arXiv subjects

M. Zubair

Publications and source records attributed to M. Zubair.

At least 19 recordsLinked to original sources

arXiv2025

Rotating Einstein-Maxwell-Dilaton Black Hole as a Particle Accelerator

Similar to particle accelerators, black holes also have the ability to accelerate particles, generating significant amounts of energy through particle collisions. In this study, we examine the horizon and spacetime structures of a rotating black hole within the framework of Einstein-Maxwell-Dilaton gravity. Additionally, we extend the analysis to explore particle collisions and energy extraction near this black hole using the Banados-Silk-West mechanism. Our findings reveal that the mass and angular momentum of the colliding particles significantly influence the center of mass energy, more so than the parameters of the black hole itself. Furthermore, we apply the Banados-Silk-West mechanism to massless particles, particularly photons, while disregarding their intrinsic spin in plasma; an aspect that has not been previously explored. The Banados-Silk-West mechanism cannot be directly applied, as the refractive index condition only permits photon propagation, meaning that massive particles in vacuum cannot be included in this study. We derive the propagation conditions for photons and analyze photon collisions by treating them as massive particles in a dispersive medium. The impact of the plasma parameter on the extracted center of mass energy is also examined. Our results show that the plasma parameter has a relatively weak and unchanged effect on the center of mass energy across all cases, indicating that energy losses due to friction within the medium are a contributing factor.

gr-qc↗

arXiv2025

Implications of Complexity Factor on Evolution of New Dynamical and Static Wormholes in $f(R, T)$ Gravity

This study presents new spherically symmetric and dynamical wormhole solutions supported by ordinary matter modeled as an anisotropic fluid, exhibiting a traversable nature. To achieve this goal, we adopt different approaches to obtain both evolving static and genuinely dynamical solutions, such as imposing a viable condition on the Ricci scalar, considering an anisotropic equation of state, and choosing a suitable energy density profile. For each derived shape function, we analyze the corresponding $2D$ and $3D$ embedding diagrams and verify their compatibility with the weak energy condition through density plots. The equilibrium conditions are also explored graphically to assess the stability of the obtained solutions, which are shown to be stable within the analyzed framework. Additionally, we investigate the complexity factor associated with each configuration, examining its dependence on both temporal evolution and the coupling parameter $λ$ of the $f(R,T)$ theory.

gr-qc↗

arXiv2025

Hot Casimir wormholes in Einstein-Gauss-Bonnet gravity

In this work, we explore the thermal effects on Casimir wormholes in the context of higher-dimensional Einstein-Gauss-Bonnet gravity. Motivated by the fundamental role of EGB gravity in describing a wide range of gravitational phenomena, we investigate how thermal fluctuations affect the quantum vacuum energy density associated with the Casimir effect and its impact on the global structure of traversable wormholes. By deriving the shape function from the EGB field equations with thermally corrected Casimir energy, we verify that all necessary conditions for wormhole formation are satisfied, including asymptotic flatness and throat stability. Our results indicate that thermal corrections modify of the wormhole geometry, increasing spatial curvature in the throat region and influencing its traversability. Furthermore, we analyze gravitational Casimir effects and discuss their possible role in modified gravity theories. Expanding on the approach of reference \cite{M. Zubair1, Mushayydha, Mushayydha2}, we adopt here the appropriate formulation for Casimir wormholes in Einstein-Gauss-Bonnet gravity, taking into account the Casimir energy density in higher dimensions. This approach allows us to obtain more accurate results compared to the simplified approximation previously used.

hep-th↗

arXiv2025

Influence of Quantum Correction on Kerr Black Hole in Effective Loop Quantum Gravity via Shadows and EHT Results

Recently, a study on shadow of quantum corrected Schwarzschild black hole in loop quantum gravity appeared in [Ye et al., Phys. Lett. B 851, 138566, (2024)] assuming a fixed value of Barbero-Immirzi parameter $γ$. Following this approach, we considered its rotating counterpart being a quantum corrected Kerr black hole in effective loop quantum gravity and studied its deviation from Kerr black hole for a fixed value of $γ$. We proposed and proved a theorem describing the location of unstable circular null orbits for all such Kerr-like metrics. The deviation between the shadows of the Kerr and quantum corrected Kerr black holes has also been studied, and parameters are constrained by comparison with the EHT results for M87* and Sgr A* to precisely probe the quantity of deviation due to quantum correction. Lastly, we immersed the quantum corrected Kerr black hole in an inhomogeneous plasma and studied its impact on the shadow size. We found that the unstable null orbits for the quantum corrected Kerr black hole are always smaller than the unstable null orbits for Kerr black hole. The effect of Barbero-Immirzi parameter allows the quantum corrected Kerr black hole to mimic Sgr A* with a higher probability than the Kerr black hole. However, the quantum corrected Kerr black hole does not mimic M87*. The plasma reduces the size of the shadow of quantum corrected black hole, and the plasma parameter in the case II is more sensitive than that in case I.

