Matter Profile

We study a Schwarzschild-like black hole embedded in an exponential-sphere (ESM) dark matter halo, characterized by a halo mass $M_0$ and a scale radius $r_0$. We first show that the halo's effect on the marginally bound orbit (MBO) and innermost stable circular orbit (ISCO) is richer than a simple shift: the characteristic radii and energies vary non-monotonically with $r_0$, dipping below their Schwarzschild values before recovering, while the angular momenta decrease monotonically; as a function of $M_0$, the response can even reverse sign depending on how extended the halo is, with only the ISCO energy remaining monotonic throughout. We classify periodic orbits by the rational frequency ratio $q=ω_ϕ/ω_r-1=w+v/z$ and find that $r_0$ and $M_0$ leave clearly distinguishable imprints on the orbit spectrum. Using the numerical kludge framework, we compute the corresponding extreme-mass-ratio-inspiral (EMRI) waveforms and show that varying $r_0$ produces a strong, monotonic effect enlarging the orbits, lengthening the radial period, and introducing a clear dephasing while comparable variations in $M_0$ leave the signal nearly unchanged unless $M_0$ becomes a sizable fraction of the black hole mass. Together, these results indicate that EMRI waveforms can, in principle, disentangle the total mass of a dark matter halo from its spatial extent, offering a strong-field probe of the dark matter distribution around supermassive black holes.

gr-qc↗

arXiv2026

Geometry-Induced Termination of the Repetitive Penrose Process in Rotating Simpson-Visser Black Holes

We examine the Repetitive Penrose Process in the rotating Simpson-Visser spacetime, whose parameter space includes regular black holes, black-bounce geometries, and traversable wormholes. Assuming that the regularization parameter remains unchanged throughout the evolution, the analysis shows that the endpoint of the repetitive process is not always determined solely by the conventional minimum spin condition. Instead, for sufficiently large values of the regularization parameter, the evolving solution may leave the parameter domain corresponding to the original two horizon black hole branch before the dynamical spin limit is reached. Within the present framework, this provides an additional geometry-induced condition that limits the continuation of the iterative sequence. The numerical results further show that the relative importance of the dynamical and geometry-induced termination mechanisms depends sensitively on the regularization parameter. For small deformations, the evolution remains qualitatively similar to that of the Kerr spacetime. As the regularization parameter increases, however, the cumulative extracted energy, the number of admissible Penrose iterations, and the efficiency of the process are progressively reduced. We also examine how the extracted energy, the final irreducible mass, and two complementary efficiency measures vary with both the regularization parameter and the particle decay radius. Overall, the present analysis indicates that, within the RSV geometry, the underlying spacetime structure influences not only the cumulative efficiency of the repetitive Penrose process but also the parameter range over which the iterative evolution remains self-consistent. These results highlight the role that spacetime geometry can play in shaping the long-term evolution of idealized Penrose-type energy extraction processes in regular rotating black-hole spacetimes.

gr-qc↗

arXiv2026

Periodic Orbits and Gravitational Wave Signatures around the Bonanno--Reuter Regular Black Hole

Timelike geodesics, periodic orbits, and their associated gravitational-wave signatures are examined in the spacetime of a Bonanno--Reuter regular black hole, a geometry arising from Asymptotically Safe Gravity in which a running Newton coupling replaces the central singularity with a de Sitter core. The dimensionless parameter $α/M^2$ completely determines the strong-field dynamics. Increasing $α/M^2$ shifts the marginally bound and innermost stable circular orbits inward, systematically reducing their characteristic radii, angular momenta, and energies; the allowed phase space for bound motion contracts accordingly. Classifying trajectories via the rational frequency ratio $q = w + v/z$ reveals that periodic orbits experience a mild inward contraction, which reduces the energy necessary to sustain a specific topology. Within the numerical kludge framework, we calculate the gravitational-wave polarizations for extreme mass-ratio inspirals. The asymptotically safe correction induces a leftward phase shift that reflects shorter orbital periods, while mildly enhancing peak amplitudes owing to the smaller periastron distances reached in the deep strong-field regime. Waveform sensitivity displays a strong dependence on topology, with high-whirl orbits, which persist longer in the strong-field region near the horizon, showing markedly more pronounced deviations. Unlike environmental effects that inflate orbital scales, intrinsic quantum-gravity modifications generate distinct, observationally detectable signatures for future space-based detectors such as LISA, Taiji, and TianQin.

