arXiv subjects

M. Yousaf

Publications and source records attributed to M. Yousaf.

13 recordsLinked to original sources

arXiv2026

Study of Dynamical Instability of Collapsing Charged Spherically Symmetric Anisotropic Matter Configurations within Non-Minimally Coupled Gravity

We investigate the dynamical instability and gravitational collapse of charged, spherically symmetric anisotropic matter configurations within $f(R,\mathcal{L}_{m})$ gravity. The analysis focuses on the combined effects of anisotropic pressure, perturbations, electric charge, and modified-gravity source terms on the stability of compact objects. A specific equation of state is adopted to relate the static and perturbed variables through the adiabatic index. Using a perturbation scheme, we derive the modified hydrostatic equilibrium and collapse equations and obtain instability constraints in both Newtonian and post-Newtonian regimes. The results show that the stability of the system is governed by the competition between inward gravitational attraction and outward pressure support. In addition, the dark source terms generated by the non-minimal matter-geometry coupling modify the collapse conditions and can enhance the stability of the charged fluid. These findings provide a useful framework for understanding the evolution and collapse of dense self-gravitating compact objects in non-minimally coupled gravity.

gr-qc↗

arXiv2026

Lorentz Symmetry Breaking Traversable Wormhole Models Supported by Einasto Dark Matter

We investigate static, spherically symmetric traversable wormholes in Kalb Ramond gravity, where spontaneous Lorentz symmetry breaking arises from a non-vanishing vacuum expectation value of the antisymmetric Kalb Ramond field. The matter sector is modeled by the Einasto dark matter density profile, yielding an analytical shape function expressed via the incomplete Gamma function. The resulting geometry satisfies the throat, flare-out, and asymptotic-flatness requirements, with embedding diagrams providing a geometric visualization of the wormhole structure. The energy density stays positive throughout the considered domain, while the radial null energy condition is violated near the throat, showing that the exotic matter required for traversability can be localized to a restricted region. The internal structure is further characterized through the complexity factor, which is most pronounced near the throat and gradually decays to zero at larger radii. We also examine the total gravitational energy, active gravitational mass, average pressure, and equilibrium behavior via the generalized TOV equation and pressure anisotropy; for the constant-redshift configuration, equilibrium is maintained by a balance between the hydrostatic and anisotropic forces. The optical properties of the spacetime are further explored through the photon sphere, critical impact parameter, light deflection angle, and echo time. Finally, the volume integral quantifier is used to estimate the total amount of null-energy-condition-violating matter, showing that the exotic contribution remains concentrated near the wormhole throat. The analysis highlights the combined role of the Kalb Ramond gravitational parameter and the Einasto matter distribution in shaping the geometric, energetic, and observational characteristics of the resulting wormhole configurations.

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 Poincar\'e sections, Lyapunov exponents, Kolmogorov-Sinai (KS) entropy and weighted Birkhoff averages (WBA). In particular, we examine the effect of quantum-improved parameter $(\tilde{\omega})$ and PFDM parameter $(\zeta)$ on the null geodesic motion hence stability of circular orbit. Poincar\'e 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

Beyond Classical Instability Limits of Anisotropic Self-gravitating Fluid Configurations in Hu-Sawicki Inspired f(R) Gravity

In this draft, we investigate the dynamical instability of a restricted class of non-static, axially symmetric, self-gravitating fluid configurations within a Hu-Sawicki inspired f(R) gravity model. The matter source is described by an anisotropic energy-momentum tensor containing three principal stresses and an off-diagonal stress component. For the adopted vorticity free geometry, conservation equations are formulated, and a linear perturbation scheme is applied to separate the equilibrium and time-dependent sectors. This procedure yields a collapse equation that governs the evolution of the perturbed compact configuration. The associated instability conditions are then derived in terms of the adiabatic index {\Gamma} under the Newtonian and post-Newtonian approximations, whereas the resulting bounds show that the onset of instability depends not only on the stiffness of the fluid, but also on the background energy density, directional pressure anisotropies, metric perturbations, and higher-curvature contributions generated by the Hu-Sawicki model. The general relativistic limit is recovered by suppressing the modified gravity parameters, while the isotropic limit reproduces the classical Chandrasekhar threshold. These results demonstrate that curvature corrections and anisotropic stresses can appreciably modify the conventional instability conditions of axially symmetric compact systems.

