arXiv subjects

Farook Rahaman

Publications and source records attributed to Farook Rahaman.

At least 37 records · Page 2Linked to original sources

arXiv2025

Godel Universe in $f(Q,T)$ gravity: Exploring causality violation and closed time-like curves

In this work, the classical Godel solution from general relativity is extended into the framework of modified gravity theories based on non-metricity $Q$ and the trace of the energy-momentum tensor $T$ in the context of $f(Q,T)$ gravity. The main feature of the Godel solution is the existence of closed time-like curves, which allow for causality violation and time travel. Since general relativity and its extensions do not demand spacetime to be globally causal, there is good motivation to explore such solutions. We have found classes of solutions with different matter content, like perfect fluid, cosmological constant, massless scalar field, etc. It is observed that, for suitable initial conditions, there is always a possibility of obtaining feasible solutions that violate causality in our setup. The presence of non-metricity in such solutions produces crucial deviations that are noteworthy.

gr-qc↗

arXiv2025

Deflection Angle of Regular Black Holes in Nonlinear Electrodynamics: Gauss-Bonnet Theorem, Time Delay, Shadow, and Greybody Bound

In this article, we study the weak gravitational lensing in the background of regular, static, spherically symmetric black hole solutions of Einstein's standard general relativity coupled with nonlinear electrodynamics. The weak deflection angles are estimated in the context of vacuum medium and plasma medium using the Gauss-Bonnet method. The obtained deflection angles decrease as the impact parameter $b$ and the charge parameter $q$ increase, while the deflection angles increase gradually with increasing values of the black hole mass $m$. Moreover, the effect of a plasma medium has increased the deflection angle than the vacuum medium scenario. We also estimate the time delay in the field of the described black holes that vanishes for $m = q =0$, i.e., the absence of the black holes. The shadow cast of the present black holes is also analyzed with respect to the impact $q$ and mass $m$, which ensures that the shadow region shrinks for increasing values of $q$ and expands for increasing values of $m$. In addition, we estimated the rigorous bounds of the greybody factor $\mathcal{T}_b$ for the described black holes and the graphical analysis ensures that the increasing charge parameter $q$ decreases the rigorous bound of $\mathcal{T}_b$ and the increasing mass $m$ increases the rigorous bound of $\mathcal{T}_b$.

gr-qc↗

arXiv2025

Testing the Generalized Second Law in $(2+1)$-Dimensional Cosmology: Holographic Entropy Bounds and Observational Constraints

We investigate the validity of the Generalized Second Law (GSL) of thermodynamics in a $(2+1)$-dimensional holographic cosmological model with a negative cosmological constant. Adopting a horizon thermodynamics framework, we examine two prominent entropy bounds, the Fischler--Susskind (FS) bound and the Hubble Entropy (HE) bound, in both expanding and contracting universes, including the effects of quantum entropy corrections. Our theoretical analysis shows that the FS bound is intrinsically incompatible with the GSL in contracting $(2+1)$-dimensional universes, regardless of spatial curvature or exotic matter content, and that this incompatibility persists even when quantum corrections are considered. In contrast, the HE bound is consistent with the GSL in expanding universes under classical conditions and can also be reconciled in certain contracting scenarios when quantum effects are included. To complement the theoretical study, we perform a Markov Chain Monte Carlo (MCMC) analysis using recent Baryon Acoustic Oscillations (BAO), Cosmic Chronometer (CC), and Hubble parameter datasets to constrain the model parameters. The best-fit results reveal good cross-dataset consistency, with the cosmological constant parameter $ψ$ remaining stable across all probes. These findings identify the HE bound as a more robust candidate for holographic constraints in lower-dimensional cosmology, while demonstrating the limitations of the FS bound. Our results not only clarify the status of the GSL in $(2+1)$-dimensional settings but also provide a framework for testing entropy bounds with future high-precision cosmological data.

gr-qc↗

arXiv2025

Constraints on multi-fluid cosmology in modified Gauss-Bonnet gravity models with different observational data sets