gr-qc↗

arXiv2026

The Influence of Stable Photon Sphere Advent on Orbital Precession in moving towards the Extremality

In this work, we investigate the behaviour of the periapsis shift in static charged black-hole spacetimes, focusing on the interplay between extremality and the Aschenbach-like effect. We first examine the evolution of orbital dynamics as the extremal limit is approached in representative charged black-hole models. We then extend the analysis to black-hole geometries that admit a stable photon sphere outside the event horizon, where the Aschenbach-like effect has previously been identified. Our results show that the periapsis shift remains a well-defined dynamical quantity in the extremal regime and continuously reflects changes in the underlying spacetime geometry. For black-hole solutions possessing an external stable photon sphere, the associated minimum of the effective potential produces characteristic modifications in both the angular-velocity profile and the periapsis shift. When extremality and the Aschenbach-like effect coexist, the orbital dynamics undergo a qualitative transition, giving rise to a three-region structure consisting of an inner prograde region, an intermediate retrograde region, and an outer prograde region. Within the black-hole models investigated in this work, such behaviour is absent from the other configurations considered. These results demonstrate that the periapsis shift provides a sensitive dynamical probe of strong-field black-hole spacetimes, encoding both geometric and orbital information beyond that obtained from photon-sphere analyses alone. Our findings highlight the rich orbital structure that can emerge from the combined presence of extremality and an external stable photon sphere and provide a useful framework for future studies of orbital dynamics in modified theories of gravity.

gr-qc↗

arXiv2026

Repetitive Penrose Process in Rotating 4D Einstein-Gauss-Bonnet Black Holes

We investigate the repetitive Penrose process for neutral particles in a rotating four-dimensional Einstein--Gauss--Bonnet black hole obtained through the modified Newman--Janis algorithm, developing a nonlinear iterative scheme in which the mass, angular momentum, and irreducible mass are updated after each extraction event. Imposing the triple turning-point condition, we obtain a closed-form solution of the conservation equations for energy, angular momentum, and radial momentum that reduces to the Kerr result in the limit of vanishing coupling. The distinctive feature of this background is that, although the Gauss--Bonnet coupling $α$ is a fixed constant of the action and is not carried by the infalling fragments, the dimensionless coupling $\hatα=α/M^{2}$ grows at every iteration as the mass decreases, so that the effective Gauss--Bonnet correction is self-amplified along the sequence. We find that a larger coupling lowers the extremal spin, contracts the ergosphere, and reduces the number of admissible decays, forbidding the process near the horizon at strong coupling while permitting it at larger decay radii; the termination is controlled throughout by the incident particle. The energy return on investment decreases monotonically with $\hatα$ and the growth of the irreducible mass is suppressed relative to Kerr, whereas the energy utilization efficiency is non-monotonic: below a critical coupling the parameter space exhibits a four-region structure with a bounded window in which the EGB black hole is more efficient than Kerr; this four-region structure collapses into a three-region one above a critical coupling. This coupling-driven reorganization of the efficiency landscape has no analogue in the Kerr, Reissner--Nordström, Kerr--de~Sitter, accelerating Kerr, or Kerr--Newman cases.

gr-qc↗

arXiv2026

Beyond the Bounce: Multiple Tidal Sign Reversals and Turning-Point Bifurcations in Multi-Horizon Black Holes