gr-qc↗

arXiv2024

Some optical properties of rotating wormhole in Bopp-Podolsky electrodynamics

In this work, we consider a static wormhole in Bopp-Podolsky electrodynamics and convert it into its rotating counterpart by reducing it into Morris-Thorne form. We further study the null geodesics and effective potential along with the shadows for inner and outer unstable orbits for specific choices of parameters. It is found that for some cases smooth shadow curves are formed and for a few cases, the shadows formed are cuspy. All parameters have a significant impact on the shadows except for the parameter $b$ when either $a$ or $Q$ are kept small. We also analyze the gravitational lensing in the strong regime, considering that the observer and the source are on opposite sides of the throat. For this situation, we explore in detail the behavior of the deflection angle, Einstein rings and lensing observables.

gr-qc↗

arXiv2024

Thermodynamics and Perturbative Analysis of Some Newly Developed $\mathcal{F}(R,L_m, T)$ Theories Under the Scenario of Conserved Energy-momentum Tensor

The present work is devoted to explore some interesting cosmological features of a newly proposed theory of gravity namely $\mathcal{F}(R,L_m,T)$ theory, where $R$ and $T$ represent the Ricci scalar and trace of energy momentum-tensor, respectively. Firstly, a non-equilibrium thermodynamical description is considered on the apparent horizon of the Friedmann's cosmos. The Friedmann equations are demonstrated to be equivalent to the first law of thermodynamics, i.e., ${T_{Ah}d\varepsilon_{h}^\prime+T_{Ah}d_{i}\varepsilon_{h}^\prime=-d\hat{E}+\hat{W}dV}$, where ${d_{i}\varepsilon_{h}^\prime}$ refers to entropy production term. We also formulate the constraint for validity of generalized second law of thermodynamics and check it for some simple well-known forms of generic function $\mathcal{F}(R,L_m,T)$. Next, we develop the energy bounds for this framework and constraint the free variables by finding the validity regions for NEC and WEC. Further, we reconstruct some interesting cosmological solutions namely power law, $Λ$CDM and de Sitter models in this theory. The reconstructed solutions are then examined by checking the validity of GSLT and energy bounds. Lastly, we analyze the stability of all reconstructed solutions by introducing suitable perturbations in the field equations. It is concluded that obtained solutions are stable and cosmologically viable.

gr-qc↗

arXiv2024

Influence of plasma on the optical appearance of spinning black hole in Kalb-Ramond gravity and its Existence around M87* and Sgr A*

The visible universe is filled with different types of plasma media in the form of stars, nebulas and other forms of excited gases. These matter fields have a high influence on the gravity and are likely to be present around the black holes due to the immense gravitational attraction. Since a plasma medium affects the speed of light, therefore we investigated the null geodesics and various optical features around the rotating black hole in Kalb-Ramond gravity immersed in plasma medium. Various plasma distributions are considered to develop a comparative study for their influence on unstable null circular orbits, shadows and evaporation rate of the black hole in the presence of a plasma medium. Moreover, the shadow results are also compared with Event Horizon Telescope data for M78* and Sgr A* in order to estimate the parametric bounds for which the rotating black hole in Kalb-Ramond gravity is considered either M87* or Sgr A* under the different values of plasma parameters. From this analysis, we also found the distribution of plasma that has a significant impact on the above mentioned features and is most likely to be present around M87* and Sgr A*.

gr-qc↗

arXiv2023

Rotating Black Hole in Kalb-Ramond Gravity: Constraining Parameters by Comparison with EHT Observations of Sgr A* and M87*

This article deals with the study of some properties of the static and rotating black holes in Kalb-Ramond gravity in four dimensional spacetime. First, we discuss the action of the corresponding theory and the static black hole metric. Then we investigate the light sphere for the static black hole by using the Hamiltonian formalism and the corresponding linear radius of the shadow, angular velocity and Lyapunov exponent. For the rotating black hole, we discuss the horizon structure. Moreover, we study the effective potential to discuss the structure of null sphere and unstable circular null orbits around the rotating black hole. The properties such as energy emission rate and distortion are calculated and analyzed by using the numerical data for the shadows calculated by appropriately chosen parametric values for two different angular locations of the observer off the equatorial plane. We also obtain the constraints on the black hole parameters by comparing the shadow sizes of the black hole in Kalb-Ramond gravity and the supermassive black holes M87* and Sgr A*. Finally, we investigate the effect of mass, energy, angular momentum and the black hole parameters on the center of mass energy of two colliding particles that are accelerated in the vicinity of the black hole.