In the present work, we incorporate redshift-space distortion measurement to investigate the growth of large scale structure within the framework of multi-fluid cosmology in the context of modified Gauss-Bonnet gravity. Using three different modified Gauss-Bonnet gravity models, we compare the predictions of modified Gauss-Bonnet gravity expansion history-through the Friedmann equation with Hubble and BAO data sets and constrain models parameters. Within the context of multi-fluid cosmology in modified Gauss-Bonnet gravity, we obtain the structure growth equation. This equation is then combined with Sigma_8 to get f_Sigma_8 predictions-which is compared with redshift-space distortion data to constrain models parameters to obtain best-fit values including Sigma_8. This involves performing a Markov Chain Monte Carlo (MCMC) analysis for these specific forms of modified Gauss-Bonnet models.

gr-qc↗

arXiv2025

Cosmic Hysteresis in Reconstructed $f(R)$ Bounce Models: A Thermodynamic Study

We study the emergence of cosmic hysteresis in cyclic bouncing universes within the framework of analytically reconstructed $f(R)$ gravity. Using exact bouncing scale factor solutions of exponential and power-law forms, we reconstruct the corresponding $f(R)$ models and investigate the thermodynamic behavior of a minimally coupled scalar field in these geometries. The pressure evolution during expansion and contraction phases is shown to be asymmetric, leading to a non-vanishing thermodynamic work integral over each cycle, defined by $\oint p_ϕ\, dV$. We identify closed hysteresis loops in the equation-of-state space and quantify the net energy transfer per cycle. Our results reveal that such reconstructed $f(R)$ models generically support irreversible evolution, demonstrating a natural emergence of the thermodynamic arrow of time. These findings provide new insight into the dissipative features of modified gravity and the long-term dynamics of cyclic cosmological scenarios.

gr-qc↗

arXiv2025

Study on deflection angle, shadow, quasinormal modes, and greybody factor of the black hole surrounded by quintessence in Rastall gravity

The present study focuses on investigating the deflection angle in the weak-field approximation, shadow, quasinormal modes using Lyapunov exponents, and lower bound of the greybody factor for a charged black hole surrounded by a quintessence field in Rastall gravity. The weak deflection angles are calculated using the Gauss-Bonnet method. They decrease with increasing impact parameter b and charge Q, but gradually increase with increasing black hole mass m. Notably, the presence of a surrounding quintessence field in Rastall gravity leads to a higher deflection angle compared to Schwarzschild or Reissner-Nordstrom black holes with positive Nq. The photon sphere and shadow of the black hole are analysed concerning the charge Q and mass m; they shrink as Q increases and expand with increasing m. We further analyse the quasinormal modes of the black hole, explicitly derive the coordinate time Lyapunov exponent λc and the quasinormal frequency ω. In the eikonal limit, the Lyapunov exponent ensures that the real and imaginary parts of the quasinormal modes can be expressed by the frequency and instability time scale of the unstable null circular geodesics. Additionally, we derive the lower bounds of the greybody factor Gb; it decreases for increasing charge Q, while the increasing mass m enhances it. Importantly, all the findings reduce to those of the Reissner-Nordstrom black hole for Nq=0 and to the Schwarzschild black hole for Nq=Q=0.

gr-qc↗

arXiv2025

Cosmological model with Gong-Zong Parametrization in $f(R,L_m)$ gravity

We present the cosmic expansion scenarios in the $f(R, L_m)$ gravity studied by using the dark energy equation of state (EoS) parameters. We proceed with the specific form of $f(R, L_m)$ gravity termed as $f(R, L_m)=\frac{R}{2}+L_{m}^α$. We derive the expansion rate in terms of the red-shift for two different forms of EoS parameter. In first model, EoS parameter varies inversely with the redshift and in second model, it involves the exponential form with the redshift. By using the Bayesian methods based on the $χ^{2}$-minimization technique, the median values of model parameters are determined for the cosmic chronometer(CC) and Joint (CC+Pantheon) data sets. The behavior of fundamental cosmological parameters such as the deceleration parameter, energy density and pressure are thoroughly examined. Additionally, the nature of Statefinder diagnostics and the present age of universe exemplifies the compatibility with the late-time astronomical observations.