gr-qc↗

arXiv2026

Equatorial Periodic Orbits and Gravitational Wave Phenomenology around Spherically-symmetric vacuum solution in Freund-Nambu scalar-tensor gravity

We investigate test particle dynamics and gravitational wave (GW) phenomenology in an exact spherically symmetric vacuum solution of Freund - Nambu scalar - tensor gravity. This framework generalizes the Janis - Newman - Winicour (JNW) naked singularity via a geometric non - linear coupling $q$ and a direct scalar - particle coupling $g_s$. We demonstrate that these parameters systematically modify the Innermost Stable Circular Orbit (ISCO) - which shifts inward for $g_s > 0$ - and the Marginally Bound Orbit (MBO). Furthermore, we classify bound periodic trajectories to isolate extreme zoom - whirl orbits exhibiting intense periapsis precession. By applying the Numerical Kludge method to Extreme Mass - Ratio Inspirals (EMRIs), we reveal that scalar - tensor corrections induce a macroscopic temporal dephasing in high - frequency GW bursts, even when the orbit's spatial topology is preserved. These unique phase shifts offer a robust diagnostic signature for future space-based observatories like LISA to probe the strong - field regime and constrain scalar - tensor extensions of general relativity.

gr-qc↗

arXiv2026

QPO-like Signatures and Hydrodynamical Variability in Accretion around a JNW-type Compact Spacetime in Freund-Nambu Scalar-Tensor Gravity

Scalar tensor theories of gravity provide a broad as well as physically rich extension of general theory of relativity by allowing the gravitational interaction to be mediated not only by the spacetime metric but also by scalar degrees of freedom. In this manuscript, we present a new exact solution in the Freund-Nambu scalar-tensor (FNST) gravity scenario, representing a nontrivial scalar-tensor generalization of the Janis-Newman-Winicour naked-singularity geometry, characterized by an additional coupling parameter q in the scalar sector. We also numerically solve the general relativistic hydrodynamic equations in order to investigate the shock-cone mechanism formed by Bondi-Hoyle-Lyttleton accretion around this compact spacetime on the equatorial plane. We show that stronger scalar-tensor deviations modify the shock-cone morphology, significantly increase the amount of matter accumulated near the central compact object, and enhance the oscillatory behavior of the shock cone. The Lorentzian-like peaks obtained from the numerically computed power spectral density are interpreted as hydrodynamically generated QPO-like modes. These modes are driven by shock cone oscillations and by the compression and rarefaction of the plasma trapped inside the cone. Finally, for a compact object with mass parameter M = 10M_sun, the numerically extracted frequencies are found mainly in the range from a few Hz up to approximately 100 Hz. These frequencies overlap with the QPO ranges reported in stellar-mass black-hole-candidate systems. In particular, the frequencies obtained for the FNST2-FNST4 models fall within the range of timing features reported for the source GRS 1915+105. These results suggest that the exterior hydrodynamical variability of FNST compact spacetimes may provide phenomenological diagnostics of scalar-field-induced deviations from the Schwarzschild reference case.

astro-ph.HE↗

arXiv2026

Disformal Kerr Imprints on BHL Accretion: Shock Morphology, PSD Signatures, and Observational QPO Counterparts

We reveal the effect of the spacetime parameters on the accretion morphology formed through the BHL mechanism around a slowly rotating disformal Kerr black hole. Thus, we investigate the measurable signatures of these parameters on the hydrodynamical morphology and the timing behavior of the accreting flow. It is shown that even weak disformal deviations from the Kerr solution modify the shock-cone structure, enhance the density in the post-shock region, and produce coherent oscillations in the accretion rate. The Kerr model produces coherent peaks at 42.99 Hz and 68.13 Hz, and these frequencies are consistent with the high-frequency QPOs observed from the source GRS 1915+105. In the models where the deviations from the Kerr solution are weak, low-frequency QPOs are produced and found to be coherent. These frequencies also fall within the frequency range observed in Galactic black-hole binaries. On the other hand, the models with large deviations from Kerr can be used to explain observational results that are more irregular, broad-band, and contain multiple peaks. In addition, by using inverse-mass scaling in this work, the numerically calculated frequencies are also compared with observations of intermediate-mass and supermassive black holes. In particular, the disformal black-hole models are found to be consistent with the observational results obtained from the sources M82 X-1, NGC 5408 X-1, and RE J1034+396. This comparison also allows the possible black-hole mass range of observed sources to be inferred from the relation between simulated and observed frequencies. This makes BHL accretion in disformal Kerr geometry a powerful framework for connecting modified-gravity black-hole spacetimes with observable QPO phenomenology.