We investigate the radial motion and tidal forces experienced by neutral test particles in multi-horizon black hole solutions arising from Einstein gravity coupled to nonlinear electrodynamics (NED). Focusing on the three- and four-horizon configurations, we examine how nonlinear electromagnetic corrections modify the causal structure, radial geodesic motion, and tidal-force profiles in comparison with the Schwarzschild and Reissner-Nordstrom (R-N) spacetimes. Our analysis shows that the NED field gives rise to multiple zero crossings in both the radial and angular tidal-force components, leading to successive transitions between stretching and compressive tidal regimes. More importantly, the radial equation of motion contains classically forbidden regions bounded by bounce-back points. For the class of trajectories considered in this work, these forbidden regions prevent particles from entering the spacetime domain where the tidal forces become divergent. In the super-extremal regime admitted by these solutions, the forbidden region may extend beyond the event horizon, preventing particles released from rest at sufficiently large distances from crossing the horizon. We further identify a systematic ordering of the critical charge values associated with the appearance of tidal-force zero crossings, additional horizons, and bounce-back points. For both the three- and four-horizon configurations, these critical values satisfy a hierarchical ordering, indicating that changes in the tidal-force structure precede the corresponding modifications of the horizon configuration. These results demonstrate that nonlinear electrodynamics can substantially modify the classical dynamics of neutral particles in multi-horizon black hole spacetimes through the combined effects of forbidden regions, multiple tidal transitions, and changes in the horizon structure.

gr-qc↗

arXiv2026

Repetitive Penrose process for charged particles in Kerr-Newman black holes

We investigate the repetitive Penrose process for charged particles in an initially extremal Kerr--Newman black hole and develop a nonlinear iterative framework in which the black-hole mass, angular momentum, electric charge, and irreducible mass are updated after every extraction event. By imposing the triple turning-point condition, we obtain an analytic solution of the conservation equations, allowing the entire extraction sequence to be followed self-consistently. The dynamics are governed by two electromagnetic couplings. The coupling $\hat Q\hat q_0$ determines whether the incident particle can continue to access the ergoregion and therefore controls the termination of the repetitive process, whereas $\hat Q\hat q_1$ governs the depth of the negative-energy states and the extraction efficiency. An attractive interaction ($\hat Q\hat q_1<0$) significantly enhances both the energy return on investment and the energy utilization efficiency and, above a critical charge, produces a transient increase of the dimensionless spin despite the continuous loss of angular momentum. We identify a four-region structure in the captured-particle charge parameter space. Near the critical charge $\hat q_1\simeq-54.85405$, the evolution approaches the reversible Christodoulou--Ruffini limit with the energy utilization efficiency approaching unity while the black hole remains sub-extremal. Beyond this point the irreducible mass decreases, indicating the breakdown of the test-particle approximation. Unlike the repetitive Penrose process in the extremal Reissner--Nordström spacetime, the Kerr--Newman black hole can evolve through the neutral state and reverse the sign of its electric charge without violating the area theorem or cosmic censorship, demonstrating that the discharge barrier found in the Reissner--Nordström case is not a generic property of charged black holes.

gr-qc↗

arXiv2026

Thermodynamic Topology and Photon Spheres Analysis of Black Holes in Brane-World: Insights from Barrow Entropy

We explore the thermodynamics and geothermodynamics of black holes with Barrow entropy in a brane-world scenario, where the horizon geometry of the black hole is regarded as a fractal structure. Our analysis reveals the behavior of heat capacity, identifying both bound and divergence points. For the Bekenstein-Hawking entropy, the divergence point exhibits smooth behavior, indicating no phase transition. In contrast, we observe divergence with Barrow entropy as the deformation parameter increases, confirming the presence of a zero point in heat capacity through various thermodynamic geometry formalisms. Additionally, we delve into thermodynamic topology, detailing the classification of black holes in the brane-world context and comparing their characteristics determined from the Bekenstein-Hawking and the Barrow entropy. Notably, fixing the deformation and cosmological parameters results in a topological charge $-1$ predominately by the dark matter parameter, which remains unaffected despite variations in other parameters. In the dS model, the cosmological horizon prevents stable photon spheres, making topological charges of $0$ and $+1$ unattainable. Incremental increases in the cosmological parameter reduce the dark matter parameter-dominated region.