gr-qc↗

arXiv2023

Influence of interface-induced valley-Zeeman and spin-orbit couplings\\ on transport in graphene-on-WSe$_{2}$ heterostructures

We investigate the electronic dispersion and transport properties of graphene/WSe$_{2}$ heterostructures in the presence of a proximity induced spin-orbit coupling (SOC) using a low-energy Hamiltonian, with different types of symmetry breaking terms, obtained from a four-band, first and second nearest-neighbour tight-binding (TB) one. The competition between different perturbation terms leads to inverted SOC bands. Further, we study the effect of symmetry breaking terms on ac and dc transport by evaluating the corresponding conductivities within linear response theory. The scattering-independent part of the valley-Hall conductivity, as a function of the Fermi energy $E_{F}$, is mostly negative in the ranges $-λ_{R}\leqslant E_{F}$ and $E_{F}\geqslantλ_{R}$ when the strength $λ_{R}$ of the Rashba SOC increases except for a very narrow region around $E_{F}=0$ in which it peaks sharply upward. The scattering-dependent diffusive conductivity increases linearly with electron density, is directly proportional to $λ_{R}$ in the low- and high-density regimes, but weakens for $λ_{R}=0$. We investigate the optical response in the presence of a SOC-tunable band gap for variable $E_{F}$. An interesting feature of this SOC tuning is that it can be used to switch on and off the Drude-type intraband response. Furthermore, the ac conductivity exhibits interband responses due to the Rashba SOC. We also show that the valley-Hall conductivity changes sign when $E_F$ is comparable to $λ_R$ and vanishes at higher values of $E_F$. It also exhibits a strong dependence on temperature and a considerable structure as a function of the frequency.

cond-mat.mes-hall↗

arXiv2023

Valley-controlled transport in graphene/ WSe$_{2}$ heterostructures under an off-resonant polarized light

We investigate the electronic dispersion and transport properties of graphene/WSe$_{2}$ heterostructures in the presence of a proximity-induced spin-orbit coupling $λ_{v}$, sublattice potential $Δ$, and an off-resonant circularly polarized light of frequency $Ω$ that renormalizes $Δ$ to $\barΔ_{ηp} = Δ+ηp Δ_Ω $ with $η$ and $p$ the valley and polarization indices, respectively, and $ Δ_Ω $ the gap due to the off-resonant circularly polarized light. Using a low-energy Hamiltonian we find that the interplay between different perturbation terms leads to inverted spin-orbit coupled bands. At high $Ω$ we study the band structure and dc transport using the Floquet theory and linear response formalism, respectively. We find that the inverted band structure transfers into the direct band one when the off-resonant light is present. The valley-Hall conductivity behaves as an even function of the Fermi energy in the presence and absence of this light. At $Δ_Ω$ = $λ_{v}$ - $Δ$ a transition occurs from the valley-Hall phase to the anomalous Hall phase. In addition, the valley-Hall conductivity switches sign when the polarization of the off-resonant light changes. The valley polarization vanishes for $Δ_Ω$ = 0 but it is finite for $Δ_Ω$ $\neq$ 0 and reflects the lifting of the valley degeneracy of the energy levels, for $Δ_Ω \neq 0$, when the off-resonant light is present. The corresponding spin polarization, present for $Δ_Ω$ = 0, increases for $Δ_Ω$ $\neq$ 0. Further, pure $K$ or $K^{\prime}$ valley polarization is generated when $Δ_Ω$ changes sign. Also, the charge Hall conductivity is finite for $Δ_Ω\neq 0$ and changes sign when the handedness of the light polarization changes.

cond-mat.mes-hall↗

arXiv2023

Bouncing behaviours in four dimensional Einstein Gauss-Bonnet gravity with Cosmography and Observational constraints

