arXiv subjects

Susmita Sarkar

Publications and source records attributed to Susmita Sarkar.

At least 19 recordsLinked to original sources

arXiv2025

Non-exotic traversable wormholes with strong deflection angle in King and Dekel-Zhao dark matter halos under f(R,Lm) gravity

In this article, we investigate asymptotically flat non-exotic traversable wormhole geometries within the King and Dekel-Zhao dark matter halos in the framework of $f(R, L_m)$ gravity. Two functional forms of the theory are considered: Model-I: $f(R, L_m)=(R/2) + L_m^{\alpha}$ and Model-II: $f(R, L_m)=(R/2) + (1 + \lambda R)L_m$. For both models, wormhole solutions are obtained and analyzed using the King and Dekel-Zhao dark matter density profiles, allowing us to explore how the underlying matter distribution influences the wormhole structures. The energy conditions are examined to verify the feasibility of sustaining the wormhole geometries with non-exotic matter, while embedding surfaces, proper radial distance, and total gravitational energy are studied to illustrate the wormhole's physical viability and traversability. Moreover, we test the strong deflection angle and its implications for gravitational lensing and show possible observational signatures of such wormhole configurations. Our results indicate that within $f(R, L_m)$ gravity, and for appropriate parameter choices, dark matter environments can sustain physically consistent non-exotic traversable wormhole geometries with distinct gravitational lensing signatures, providing new insights into the interplay between modified gravity, dark matter, and astrophysical observations.

gr-qc↗

arXiv2025

Yukawa-Casimir wormholes within Einstein-Cartan gravity framework

The Einstein-Cartan (EC) theory of gravity provides a natural extension of general relativity by incorporating spacetime torsion to account for the intrinsic spin of matter. In this work, we investigate Yukawa-Casimir traversable wormholes supported by three distinct Yukawa-Casimir energy density profiles within the framework of EC gravity. The resulting shape functions are shown to satisfy all the fundamental requirements for traversable wormhole geometries. Our analysis reveals that the presence of exotic matter is unavoidable in sustaining these wormholes, and we quantify its total amount through the volume integral quantifier. Furthermore, the equilibrium of the wormhole configurations is established by examining the Tolman-Oppenheimer-Volkoff equation. To enhance the physical relevance of the present work, we study several key features of the wormholes, including the embedding surface, proper radial distance, tidal forces, and total gravitational energy. In addition, we analyze the optical properties of wormholes by examining both the shadow and the strong deflection angle. All the findings collectively demonstrate the physical plausibility of Yukawa-Casimir traversable wormholes within the EC gravity framework.

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

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 {\lambda}c and the quasinormal frequency {\omega}. 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

Relativistic compact stars coupled with dark energy in Heintzmann spacetime

The literature suggests that dark energy is responsible for the accelerating expansion of the universe due to its negative pressure, therefore, dark energy can be used as a possible option to prevent the gravitational collapse of compact objects into singularities. In this regard, there is a great possibility that dark energy can interact with the compact stellar matter configuration [Phys. Rev. D 103, 084042 (2021)]. In this article, we introduce a physically viable model for celestial compact stars made of isotropic baryonic matter and isotropic dark energy with Heintzmann's ansatz [Zeitschrift f\"ur Physik 228, 489-493 (1969)] in the context of Einstein's gravity. Here, the density of dark energy is assumed to be proportional to the density of baryonic matter. The main focus of the present article is to see the effects of dark energy on the physical properties of the stars. We perform an in-depth analysis of the physical attributes of the model, such as metric function, density, pressure, mass-radius relation, compactness parameter, gravitational and surface redshifts, along with the energy conditions for three well-known compact stars. We analyse the equilibrium of the present model via the generalised Tolman-Oppenheimer-Volkoff equation and the stability with the help of the adiabatic index and Harrison-Zeldovich-Novikov's static stability condition. Moreover, we estimate the solutions representing the maximum masses and the predicted surface radii from the M-R graph for different values of the coupling parameter {\alpha}. All the analyses ensure that the present model is non-singular and physically viable by satisfying all the essential conditions.

gr-qc↗

arXiv2025

Deflection of light by dark matter supporting traversable wormholes in the framework of Kalb-Ramond gravity

