arXiv subjects

Farook Rahaman

Publications and source records attributed to Farook Rahaman.

At least 55 records · Page 3Linked to original sources

arXiv2023

Reconstruction of $f(T,\mathcal{T})$ Lagrangian for various cosmological scenarios

In this paper, we explore a reconstruction scheme in the background of the $f(T,\mathcal{T})$ gravity theory for different cosmological scenarios, where $T$ is the scalar torsion and $\mathcal{T}$ is the trace of the energy-momentum tensor. Using the reconstruction technique $f(T, \mathcal{T})$ Lagrangian is constructed for different cosmological eras such as dust, $ΛCDM$, perfect fluid, etc. Both minimal and non-minimal matter-coupled models are considered for this purpose. Different cosmological scenarios such as power law expansion, de-Sitter expansion, etc. have been considered, and using them Lagrangian functionals are constructed. Mathematical viabilities of all the constructed functionals have been investigated. The physical implications of the obtained solutions are discussed in detail. To check the cosmological compatibility of the constructed $f(T,\mathcal{T})$ functionals we have generated plots of important parameters like the equation of state parameter and deceleration parameter. It is seen that the reconstructed models are perfectly compatible with the late-time accelerated expansion of the universe.

gr-qc↗

arXiv2023

Testing of K(R,T)-gravity through gravastar configurations

In this article, we are reporting for the first time the existence of gravastar configurations in the framework of K(R,T)-gravity, which can be treated as an alternative to a black hole (Mazur and Mottola). This strengthens how much this new gravity theory may be physically demanding to the gravity community in the near future. We first develop the gravastar field equations for a generic K(R,T) functional and then we study four different models within this theory. We find that the solutions for the interior region are regular everywhere regardless of the exact form of the K(R,T) functional. The solutions for the shell region indicate that two of the four models subjected to the study are physically feasible. In addition, the junction conditions are considered at each interface by using the Lanczos equations that yield the surface density and pressure at the thin shell. We investigate various characteristics of the gravastar structure such as the proper length, energy, and entropy of the spherical distribution.

gr-qc↗

arXiv2023

On some properties of M-projective curvature tensor in spacetime of general relativity

In this paper, we investigate the connection between the M-projective curvature tensor and other tensors. Also, we obtain the divergence of M-projective curvature tensor. A symmetry of spacetime known as M-projective collineation has been presented, and it has been possible to determine the conditions under which the general relativity spacetimes can admit such collineations.

gr-qc↗

arXiv2022

Possible existence of stable compact stars in $κ(\mathcal{R},\mathcal{T})-$gravity

We present the first interior solutions representing compact stars in $κ(\mathcal{R},\mathcal{T})$ gravity, by solving the modified field equations in isotropic coordinates. Further, we have assumed the metric potentials in Schwarzschild's form and a few parameters along with the isotropic condition of pressure. For solving, we use specific choice of the running gravitational constant as $κ(\mathcal{R},\mathcal{T})=8π-λ\mathcal{T} ~~(G=\tilde{c}=1)$. Once arrived at the reduced field equations, we investigate two solutions with $c=1$ and $c \neq 1$, where $c$ denotes here another constant that should not be confused with the speed of light. Then, we investigate each solution by determining the thermodynamics variable {\it viz} pressure, density, speed of sound, and adiabatic index. We found that these solutions satisfy the Bondi criterion, causality condition, and energy conditions. We also found that the $M-R$ curves generated from these solutions satisfy the stringent constraints provided by the gravitational wave observations due to the neutron star merger GW 170817.

gr-qc↗

arXiv2022

Wormholes in $κ(R,T)$ gravity

The wormhole solution could be found by solving the Einstein field equations with violating the null energy condition (NEC). We represent wormhole solutions in $κ(R,T)$ gravity in two different ways. At first, we find the shape function by considering a redshift function and linear equation of state (EoS). The solution represents a wormhole for the real feasible matter. In the second part, we consider four pairs of two redshift functions and two shape functions and analyze the obtained solutions. Some of the models suggest that for particular values of the parameters, the existence of wormholes are supported by an arbitrarily small quantity of exotic matter.