This manuscript is based on an investigation of bouncing cosmology in a 4D Einstein Gauss-Bonnet gravity. Various bouncing models such as symmetric bounce, matter bounce, super bounce, and oscillatory bounce have been examined. Expressions for energy density, pressure, equation of state parameter have been derived in the most general manner and then reduced to 4D Einstein Gauss-Bonnet gravity for isotropic, homogenous, FLRW cosmos. Physical interpretation of Hubble and deceleration parameters has also been discussed and plotted for each model from non-vanishing scale factors. Non-singular bouncing models indulge in accelerating late-time cosmic acceleration phenomenon. It has been analysed that the Gauss-Bonnet coupling parameter has a lesser contribution to the dynamics of modified gravity while the bouncing parameter has noticeable effects. We have examined various energy conditions and witnessed the violation of strong and null energy conditions in bouncing models. Analytical expressions for jerk and snap parameters have also been calculated in terms of cosmic time and redshift. We have explored bouncing models through specific cosmographic tests to check their validity. Also, through stability analysis, matter bounce becomes the most stable model by increasing the value of the bouncing parameter. To find best-fit values, bouncing models have been constrained with Hubble data set and $Λ$CDM. We have calculated the values of parameters by applying the least-square fitting method. To make this analysis quantified, we have employed reduced chi-squared method on $H(z)$ data sets for each model.

gr-qc↗

arXiv2023

$4D$ Einstein-Gauss-Bonnet Black Hole in Power-Yang-Mills Field: A Shadow Study

We consider a static black hole immersed in the Power-Yang-Mills field in four dimensional Einstein-Gauss-Bonnet gravity and investigate the effect of various parameters on the radius of the photon sphere. The modified form of the Newman-Janis algorithm is used for obtaining a rotating black hole solution in this gravity. Further, we try to explore the influence of the Yang-Mills magnetic charge $Q$ with power $q$, Gauss-Bonnet parameter $α$ and spin $a$ on the horizon radius. The geodesic equations are constructed by incorporating the Hamilton-Jacobi formalism. The radial component of the geodesic equations gives the effective potential which is further used in deriving the mathematical structure for the shadows by using Bardeen's procedure for a fixed observer at infinity. The shadows are calculated and plotted in terms of two celestial coordinates for an equatorial observer. It is observed that all the parameters have a very significant effect on the shadow and related physical observables. {We also obtain the constraint values for the spin, magnetic charge and Gauss-Bonnet parameters, using the shadow size of supermassive black holes Sagittarius A$^*$ and M$87$* from the EHT observations for the cases of $q=0.6$ and $0.9$. It is shown that there are upper and lower bounds for the charge and spin of M$87$* at $q=0.6$, while only the upper bounds for charge and spin of Sagittarius A$^*$. Finally, we investigate the energy emission rate in the Hawking radiation around the $4D$ Einstein-Gauss-Bonnet black hole in the Power-Yang-Mills field.}

gr-qc↗

arXiv2022

Optical Features of Rotating Black Hole with Nonlinear Electrodynamics

In this article, we considered the strong field approximation of nonlinear electrodynamics black hole and constructed its rotating counterpart by applying the modified Newman-Janis algorithm. The corresponding metric function in the strong field limit of the static black hole is identified in order to study the radius of photon sphere. However, the metric function for the rotating counterpart in the strong field limit is considered in order to study the horizon radius w.r.t spin parameter. We considered the Hamilton-Jacobi method to derive the geodesic equations for photon and constructed an orthonormal tetrad for deriving the equations for celestial coordinates in the observer's sky. Shadows, distortions and energy emission rates are investigated and the results are compared for different values of nonlinear electrodynamics parameter, charge and spin. It is found that the presence of the nonlinear electrodynamics parameter affects the shape and size of the shadows and thus the distortion in the case of rotation. It is also found that the nonlinearity of electrodynamics diminishes the flatness in the shadow due to the effect of spin and other parameters.

gr-qc↗

arXiv2022

Energy Constraints for Evolving Spherical and Hyperbolic Wormholes in $f(R,T)$ Gravity

The primary objective of this article is to study the energy condition bounds for spherical and hyperbolic wormholes in well-known $f(R,T)$ theory of gravity. For this purpose, we formulate the field equations for spherically and pseudospherically geometries using anisotropic matter and linear form of generic function $f(R,T)$. By imposing different conditions on radial and tangential pressures or by adopting some known choices for red shift and shape functions, we present the graphical analysis of energy conditions for both spherically and pseudospherically symmetric wormholes. It is seen that energy density for spherically symmetric wormhole is always positive for $λ>-4π$ and $λ<-8π$, while the energy conditions for radial pressure are negative at throat. Likewise, in case of pseudospherically symmetric wormhole, it is observed that energy density is always positive for negative $λ$, however conditions based on radial pressure may be positive or negative for the considered different cases.

gr-qc↗

arXiv2022

Complexity analysis of Cylindrically Symmetric Self-gravitating Dynamical System in $f(R,T)$ Theory of Gravity