gr-qc↗

arXiv2026

Holographic CFT Phase Transitions and Criticality for Einstein-Maxwell-Power-Yang-Mills AdS Black Holes

We present a comprehensive study of the thermodynamic phase structure for Anti-de Sitter black holes in Einstein-Maxwell-power-Yang-Mills gravity, reformulated through holographic duality as an ensemble problem in the dual conformal field theory (CFT). By deriving an extended first law where the central charge $C$ is a thermodynamic variable, we systematically explore both canonical and mixed ensembles. In the canonical ensemble with fixed charges, we identify a van der Waals-like phase transition between small and large black holes, marked by a characteristic swallowtail structure and coexistence curves with a negative slope. In contrast, within the mixed ensemble of fixed electric potential, the system exhibits a Hawking-Page transition between confined and deconfined phases of the boundary CFT. Our key finding is the suppressive role of the non-Abelian Yang-Mills charge $\tilde{q}$: increasing $\tilde{q}$ lowers both the minimum and the Hawking-Page transition temperatures, significantly narrowing the stability window of the confined phase. These results, supported by detailed numerical analysis, reveal a rich, ensemble-dependent phase landscape and establish the non-linear Yang-Mills sector as a critical controller of confinement physics in strongly coupled holographic systems.

hep-th↗

arXiv2025

A Deep Dive into classical and Topological CFT Thermodynamics in Lifshitz and Hyperscaling Violating Black Holes

To effectively utilize the AdS/CFT correspondence, a precise set of rules must be established to guide the translation of computed quantities in the gravitational sector into their CFT counterparts, and vice versa. This framework is commonly referred to as the holographic dictionary. The formulation of such dictionaries opens a two-way gateway, allowing researchers to extend theoretical principles and findings from one domain into the other for further exploration and study. The development of a holographic dictionary for Lifshitz black holes and hyperscaling violation (HSV) models \cite{6} has provided an essential foundation for studying CFT thermodynamics and phase behavior of these black holes. Based on this framework, we will investigate their thermodynamic properties using two distinct approaches. In the first step, we adopt the classical and traditional method, identifying critical points to examine the behavior of the free energy function as a function of temperature near the critical boundary. By analyzing its behavior, we will study phase transitions and then proceed to evaluate the stability of the models. In the next step, to compare both methodologies and highlight their equivalence, particularly demonstrating the accessibility of the topological method compared to the classical approach, we will analyze phase behavior through the lens of topological charges.

hep-th↗

arXiv2025

Overcoming Barriers: Kramers' Escape Rate Analysis of Metastable Dynamics in First-Order Multi-Phase Transitions

The expanding application of classical thermodynamic methods to black hole physics has yielded significant advances in characterizing phase transition behavior. Among these approaches, thermodynamic analysis -- particularly kinetic formulations like the Kramers' escape rate -- provides a robust framework for probing black hole phase transitions with minimal relativistic constraints. This study investigates the kinetics and dynamic evolution of first-order phase transitions in black holes exhibiting multiple critical points, employing a particle-based escape rate model. The distinct free energy landscapes inherent to multi-critical systems, which can simultaneously support multiple local minima under specific thermodynamic conditions (temperature and pressure) within a given reference frame, raise fundamental questions regarding transition pathways. We rigorously assess whether the Kramers' escape rate retains its predictive validity in these complex multi-minima systems, as established for conventional single-minimum configurations. Furthermore, we examine whether transitions proceed via a sequential, stepwise mechanism between adjacent minima, or if pathways exist that bypass intermediate states through direct descent to the global minimum. Our analysis of black holes undergoing multiphase transitions reveals both parallels and significant deviations from single-transition models. Crucially, we demonstrate that the Kramers' escape rate remains a quantitatively reliable indicator of first-order phase transitions in black holes, even within multi-critical frameworks. This approach offers deeper insights into the governing energetic landscapes and kinetic processes underlying these phenomena.