gr-qc↗

arXiv2022

Quasi Normal Modes of Ayon-Beato Garcia Regular Black Holes For Scalar Field

In this paper, we compute quasi-normal modes of ABG black holes (which has a non-linear electrodynamical source) using the WKB methods and AIM. A comparison between the spectrum of QNMs calculated by both methods is made. We analyse how the spectrum of QNMs depends on the black hole parameters, multipole number and overtone number and establish that the ABG black hole is stable against the scalar field.

gr-qc↗

arXiv2022

Physical Implications of Pure Lovelock Geometry on Stellar Structure

We construct an exact anisotropic star model with a linear barotropic equation of state and with Finch-Skea potential within the framework of pure Lovelock gravity. A comparison with the corresponding Einstein model in a suitable limit is easily deduced. Evidently higher curvature effects induced by the Lovelock contributions generate lower densities, pressures, surface tensions and anisotropy factors when compared to its Einstein counterpart. The maximum moment of inertia is attained for the Einstein case and hence it may be inferred that Lovelock effects soften the equation of state. The model satisfies various stability tests.

gr-qc↗

arXiv2022

Geometrical Origin of Growth of Matter Perturbations

The density perturbation of the universe has been considered in the framework of a Finslerian cosmological model in which the background spacetime of the universe is taken as the spatially flat FLRW spacetime with a Finslerian perturbation. The inhomogeneity in the matter(energy) density arises naturally in this consideration of gravity in the $( α, β)$ - Finsler space for the background spacetime of the universe. The resulting inhomogeneous matter density indicates matter perturbation departing from the uniform density, and it is caused by the Finslerian perturbation in the Riemannian spacetime, thus ascertaining the geometrical origin of the growth of matter perturbation.

gr-qc↗

arXiv2022

A note on shock wave in the dark matter medium

Employing Newton's stellar balance equation and using the flat rotation velocity of satellite galaxies, we have found the velocity of sound in the dark matter medium. What is interesting is that the velocity of satellite galaxies is very much larger than the velocity of sound in the dark matter medium. This indicates that there will be shock wave in the dark matter medium due to the supersonic movement of satellite galaxies.

gr-qc↗

arXiv2021

Wormhole geometries in $f(Q)$ gravity and the energy conditions

Following the recent theory of $f(Q)$ gravity, we continue to investigate the possible existence of wormhole geometries, where $Q$ is the non-metricity scalar. Recently, the non-metricity scalar and the corresponding field equations have been studied for some spherically symmetric configurations in [ Phys. Lett. B 821, 136612 (2021) and Phys. Rev. D 103, 124001 (2021) ]. One can note that field equations are different in these two studies. Following [Phys. Rev. D 103, 124001 (2021)], we systematically study the field equations for wormhole solutions and found the violation of null energy conditions in the throat neighborhood. More specifically, considering specific choices for the $f(Q)$ form and for constant redshift with different shape functions, we present a class of solutions for static and spherically symmetric wormholes. Our survey indicates that wormhole solutions could not exist for specific form function $f(Q)= Q+ αQ^2$. To summarize, exact wormhole models can be constructed with violation of the null energy condition throughout the spacetime while being $ρ\geq 0$ and vice versa.

gr-qc↗

arXiv2021

A Reissner-Nordström$+Λ$ black hole in the Friedman-Robertson-Walker universe

A charged, non-rotating, spherically symmetric black hole which has cosmological constant $Λ$ (Reissner-Nordström+$Λ$ or RN+$Λ$), active gravitational mass $M$ and electric charge $Q$ is studied with exterior Friedman-Robertson-Walker (FRW) universe in (2+1) dimensional spacetime. We find a new classes of exact solutions of the charged black hole where the generalized Birkoff's theorem is assumed to be valid. It is found that the cosmological constant is negative inside the black hole. We confirm it from the geodesic equations too. The cosmological constant is found to be dependent on $R, Q$ and $a(v)$ which correspond to the areal radius, charge, of the black hole and the scale factor of the universe respectively. We note that the expansion of the universe affects the size and the mass of the black hole. An important observation is that, for an observer at infinity, both the mass and charge of black hole increase with the contraction of the universe and decrease with the expansion of the universe.