This study explores the possible formation of asymptotically flat traversable wormholes within dark matter halos under the framework of Kalb-Ramond gravity. The wormhole solutions are derived based on the King and Navarro-Frenk-White dark matter density profiles associated with anisotropic matter sources. For a particular set of parameters, the proposed shape functions are found to be positively increasing and satisfy all the essential geometric conditions along with the flare-out condition, thereby supporting asymptotically flat traversable wormholes. To study the underlying matter content responsible for the wormhole structures, we analyze the null energy condition at the wormhole throat and provide graphical representations of various energy conditions, highlighting both the regions where they are satisfied and where they are violated. The stability of the reported wormhole solutions is confirmed through the generalized Tolman-Oppenheimer-Volkoff equation. In addition, we explore several physical features of the wormhole configurations, including the embedding surface, complexity factor, active gravitational mass, and total gravitational energy. Moreover, we investigate the deflection of light by these wormholes, finding that the deflection angle approaches zero at large distances, where the wormhole's gravity is negligible, and diverges near the throat, where the gravitational influence is extremely strong.

gr-qc↗

arXiv2025

Modeling Cost-Associated Cooperation: A Dilemma of Species Interaction Unveiling New Aspects of Fear Effect

With limited resources, competition is widespread, yet cooperation persists across taxa, from microorganisms to large mammals. Recent observations reveal contingent factors often drive cooperative interactions, with the intensity heterogeneously distributed within species. While cooperation has beneficial outcomes, it may also incur significant costs, largely depending on species density. This creates a dilemma that is pivotal in shaping sustainable cooperation strategies. Understanding how cooperation intensity governs the cost-benefit balance, and whether an optimal strategy exists for species survival, is a fundamental question in ecological research, and the focus of this study. We develop a novel mathematical model within the Lotka-Volterra framework to explore the dynamics of cost-associated partial cooperation, which remains relatively unexplored in ODE model-based studies. Our findings demonstrate that partial cooperation benefits ecosystems up to a certain intensity, beyond which costs become dominant, leading to system collapse via heteroclinic bifurcation. This outcome captures the cost-cooperation dilemma, providing insights for adopting sustainable strategies and resource management for species survival. We propose a novel mathematical approach to detect and track heteroclinic orbits in predator-prey systems. Moreover, we show that introducing fear of predation can protect the regime shift, even with a type-I functional response, challenging traditional ecological views. Although fear is known to resolve the "paradox of enrichment," our results suggest that certain levels of partial cooperation can reestablish this dynamic even at higher fear intensity. Finally, we validate the system's dynamical robustness across functional responses through structural sensitivity analysis.

q-bio.PE↗

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 $\chi$. 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 $\omega(r) \in $ (0, 1), causality condition, adiabatic index $\Gamma(r) > 4/3$. 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 $\chi$ 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↗

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↗

arXiv2020

Tolman IV fluid sphere in bigravity

We present Tolman IV spacetime representing compact fluid sphere in bigravity. Here we have explored the effect of scale parameter $k$ in the local matter distribution of compact stars. We have model for three well-known compact stars and it shows that for lower values of $k$ leads to stiffer EoS. This claim is also supported by the graphical analysis. It can be observed that the sound speed and the adiabatic index are more for lower values of $k$. It is also seen that all the solutions of Einstein's field equations are still satisfying the field equations in the presence of a background metric $γ_{μν}$. However, the density and pressure does modified by extra term from the constant curvature background, thus affecting the EoS. One can also think that the parameter $α\equiv 1/k^2$ as coupling constant between the $g_{μν}$ and $γ_{μν}$ and consequently more the coupling stiffer is the EoS. As $k\rightarrow \infty$, the background de-Sitter spacetime reduces to Minkowski's spacetime and the coupling vanishes. The solution satisfy the causality condition, all the energy conditions and equilibrium under gravity and hydrostatic forces. The stability of the local stellar structure is enhanced by reducing the scalar curvature of the background spacetime.

gr-qc↗

arXiv2019

Possible formation of wormholes from dark matter in an isothermal galactic halo and void

It is well known that traversable wormholes are valid solutions of the Einstein field equations, but these structures can only be maintained by violating the null energy condition. In this paper, we have obtained such wormhole solutions in an isothermal galactic halo, as well as in a void. We have shown that the null energy condition is violated, with the help of a suitable redshift function obtained from flat galactic rotation curves.

physics.gen-ph↗

arXiv2019

Study of memory effect in an EOQ model for completely backlogged demand during shortage