gr-qc↗

arXiv2025

Assessing WGC Compatibility in ModMax Black Holes via Photon Spheres Analysis and WCCC Validation

It seems that the regime of Hawking radiation and evaporation ultimately drives charged black holes toward super-extremality of the charge parameter and the dominance of extremal conditions. This progression, in turn, lays the groundwork for satisfying the necessary conditions for the Weak Gravity Conjecture (WGC). Preliminary studies indicate that black holes such as the Reissner-Nordstr$ö$m (RN) model, in their initial form, lack the capacity to sustain super-extremality of the charge parameter. If such conditions arise, these black holes transition into naked singularities-a scenario that is highly undesirable due to the loss of causality and the breakdown of space-time geometry. This raises whether the inability to sustain super-extremality is an inherent property of the model or a consequence of the approximations and precision limitations employed in its construction. To address this, we turned to the ModMax model, which represents an extension of the RN model. Our analysis revealed that the ModMax model not only accommodates super-extremality of the charge parameter but also, under certain conditions, emerges as a promising candidate for investigating the WGC. Furthermore, we independently observed how the inclusion of the de Sitter radius ($\ell$) in the AdS model and $f(R)$ gravitational corrections-both of which enhance and complicate the model-can have a direct impact on the range of super-extremal charge tolerance which, in turn, provides the realization of the conditions necessary for the WGC.

hep-th↗

arXiv2025

Cooling and heating regions of Joule-Thomson expansion for AdS black holes: Einstein-Maxwell-Power-Yang-Mills and Kerr Sen black holes

In this paper, we study the Joule-Thomson Expansion (JTE) process for two types of black holes: AdS-Einstein-Maxwell-Power-Yang-Mills (AEMPYM) and AdS-Kerr-Sen (AKS). Our study focuses on understanding how various parameters influence the Joule-Thomson Coefficient (JTC), the inversion curve, and the ratio of minimum inversion temperature to critical temperature. For the AKS black hole, we observe that the isenthalpic curves can exhibit either cooling or heating behavior. This behavior is determined by the inversion curve, which is affected by the black hole's mass and specific parameters such as $b$ (parameter signifies the ionic charge of the black hole) and $a$ (rotation parameter). In the case of the AEMPYM black hole, our findings reveal that the ratio of minimum inversion temperature to critical temperature approaches a specific value as Maxwell's charge increases. This ratio remains constant for certain parameter values, while it varies for others. Specifically, when the parameter $q$ (real positive parameter of AEMPYM black hole) is greater than 1, the ratio is almost equal to 1/2 as Maxwell's charge (C) increases. When q equals 1/2, the ratio is exactly 1/2 for all values of (C). For values of (q) between 1/2 and 1, the ratio is close to 1/2, and for values of (q) between 0 and 1/2, the ratio decreases, moving away from 1/2. For the AKS black hole, we find that specific parameter values, such as (a = 0.00951) and (b = 0.00475 ), yield a ratio of minimum inversion temperature to a critical temperature that is approximately 1/2. This consistency across different parameter values highlights the robustness of our findings. Finally, we compare our results with those reported in the existing literature, providing a comprehensive summary in detailed tables.

hep-th↗

arXiv2025

Thermodynamic topology of Black Holes in $F(R)$-Euler-Heisenberg gravity's Rainbow