physics.gen-ph↗

arXiv2021

Shadows of Lorentzian traversable wormholes

The prospect of identifying wormholes by investigating the shadows of wormholes constitute a foremost source of insight into the evolution of compact objects and it is one of the essential problems in contemporary astrophysics. The nature of the compact objects (wormholes) plays a crucial role on shadow effect, which actually arises during the strong gravitational lensing. Current Event Horizon Telescope observations have inspired scientists to study and to construct the shadow images of the wormholes. In this work, we explore the shadow cast by a certain class of rotating wormhole. To search this, we first compose the null geodesics and study the effects of the parameters on the photon orbit. We have exposed the form and size of the wormhole shadow and have found that it is slanted as well as can be altered depending on the different parameters present in the wormhole spacetime. We also constrain the size and the spin of the wormhole using the results from M87* observation, by investigating the average diameter of the wormhole as well as deviation from circularity with respect to the wormhole throat size. In a future observation, this type of study may help to indicate the presence of a wormhole in a galactic region.

gr-qc↗

arXiv2021

Thin accretion disks around traversable wormholes

In this paper, we aim to investigate various physical properties and characteristics of radiation emerging from the surface of accretion disks, in a rotating traversable axially symmetric wormhole spacetime of the Teo class. We have studied the marginally stable orbits and accretion efficiency graphically, corresponding to different values of dimensionless spin parameter $J/M^2$ ranging from 0.2 to 1.5 and some values of the throat radius $r_0$, in comparison to the Kerr black hole with the same parameter values, and also tabulated the results. The energy flux radiated by the accretion disk $F(r)$, the temperature distribution $T(r)$ and the emission spectra $νL(ν)$ is plotted, corresponding to varying values of the dimensionless spin parameter $J/M^2$ and throat radius $r_0$. Also, the critical frequency at which the luminosity attains its maximum value, for various values of the angular momentum of the wormhole $J/M^2$ and $r_0$ is tabulated. Lastly, we have employed ray-tracing technique, to produce the intensity map of the image of an accretion disk, as observed by an asymptotic observer, under two conditions: firstly when the disk is on the same side as the observer and we have also compared those with the images of an accretion disk in case of Kerr black hole with same parameters. Secondly, the images have been provided when the disk and observer are on opposite sides of the throat. This study may help to detect and distinguish wormhole geometries from other compact objects.

gr-qc↗

arXiv2021

Quark stars in 4-dimensional Einstein-Gauss-Bonnet gravity

The article explores the first exact solution in a four-dimensional EGB-gravity with an anisotropic matter source. The solution has been found by two assumptions, a metric potential and the MIT-bag equation of state. Further, the solution has been tested with several physical constraints. Finally, using the boundary condition we have also estimated the range of bag constant $\mathcal{B}$ by varying the mass of the structure and the Gauss-Bonnet coupling constant $α$. This allows us to check the physical viability of the solution by comparing it with the existing accepted range of bag constant $\mathcal{B}$.