gr-qc↗

arXiv2025

Study of Wormhole in $f(Q)$ gravity with some dark energy models

This study discusses the development of some particular static wormhole models in the background of an extended $f(Q)$ gravity theory. Wormhole solutions are derived by considering the radial pressure to admit an equation of state corresponding to Chaplygin gas. The Chaplygin gas equation of state is taken into consideration in two different forms: $p_{r}=-\frac{Bb(r)^{u}}{ρ^{a}}$, $p_{r}=-\frac{B}{ρ^{a}}$. Wormhole models are also generated assuming that a variable barotropic fluid may explain the radial pressure given by $p_{r} =-ωρb(r)^u$. For every model, the shape function $b(r)$ is the function that can be derived from the wormhole metric in any scenario. The stability analysis of the wormhole solutions and the shape function viability for each situation are then investigated.Since each wormhole model is shown to violate the null energy condition (NEC), it can be understood that these wormholes are traversable. More generally, we investigate whether the model is stable under the hydrostatic equilibrium state condition using the TOV equation.The physical characteristics of these models are shown under the same energy circumstances. The typical characteristic is the radial pressure $p_{r}$ near the wormhole throat, which violates the NEC $(ρ+P_{r} \geq0)$. In some models, it is possible to meet the NEC at the neck and yet violate the DEC $(ρ- P_{r}\geq0)$. In summary, precise wormhole models may be generated, provided that $(ρ\geq0)$, and there may be a potential breach of the NEC at the wormhole's throat.

gr-qc↗

arXiv2025

Accretion of Dark Energy Candidates Following Redshift Parametrization Type Equation of State : Horava-Lifshitz Gravity

In this article, accretion of particular dark energy candidates is studied. These dark energy models possess equation of state dependent on redshift and some free parameters. Central gravitating object for this accretion model is chosen to be the Kehagias Sfetsos black hole sitting in Horava Gravity. Four special redshift parametrization models, viz. Chevallier-Polarski-Linder, Jassal-Bagala-Padmanabhan, Barboza-Alcaniz and Barboza-Alcaniz-Zhu-Silva are picked to study different accretion properties. Formation of critical point and related radial infalling speed, sonic speed, different arbitrary parameters' values etc are obtained. Mass growth curves are plotted. The rate of growth is followed to depend on the dark matter's behavior as well as the free parameters of dark energy models. Future loss in mass is noted for some of the chosen dark energy models.

gr-qc↗

arXiv2025

Gravitational lensing due to charged galactic wormhole

We propose the back reaction to the charged galactic wormhole spacetime based on Yoshiaki Sofue's exponential dark matter density profile to find exact solutions. The charges act as an additional component to the static wormhole, which is primarily formed by the galactic dark matter density. Unlike traditional mass-based models, this solution incorporates charge effects within a realistic dark matter distribution, revealing unique interactions between dark matter, electromagnetic fields, and spacetime curvature. This study confirms the criteria for wormhole formation, designating it the "Charged Galactic Wormhole," and offers a new framework for investigating galactic structures, with potential observational signatures that deepen our understanding of dark matter and spacetime. Later, the proper radial distance and the embedding surface were also analyzed. Furthermore, the deflection of light around a charged galactic wormhole was investigated, along with a comprehensive review of the resulting image. The deflection of massive objects (charge less) near charged galactic wormholes is studied using the Gauss-Bonnet and Rindler-Ishak methods, with a detailed comparison of the results from both approaches. Additionally, in both the Rindler-Ishak (RI) and Gauss-Bonnet (GB) methods, when v tends to 1 i.e. when particle's velocity comparable to the speed of light , the results from these approaches converge, producing the same outcome as strong gravitational lensing.

physics.gen-ph↗

arXiv2025

Energy Conditions and Stability of Charged Wormholes in $f(R, \mathscr{L}_m)$ Gravity: A Comparative Analysis with Compact Objects