astro-ph.HE↗

arXiv2026

Accretion flow around Kerr metric in the infra-red limit of asymptotically safe gravity

We investigate accretion disk dynamics and the formation of quasi-periodic oscillations (QPOs) in the infrared limit around Kerr-like black holes in asymptotically safe gravity. Relativistic hydrodynamic solutions of Bondi-Hoyle-Lyttleton (BHL) accretion reveal that quantum corrections significantly modify the structure of the shock cone formed around the black hole. The black hole spin controls the azimuthal asymmetry of the shock cone through frame-dragging effects, whereas the quantum correction parameter effectively reduces the strength of gravitational focusing by modifying the metric coefficients in the strong-field region, resulting in a wider shock opening angle, weaker post-shock compression, and reduced density concentration within the cone. Time-dependent mass accretion rates reveal oscillation modes trapped within the shock cone. The power spectral density (PSD) investigations suggest that these modes naturally generate low-frequency QPOs, whose amplitudes, coherence, and harmonic structure depend on both the spin and the quantum correction parameter. The PSD analyses performed at different radial locations reveal that identical QPO frequencies are obtained in all cases. The numerically detected frequencies result from the excitation of global oscillation modes trapped within the post-shock region. The resulting global modes are found to consist of fundamental frequencies, their associated harmonic overtones, and near-commensurate frequency ratios such as 2:1 and 3:2. Coherent oscillations are enhanced and near-commensurate frequency ratios are produced when moderate rotation and moderate quantum corrections are coupled. Large quantum correction parameters, on the other hand, wash out unique spectral peaks and suppress oscillation amplitudes.

astro-ph.HE↗

arXiv2025

Origin of Quasi-Periodic Oscillations and Accretion Process in X-Ray Binaries around Quantum Lee-Wick Black Hole

In this study, we investigate the accretion dynamics and test particle motion around a non-rotating, spherically symmetric Lee-Wick black hole (BH) to reveal how the model parameters affect orbital stability and the quasi-periodic oscillations (QPOs) observed in X-ray binary systems. The spacetime geometry, characterized by the BH mass and the coupling parameters $S_1$ and $S_2$, includes exponential and oscillatory corrections arising from the Lee-Wick terms. Using the effective potential approach, we derive specific energy, angular momentum, epicyclic frequencies, and the locations of the innermost stable circular orbits (ISCOs) of test particles. In addition to the analytical analysis, we explore the effects of the Lee-Wick spacetime parameters on the shock-cone morphology produced by Bondi-Hoyle-Lyttleton (BHL) accretion. To this end, we perform general relativistic hydrodynamic simulations in two characteristic regimes: Block-1 (weak Lee-Wick regime) and Block-2 (strong Lee-Wick regime). The results show that Block-1 solutions closely resemble the Schwarzschild case, while Block-2 models develop denser and asymmetric shock cones accompanied by stronger QPOs activity, shifting from low-frequency to high-frequency QPOs. These variations yield distinct observational signatures that may be detectable in high-resolution X-ray timing data. Our analytical and numerical findings demonstrate that the Lee-Wick parameters $S_1$ and $S_2$ cause measurable changes in the morphology of the accretion flow and in the frequency ratios near the BH. This suggests that future multi-wavelength observations could provide an important avenue to test higher-derivative gravity theories.

gr-qc↗

arXiv2025

Relativistic accretion process onto rotating black holes in Einstein-Euler-Heisenberg nonlinear electrodynamic gravity

In this study, we uncover the accretion dynamics and oscillatory behavior around rotating black holes within the EEH nonlinear electrodynamic framework by analyzing both the motion of test particles and numerically solving the general relativistic hydrodynamic equations. Using EEH geometry, we compute the structure of circular motion, the effective potential and force, and we evaluate the orbital, radial, and vertical epicyclic frequencies together with the Lense-Thirring and periastron precession rates. Our calculations show that, compared to the Kerr model, the charge parameter $Q$ and the spin parameter $a$ significantly modify the strong gravitational field and shift the characteristic frequencies. We then model the dynamical structure formed by matter accreting toward the EEH black hole through the BHL mechanism, finding that the parameter $Q$ increases the amount of infalling matter and strengthens shock-cone instabilities near the horizon, while farther from the black hole it suppresses accretion and reduces turbulence. Time-series analysis of the accretion rate reveals robust QPOs, whose low-frequency components arise from the precession of the shock cone, while high-frequency components appear as a consequence of strong-field instabilities modified by $Q$ and $a$. A systematic parameter-space exploration identifies the regions where EEH corrections maximize QPO activity, indicating that nonlinear electrodynamics can leave observable imprints on accretion flows and may be testable with QPO and horizon-scale observations.