The topology of black hole thermodynamics is a fascinating area of study that explores the connections between thermodynamic properties and topological features of black holes. We successfully derive the field equations for $F(R)$-Euler-Heisenberg theory, providing a framework for studying the interplay between modified gravity and non-linear electromagnetic effects. We obtain an analytical solution for a static, spherically symmetric, energy-dependent black hole with constant scalar curvature. Also, our analysis of black holes in F(R)-Euler-Heisenberg gravity's Rainbow reveals significant insights into their topological properties. We identified the total topological charges by examining the normalized field lines along various free parameters. Our findings indicate that the parameters $( R_0 )$ and $( f_ε = g_ε )$ influence the topological charges. These results are comprehensively summarized in Table I. In examining the photon sphere within this model, the sign of the parameter $ R_0 $ plays a crucial role in determining whether the model adopts a dS or AdS configuration. An interesting characteristic of this model is that, in its AdS form, it avoids the formation of naked singularity regions, which sets it apart from many other models. Typically, varying parameter values in other models can result in the division of space into regions of black holes and naked singularities. However, this model consistently retains its black hole behavior by featuring an unstable photon sphere, regardless of parameter values within the acceptable range. In its dS form, the behavior of the model's photon sphere remains consistent with other dS models and does not exhibit unique differences.

gr-qc↗

arXiv2025

Mechanisms Behind the Aschenbach Effect in Non-Rotating Black Hole Spacetime

General relativity predicts that a rotating black hole drags the spacetime due to its spin. This effect can influence the motion of nearby objects, causing them to either fall into the black hole or orbit around it. In classical Newtonian mechanics, as the radius of the orbit increases, the angular velocity of an object in a stable circular orbit decreases. However, Aschenbach discovered that for a hypothetical non-rotating observer, contrary to usual behavior, the angular velocity increases with radius in certain regions. Although the possibility of observing rare and less probable rotational behaviors in a rotating structure is not unlikely or impossible. However, observing such behaviors in a static structure is not only intriguing but also thought-provoking, as it raises questions about the factors that might play a role in such phenomena. In seeking answers to this question, various static models, particularly in the context of nonlinear fields, were examined, with some results presented as examples in the article. Among the models studied, the model of Magnetic Black Holes in 4D Einstein Gauss Bonnet Massive Gravity Coupled to Nonlinear Electrodynamics (M-EGB-Massive) appears to be a candidate for this phenomenon. In the analysis section, we will discuss the commonalities of this model with previous models that have exhibited this phenomenon and examine the cause of this phenomenon. Finally, we will state whether this phenomenon is observable in other black holes and, if not, why

gr-qc↗

arXiv2025

Thermodynamic Topology of Kiselev-AdS Black Holes within f (R, T) gravity

In this paper, we investigate the topological charge and the conditions for the existence of the photon sphere (PS) in Kiselev-AdS black holes within $f(R, T)$ gravity. We employ two different methods based on Duan's topological current $ϕ$-mapping theory viz analize of temperature and the generalized Helmholtz free energy methods to study the topological classes of our black hole. By considering the mentioned black hole, we discuss the critical and zero points (topological charges and topological numbers) for different parameters. Our findings reveal that the Kiselev parameter $ω$ and the $f(R, T)$ gravity parameter $γ$ influence the number of topological charges of black holes, leading to novel insights into topological classifications. We observe that for given values of the free parameters, there exist total topological charges ($Q_{total} = -1$) for T-method and total topological numbers ($W = +1$) for the generalized Helmholtz free energy method. Our research findings elucidate that, in contrast to the scenario where $ω= 1/3$, in other cases, increasing the parameter $γ$ increases the number of total topological charges for the black hole. Interestingly, for the phantom field ($ω= -4/3$), we observed that decreasing the parameter $γ$ increases the number of topological charges. Additionally, we study the results for the photon sphere. The studied models clearly reveal that the simultaneous presence of $γ$ and $ω$ effectively expands the permissible range for $γ$. In other words, the model can exhibit black hole behavior over a larger domain. Additionally, it is evident that with the stepwise reduction of $ω$, the region covered by singularity also diminishes and becomes more restricted. However, An interesting point about all three ranges is the elimination of the forbidden region in this model.

gr-qc↗