In this paper, we study the energy conditions of charged traversable wormholes in the framework of $f(R, \mathscr{L}_m)$ modified gravity. In the first case, we derive the shape functions (SFs) for two different choices of the charge function $\mathcal{E}^2$ by considering the Exponential Spheroid (ES) model and analyze the null energy condition (NEC). In the second case, we consider a particular shape function and study its implications for the energy conditions. In both cases, we obtain expressions for energy density and pressure in radial and tangential directions. Our findings show that the radial NEC remains satisfied across a wide range of charge parameters $\mathcal{E}$ consistent with established physical laws. However, the tangential NEC is only sustained in the range $0.1 \leq \mathcal{E} \leq 0.6$; for higher charge values, violations occur, indicating the formation of a throat-like structure necessary for wormhole stability. Additionally, we compare the pressure-density profiles of these charged wormholes with those of compact objects such as neutron stars, revealing distinct variations in matter distribution. This analysis highlights the crucial role of charge and modified gravity in determining the stability and physical characteristics of wormhole structures.

gr-qc↗

arXiv2024

Shadows and Strong Gravitational Lensing Around Black Hole-like Compact Object in Quadratic Gravity

We investigate the astrophysical consequences of black holes in quadratic gravity, characterized by the parameters $S_0$, $S_2$, $m_0$ and $m_2$, in addition to the black hole mass $M$. To evaluate the physical validity of the fundamental quadratic gravity black hole solutions, we analyze their gravitational lensing properties in the strong field regime. Specifically, we examine the shadow cast by the quadratic gravity black hole and constrain its parameters using observational data from the $M87^*$ and $Sgr A^*$ supermassive black holes. Our analysis reveals that, within the $1σ$ confidence level, a significant portion of the parameter space for quadratic gravity black holes is consistent with the Event Horizon Telescope (EHT) observations of $M87^*$ and $Sgr A^*$. This suggests that these black holes are plausible candidates for describing astrophysical black holes. As an additional observational test, we perform a detailed investigation of the strong gravitational lensing properties of these black holes. We explore the fundamental strong lensing observables in detail, including the angular positions and separations of the lensed images, the relative magnifications, the radius of the outermost Einstein ring and the relativistic time delay between images. We compare the predictions of the quadratic gravity black hole for each observable with those of the classical Schwarzschild solution using realistic astrophysical data. Our findings provide a pathway for testing quadratic gravity at the galactic and extragalactic scales, offering new insights into the observational properties of black hole solutions within this framework.

gr-qc↗

arXiv2024

Conservative wormholes in generalized $κ(\mathcal{R},\mathcal{T})$-function

We present an exhaustive study of wormhole configurations in $κ(\mathcal{R},\mathcal{T})$ gravity with linear and non-linear functions. The model assumed Morrison-Thorne spacetime where the redshift and shape functions linked with the matter contain and geometry of the spacetime through non-covariant conservation equation of the stress-energy tensor. The first solution was explored assuming a constant redshift function that leads to a wormhole (WH) which is asymptotically non-flat. The remaining solutions were explored in two cases. Firstly, assuming a linear equation of state $p(r)=ωρ(r)$ along with different forms of $κ(\mathcal{R},\mathcal{T})-$function. This proved enough to derive a shape function of the form $b(r)=r_{0}\left(\frac{r_{0}}{r}\right)^{1/ω}$. Secondly, by assuming specific choices of the shape function consistent with the wormhole configuration requirements. All the solutions fulfill flare-out condition, asymptotically flat and supported by phantom energy. Further, the embedding surface and its revolution has been generated using numerical method to see how the length of the throat is affected of the coupling parameters through $κ(\mathcal{R},\mathcal{T})$ function. At the end, we have also calculated the average null energy condition, which is satisfied by all the WH models signifying minimum exotic matter is required to open the WH throats.

gr-qc↗

arXiv2024

A new type of $f(\mathcal{T})$ gravity from Barrow entropy

In this work, two originally separate adjustments for the Friedmann equations are concurrently considered. Firstly, the fractal structure of the black hole horizon region is imposed by the Barrow entropy. The second adjustment is the $f(\mathcal{T})$ gravity, which is based on a teleparallel framework generalization of the Einstein-Hilbert action, where $\mathcal{T}$ is the scalar torsion. This can be considered as a Barrow entropy modification of the $f(\mathcal{T})$ gravity thus yielding a new model. Gravity thermodynamics hypothesis principles are used to integrate these two models under a unified framework. We derive the modified Friedmann equation and note the corrections obtained. To understand the implications of such dual modifications, an application is analyzed and a particular $f(\mathcal{T})$ toy model is chosen for the purpose. The equation of the state parameter of the resulting model, the dimensionless density parameters of matter and dark energy, and the deceleration parameter are explored to check the viability of the new model. A discussion of the dynamic evolution of the universe follows from these results. It is seen that the results comply with the observations. The newly developed model is promising and demands further study.