In this article, we have studied a cylindrically symmetric self-gravitating dynamical object via complexity factor which is obtained through orthogonal splitting of Reimann tensor in $f(R,T)$ theory of gravity. Our study is based on the definition of complexity for dynamical sources, proposed by Herrera \cite{12b}. We actually want to analyze the behavior of complexity factor for cylindrically symmetric dynamical source in modified theory. For this, we define the scalar functions through orthogonal splitting of Reimann tensor in $f(R,T)$ gravity and work out structure scalars for cylindrical geometry. We evaluated the complexity of the structure and also analyzed the complexity of the evolutionary patterns of the system under consideration. In order to present simplest mode of evolution, we explored homologous condition and homogeneous expansion condition in $f(R,T)$ gravity and discussed dynamics and kinematics in the background of a generic viable non-minimally coupled $f(R,T)=α_1 R^m T^n +α_2 T(1+α_3 T^p R^q)$ model. In order to make a comprehensive analysis, we considered three different cases (representing both minimal and non-minimal coupling) of the model under consideration and found that complexity of a system is increased in the presence of higher order curvature terms, even in the simplest modes of evolution. However, higher order trace terms affects the complexity of the system but they are not crucial for simplest modes of evolution in the case of minimal coupling. The stability of vanishing of complexity factor has also been discussed.

gr-qc↗

arXiv2021

Tolman IV fluid sphere in f(R, T) gravity

In this article, we studied the behavior of relativistic spherical objects considering Tolman IV spacetime in modified $f(R,\,T)$ gravity for the uncharged perfect fluid matter. We have chosen the matter Lagrangian as $\mathcal{L}_m=-p$ to develop our present model. In particular, for this investigation we have reported for the compact object LMC $X-4$ [Mass=$(1.04 \pm 0.09)M_{\odot}$; Radius= $8.301_{-0.2}^{+0.2}$ Km] in our paper. The effect of the coupling parameter $β$ on the local matter distribution of compact stars has been investigated in this paper. It can be seen that with greater values of $β$, the sound speed and adiabatic index are higher. On contrary, the mass function takes lower value for higher values of $β$. Our obtained solution does not admit singularities in the matter density, pressure and metric functions. According to our graphical analysis, this new stellar model satisfies all physical requirements anticipated in a realistic star.

gr-qc↗

arXiv2021

Reconstruction and Stability Analysis of Some Cosmological Bouncing Solutions in $F(\mathcal{R},T)$ Theory

The present article investigates the possibility of reconstruction of the generic function in $F(\mathcal{R},T)$ gravitational theory by considering some well-known cosmological bouncing models namely exponential evaluation, oscillatory, power law and matter bounce model, where $\mathcal{R}$ and $T$ are Ricci scalar and trace of energy-momentum tensor, respectively. Due to the complexity of dynamical field equations, we propose some ansatz forms of function $F(\mathcal{R},T)$ in perspective models and examine that which type of Lagrangian is capable to reproduce bouncing solution via analytical expression. It is seen that for some cases of exponential, oscillatory and matter bounce models, it is possible to get analytical solution while in other cases, it is not possible to achieve exact solutions so only complementary solutions can be discussed. However, for power law model, all forms of generic function can be reconstructed analytically. Further, we analyze the energy conditions and stability of these reconstructed cosmological bouncing models which have analytical forms. It is found that these models are stable for linear forms of Lagrangian only but the reconstructed solutions for power law are unstable for some non-linear forms of Lagrangian.

gr-qc↗

arXiv2021

Co-deformation Between the Metallic Matrix and Intermetallic Phases in a Creep-Resistant Mg-3.68Al-3.8Ca Alloy

The microstructure of Mg-Al-Ca alloys consists of a hard intra- and intergranular eutectic Laves phase network embedded in a soft $α$-Mg matrix. For such heterogeneous microstructures, the mechanical response and co-deformation of both phases under external load are not yet fully understood. We therefore used nano- and microindentation in combination with electron microscopy to study the deformation behaviour of an Mg-3.68Al-3.8Ca alloy. We found that the hardness of the Mg$_2$Ca phase was significantly larger than the $α$-Mg phase and stays constant within the measured temperature range. The strain rate sensitivity of the softer $α$-Mg phase and of the interfaces increased while activation volume decreased with temperature. The creep deformation of the Mg$_2$Ca Laves phase was significantly lower than the $α$-Mg phase at 170 $^{\circ}$C. Moreover, the deformation zone around and below microindents depends on the matrix orientation and is influenced by the presence of Laves phases. Most importantly, slip transfer from the $α$-Mg phase to the (Mg,Al)$_2$Ca Laves phase occurred, carried by the basal planes. Based on the observed orientation relationship and active slip systems, a slip transfer mechanism from the soft $α$-Mg phase to the hard Laves phase is proposed. Further, we present implications for future alloy design strategies.

cond-mat.mtrl-sci↗