gr-qc↗

arXiv2021

Null Geodesics and QNMs in the field of Regular Black Holes

We analyze the null geodesics of regular black holes. A detailed analysis of geodesic structure both null geodesics and time-like geodesics have been investigated for the said black hole. As an application of null geodecics, we calculate the radius of photon sphere and gravitational bending of light. We also study the shadow of the black hole spacetime. Moreover, we determine the relation between radius of photon sphere~$(r_{ps})$ and the shadow observed by a distance observer. Furthermore, We discus the effect of various parameters on the radius of shadow $R_s$. Also we compute the angle of deflection for the photons as a physical application of null-circular geodesics. We find the relation between null geodesics and quasinormal modes frequency in the eikonal approximation by computing the Lyapunov exponent. It is also shown that~(in the eikonal limit) the quasinormal modes~(QNMs) of black holes are governed by the parameter of null-circular geodesics. The real part of QNMs frequency determines the angular frequency whereas the imaginary part determines the instability time scale of the circular orbit. Next we study the massless scalar perturbations and analyze the effective potential graphically. Massive scalar perturbations also discussed. As an application of time-like geodesics we compute the innermost stable circular orbit~(ISCO) and marginally bound circular orbit~(MBCO) of the regular BHs which are closely related to the black hole accretion disk theory. In the appendix, we calculate the relation between angular frequency and Lyapunov exponent for null-circular geodesics.

gr-qc↗

arXiv2021

Quark matter supported wormhole in third order Lovelock gravity

It is generally believed that wormholes are supported by exotic matter violating Null Energy Condition (NEC). However, various studies of wormhole geometries under Lovelock theories of gravity have reported existence of wormhole supported by matter satisfying NEC. Being inspired by these results, we explore the possibility of the existence of wormhole supported by normal quark matter in third order Lovelock gravity theory. Well known MIT Bag Model Equation of state is chosen for describing the quark matter. Taking physically acceptable approximations, we solve the field equations for shape function which satisfies flare out condition. The residual of the approximate solution is studied for accuracy and found to be acceptable.

gr-qc↗

arXiv2021

Lyapunov exponent ISCO and Kolmogorov Senai entropy for Kerr Kiselev black hole

Geodesic motion has significant characteristics of space-time. We calculate the principle Lyapunov exponent (LE), which is the inverse of the instability timescale associated with this geodesics and Kolmogorov-Senai (KS) entropy for our rotating Kerr-Kiselev (KK) black hole. We have investigate the existence of stable/unstable equatorial circular orbits via LE and KS entropy for time-like and null circular geodesics. We have shown that both LE and KS entropy can be written in terms of the radial equation of innermost stable circular orbit (ISCO) for time-like circular orbit. Also, we computed the equation marginally bound circular orbit, which gives the radius (smallest real root) of marginally bound circular orbit (MBCO). We found that the null circular geodesics has larger angular frequency than time-like circular geodesics ($Q_o > Q_σ$). Thus, null-circular geodesics provides the fastest way to circulate KK black holes. Further, it is also to be noted that null circular geodesics has shortest orbital period $(T_{photon}< T_{ISCO})$ among the all possible circular geodesics. Even null circular geodesics traverses fastest than any stable time-like circular geodesics other than the ISCO.

gr-qc↗

arXiv2021

Color-flavor locked quark stars in energy-momentum squared gravity

Several attempts have been made in the past decades to search for the true ground state of the dense matter at sufficiently large densities and low temperatures via compact astrophysical objects. Focusing on strange stars, we derive the hydrostatic equilibrium assuming a maximally symmetric phase of homogeneous superconducting quark matter called the \textit{color-flavor-locked} (CFL) phase in the background of energy-momentum squared gravity (EMSG). Theoretical and experimental investigations show that strange quark matter (SQM) in a CFL state can be the true ground state of hadronic matter at least for asymptotic densities, and even if the unequal quark masses. Motivated by these theoretical models, we explore the structure of stellar objects in recently proposed EMSG, which allows a correction term $T_{μν}T^{μν}$ in the action functional of the theory. Interestingly, EMSG may be effective to resolve the problems at high energy densities, e.g., relevant to the early universe and dense compact astrophysical objects without invoking some new forms of fluid stress, such as bulk viscosity or scalar fields. Finally, we solve the complicated field equations numerically to obtain the mass-radius relations for strange stars in CFL equation of state.

gr-qc↗