The most commonly developed inventory models are the classical economic order quantity model, is governed by the integer order differential equations. We want to come out from the traditional thought i.e. classical order inventory model where the memory phenomena are absent. Here, we want to incorporate the memory effect that is based on the fact economic agents remember the history of changes of exogenous and endogenous variables. In this paper, we have proposed and solved a fractional order EOQ model with constant demand rate where the demand is fully backlogged during shortage time. Finally, a numerical example has been illustrated for this model to show the memory dependency of the system. The numerical example clears that for the considered system the profit is maximum in long memory affected system compared to the low memory affected or memory less system.

math.OC↗

arXiv2019

Generating functions of wormholes

It is known that wormhole geometry could be found solving the Einstein field equations by tolerating the violation of null energy condition (NEC). Violation of NEC is not possible for the physical matter distributions, however, can be achieved by considering distributions of "exotic matter". The main purpose of this work is to find generating functions comprising the wormhole like geometry and discuss the nature of these generating functions. We have used the Herrera et al. \cite{1} approaches of obtaining generating functions in the background of wormhole spacetime. Here we have adopt two approaches of solving the field equations to find wormhole geometry. In the first method, we have assumed the redshift function $f(r)$ and the shape function $b(r)$ and solve for the generating functions. In an another attempt we assume generating functions and redshift functions and then try to find shape functions of the wormholes.

physics.gen-ph↗

arXiv2018

Black holes/naked singularities in four-dimensional non-static space-time and the energy-momentum distributions

In this article, we discuss four dimensional non-static space-times in the background of de-Sitter and anti-de Sitter spaces with the matter-energy sources a stiff fluid, anisotropic fluid, and an electromagnetic field. Under various parameter conditions the solutions may represent models of naked singularity and/or black holes. Finally, the energy-momentum distributions using the complexes of Landau-Lifshitz, Einstein, Papapetrou, and Møller prescriptions, were evaluated.

gr-qc↗

arXiv2018

Particle motion around charged black holes in generalized dilaton-axion gravity

The behaviour of massive and massless test particles around asymptotically flat and spherically symmetric, charged black holes in the context of generalized dilaton-axion gravity in four dimensions is studied. All the possible motions are investigated by calculating and plotting the corresponding effective potential for the massless and massive particles as well. Further, the motion of massive (charged or uncharged) test particles in the gravitational field of charged black holes in generalized dilaton-axion gravity for the cases of static and non-static equilibrium is investigated by applying the Hamilton-Jacobi approach.

gr-qc↗

arXiv2018

Qualitative Analysis and Optimal Control Strategy of an SIR Model with Saturated Incidence and Treatment

This paper deals with an SIR model with saturated incidence rate affected by inhibitory effect and saturated treatment function. Two control functions have been used, one for vaccinating the susceptible population and other for the treatment control of infected population. We have analysed the existence and stability of equilibrium points and investigated the transcritical and backward bifurcation. The stability analysis of non-hyperbolic equilibrium point has been performed by using Centre manifold theory. The Pontryagin's maximum principle has been used to characterize the optimal control whose numerical results show the positive impact of two controls mentioned above for controlling the disease. Efficiency analysis is also done to determine the best control strategy among vaccination and treatment.

math.DS↗

arXiv2017

The new expansion method to solve Fractional KdV-Equations

Fractional calculus of variation plays an important role to formulate the non-conservative physical problems. In this paper we use semi-inverse method and fractional variational principle to formulate the fractional order generalized Korteweg-deVries (KdV) equation with Jumarie type fractional derivative and proposed a new method to solve the non-linear fractional differential equation named as expansion method. Using this method we obtained the solutions of fractional order generalized KdV. The obtained solutions are more general compare to other method and the solutions are expressed in terms of the generalized hyperbolic, trigonometric functions and rational functions.

math.AP↗

arXiv2017

D Alemberts solution of fractional wave equations using complex fractional transformation

Fractional wave equation arises in different type of physical problems such as the vibrating strings, propagation of electro-magnetic waves, and for many other systems. The exact analytical solution of the fractional differential equation is difficult to find. Usually Laplace-Fourier transformation method, along with methods where solutions are represented in series form is used to find the solution of the fractional wave equation. In this paper we describe the D Alembert s solution of the fractional wave equation with the help of complex fractional transform method. We demonstrate that using this fractional complex transformation method, we obtain the solutions easily as compared to fractional method of characteristics; and get the solution in analytical form. We show that the solution to the fractional wave equation manifests as travelling waves with scaled coordinates, depending on the considered fractional order value.

math.AP↗