gr-qc↗

arXiv2025

Structure and Mass-Radius Stability of Charged Compact Objects in Symmetric Teleparallel Euler-Heisenberg Gravity

In this work, we develop a new relativistic model for a charged anisotropic compact star in the framework of modified symmetric teleparallel gravity, namely $f(Q)$-Euler-Heisenberg gravity. By employing the MIT bag model equation of state, we establish a relation between the metric potentials, leading to an exact solution of the field equations for an anisotropic fluid configuration coupled with a non-linear electromagnetic source. The interior spacetime is smoothly matched with the exterior geometry calculated from the theoretical setup of $f(Q)$-Euler-Heisenberg gravity using the Darmois-Israel junction conditions, ensuring the continuity of the metric functions and their derivatives at the stellar boundary. The physical viability of the model is examined through regularity, energy, and causality conditions, all of which are satisfied throughout the stellar interior. The study highlights how the pressure anisotropy, the propagation speeds of sound, and the Tolman-Oppenheimer-Volkoff balance condition are interconnected, showing that the star remains in mechanical equilibrium only when the gravitational, hydrostatic, electric, and anisotropic contributions counterbalance one another appropriately. The dynamical stability of the configuration is further supported by the requirement $\Gamma > \tfrac{4}{3}$ for the adiabatic index, indicating resilience against small radial perturbations. The plots of compactness, surface redshift, and the mass--radius profiles confirm that all physical quantities behave regularly and vary smoothly throughout the stellar interior. We graphically plotted the mass-radius curves.

gr-qc↗

arXiv2022

Junction conditions in perfect fluid $f(\mathcal{G},~T)$ gravitational theory

This manuscript aims to establish the gravitational junction conditions(JCs) for the $f(\mathcal{G},~T)$ gravity. In this gravitational theory, $f$ is an arbitrary function of Gauss-Bonnet invariant $\mathcal{G}$ and the trace of the energy-momentum tensor $T_{\mu\nu}$ i.e., $T$. We start by introducing this gravity theory in its usual geometrical representation and posteriorly obtain a dynamically equivalent scalar-tensor demonstration on which the arbitrary dependence on the generic function $f$ in both $\mathcal G$ and $T$ is exchanged by two scalar fields and scalar potential. We then derive the JCs for matching between two different space-times across a separation hyper-surface $\Sigma$, assuming the matter sector to be described by an isotropic perfect fluid configuration. We take the general approach assuming the possibility of a thin-shell arising at $\Sigma$ between the two space-times. However, our results establish that, for the distribution formalism to be well-defined, thin-shells are not allowed to emerge in the general version of this theory. We thus obtain instead a complete set of JCs for a smooth matching at $\Sigma$ under the same conditions. The same results are then obtained in the scalar-tensor representation of the theory, thus emphasizing the equivalence between these two representations. Our results significantly constrain the possibility of developing models for alternative compact structures supported by thin-shells in $f(\mathcal{G},~T)$ gravity, e.g. gravastars and thin-shell wormholes, but provide a suitable framework for the search of models presenting a smooth matching at their surface, from which perfect fluid stars are possible examples.

gr-qc↗

arXiv2011

Evaluating Impact of Mobility on Wireless Routing Protocols

In this paper, we evaluate, analyze, and compare the impact of mobility on the behavior of three reactive protocols (AODV, DSR, DYMO) and three proactive protocols (DSDV, FSR, OLSR) in multi-hop wireless networks. We take into account throughput, end-to-end delay, and normalized routing load as performance parameters. Based upon the extensive simulation results in NS-2, we rank all of six protocols according to the performance parameters. Besides providing the interesting facts regarding the response of each protocol on varying mobilities and speeds, we also study the trade-offs, the routing protocols have to make. Such as, to achieve throughput, a protocol has to pay some cost in the form of increased end-to-end delay or routing overhead.

cs.NI↗