gr-qc↗

arXiv2024

Evolving wormhole geometry from dark matter energy density

We analyze traversable wormholes defined by the dynamic line elements that asymptotically approaches Friedmann-Robertson-Walker (FRW) universe. This dynamical wormholes is supported by the galactic dark matter as well as perfect isotropic fluid. We will discuss several evolving Lorentzian wormholes comprising with different perfect isotropic fluids in addition to various scale factors. We will speculate the various significance, features and throat energy conditions for these evolving traversable Lorentzian wormholes.

gr-qc↗

arXiv2024

Finslerian extension of an anisotropic strange star in the domain of modified gravity

In this article, we apply the Finsler spacetime to develop the Einstein field equations in the extension of modified geometry. Following Finsler geometry, which is focused on the tangent bundle with a scalar function, a scalar equation should be the field equation that defines this structure. This spacetime maintains the required causality properties on the generalized Lorentzian metric manifold. The matter field is coupled with the Finsler geometry to produce the complete action. In this work, we use modified gravity to develop the Einstein field equations from the variational principle. Developed Einstein field equations are employed on the strange stellar system to improve the study. The interior of the system is made of a strange quark, maintained by the MIT Bag equation of state. In addition, the modified Tolman-Oppenheimer-Volkov (TOV) equation is formulated. In particular, the anisotropic stress attains the maximum at the surface. The mass-central density variation justifies the stability of the system.

gr-qc↗

arXiv2024

Relativistic isotropic stellar model with Durgapal-V metric in $f({R}, {T})$ gravity

The main aim of this paper is to obtain a completely new relativistic non-singular model for static, spherically symmetric isotropic celestial compact stars in the $f(R, T)$ gravity scenario. In this regard, we have considered the isotropic Durgapal-V metric {\it ansatz} \cite{dur82} to find the solutions of Einstein's field equations in the framework of $f(R, T)$ gravity. The obtained solutions are analyzed graphically for the compact star {\it Cen X}-3 with mass $M$ = $ 1.49 \pm 0.08 ~ M_\odot$ and radius $R$ = 9.178 $\pm$ 0.13 $km$ \cite{ml11} and numerically for ten well-known different compact stars along with {\it Cen X}-3 corresponding to the different values of coupling constant $χ$. The reported solutions are singularity-free at the center of the stars, physically well-behaved, and hold the physically stable matter configurations by satisfying all the energy conditions and EoS parameter $ω(r) \in $ (0, 1), causality condition, adiabatic index $Γ(r) > 4/3$. We have also discussed hydrostatic equilibrium through the modified TOV equation to ensure the equilibrium position of the solutions representing matter distributions. Considering the several values of $χ$ we have examined the impact of this parameter on the proposed solutions that help to make a fruitful comparison of modified $f(R, T)$ gravity to the standard general relativity, and interestingly, we have found that the modified $f(R, T)$ gravity holds long-term stable compact objects than the standard Einstein gravity. All the graphical and numerical results ensure that our reported model is under the physically admissible regime that indicates the acceptability of the model.

gr-qc↗

arXiv2023

Strong Lensing of a Regular Black Hole with an Electrodynamics Source

In this paper we have investigated the gravitational lensing phenomenon in the strong field regime for a regular, charged, static, non-linear black hole having a electrodynamics source. We have obtained the angle of deflection and compared it to a Schwarzschild black hole and Reissner Nordstrom black hole with similar properties. We have also done a graphical study of the relativistic image positions and magnifications. We hope that this method may be useful in the detection of non-luminous bodies like this current black hole.

gr-qc↗