arXiv subjects

Gauranga C. Samanta

Publications and source records attributed to Gauranga C. Samanta.

At least 19 recordsLinked to original sources

arXiv2024

Generic autonomous system approach to interacting dark energy models

We explore an autonomous system analysis of dark energy models with interactions between dark energy and cold dark matter in a general systematic approach to cosmological fluids. We investigate two types of models such as local and non-local ones. In particular, a local form of interaction is directly proportional to only the energy density, while a non-local interaction is directly proportional to the energy density as well as the Hubble parameter. As a consequence, it is explicitly demonstrated that in both cases there exist the stability points in terms of cosmological parameters. This work aims at obtaining acceleration and stability using interaction models without modifying the matter or geometric component of the Universe.

gr-qc↗

arXiv2023

On the visibility of singularities in general relativity and modified gravity theories

We investigate the global causal structure of the end state of a spherically symmetric marginally bound Lemaitre-Tolman-Bondi (LTB) \cite{Lemaitre, Tolman, Bondi} collapsing cloud (which is well studied in general relativity) in the framework of modified gravity having the generalized Lagrangian $R+αR^2$ in the action. Here $R$ is the Ricci scalar, and $α\geq 0$ is a constant. By fixing the functional form of the metric components of the LTB spacetime, using up the available degree of freedom, we realize that the matching surface of the interior and the exterior metric are different for different values of $α$. This change in the matching surface can alter the causal property of the first central singularity. We depict this by showing a numerical example. Additionally, for a globally naked singularity to have physical relevance, a congruence of null geodesics should escape from such singularity to be visible to an asymptotic observer for an infinite time. For this to happen, the first central singularity should be a nodal point. We here give a heuristic method to show that this singularity is a nodal point by considering the above class of theory of gravity, of which general relativity is a particular case.

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arXiv2021

Cosmological dynamics of f(R) models in dynamical system analysis

In this work we try to understand the late time acceleration of the universe by assuming some modification in the geometry of the space and using dynamical system analysis. This technique allows to understand the behavior of the universe without analytically solving the field equations. We study the acceleration phase of the universe and stability properties of the critical points which could be compared with observational results. We consider an asymptotic behavior of two particular models $f(R) = R - μR_{c} \frac{(R/R_c)^{2n}}{(R/R_c)^{2n} + 1}$ and $f(R) = R - μR_{c} \left[ 1 - (1 + R^2/R_{c}^2)^{-n} \right]$ with $n,μ, R_c >0$ for the study. As a first case we fix the value of $μ$ and analyzed for all $n$. Later as second case, we fix the value of $n$ and calculation are done for all $μ$. At the end all the calculations for the generalized case have been shown and results have been discussed in detail.

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arXiv2021

Gravitational lensing effect in traversable wormholes

The present paper is intended for studying the effect of strong gravitational lensing in the context of charged wormhole. To study this effect, the conditions determining the existence of photon spheres at and outside the throat are obtained. The necessary and sufficient conditions for the existence of photon spheres at or outside the throat of the charged wormhole is derived. Furthermore, photon spheres are investigated in three cases for three different forms of redshift function. These three cases include the existence of effective photon spheres (i) at the throat, (ii) outside the throat and (iii) both at and outside the throat. Consequently, these provide the information about the formation of infinite number of concentric rings and may lead to the detection of wormhole geometries.

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arXiv2020

Cosmological dynamics in $R^2$ gravity with logarithmic trace term

A novel function for modified gravity is proposed, $f(R, T)=R+λR^2+2β\ln(T)$, with constants $λ$ and $β$, scalar curvature $R$, and the trace of stress energy tensor $T$, satisfying $T=ρ-3p>0$. Subsequently, two equations of state (EoS) parameters, namely $ω$ and a parametric form of the Hubble parameter $H$, are employed in order to study the accelerated expansion and initial cosmological bounce of the corresponding universe. Hubble telescope experimental data for redshift $z$ within the range $0.07\leq z \leq 2.34$ are used to compare the theoretical and observational values of the Hubble parameter. Moreover, it is observed that all the energy conditions are fulfilled within a neighborhood of the bouncing point $t=0$, what shows that the necessary condition for violation of the null energy condition, within a neighborhood of the bouncing point in general relativity, could be avoided by modifying the theory in a reasonable way. Furthermore, a large amount of negative pressure is found, which helps to understand the late time accelerated expansion phase of the universe.

physics.gen-ph↗

arXiv2020

Traversable Wormholes with Exponential Shape Function in Modified Gravity and in General Relativity: A Comparative Study

We have proposed a novel shape function on which the metric that models traversable wormholes is dependent. Using this shape function, the energy conditions, equation of state and anisotropy parameter are analyzed in $f(R)$ gravity, $f(R,T)$ gravity and general relativity. Furthermore, the consequences obtained with respect to these theories are compared. In addition, the existence of wormhole geometries is investigated.

gr-qc↗

arXiv2020

Estimation of Cosmological Parameters, Stability Analysis and Energy Conditions in Viable Modified Gravity

In the present paper, we have investigated the Friedmann Robertson Walker (FRW) model in viable $f(R,T)$ gravity with $f(R,T)$ function proposed as $f(R,T)=R +ξT^{1/2}$, where $ξ$ is an arbitrary constant, $R$ is the scalar curvature and $T$ is the trace of stress energy tensor. Defining the scale factor, the field equations are solved numerically and the energy conditions are analyzed. Further, determining Hubble parameter and deceleration parameter, their present values are estimated. Furthermore, 57 redshift data (42 redshift data from Supernova Cosmology project and 15 redshift data from Calán/ Tolono Supernova survey) are used to estimate the age of the universe and to find the best fit curves for luminosity distance and apparent magnitude.

physics.gen-ph↗

arXiv2020

Wormhole modeling in $R^2$ gravity with linear trace term

Morris and Thorne \cite{morris1} proposed traversable wormholes, hypothetical connecting tools, using the concept of Einstein's general theory of relativity. In this paper, the modification of general relativity (in particular $f(R,T)$ theory of gravity defined by Harko et al. \cite{harko}) is considered, to study the traversable wormhole solutions. The function $f(R,T)$ is considered as $f(R,T)=R+αR^2+βT$, where $α$ and $β$ are controlling parameters. The shape and red shift functions appearing in the metric of wormhole structure have significant contribution in the development of wormhole solutions. We have considered both variable and constant red shift functions with a logarithmic shape function. The energy conditions are examined, geometric configuration is analyzed and the radius of the throat is determined in order to have wormhole solutions in absence of exotic matter.

gr-qc↗

arXiv2020

Traversable wormholes in $f(R)$ gravity with constant and variable redshift functions

The present paper is aimed at the study of traversable wormholes in $f(R)$ gravity with a viable $f(R)$ function defined as $f(R)=R-μR_c\Big(\frac{R}{R_c}\Big)^p$, where $R$ is scalar curvature, $μ$, $R_c$ and $p$ are constants with $μ, R_c>0$ and $0<p<1$ \citep{Amendola}. The metric of wormhole is dependent on shape function $b(r)$ and redshift function $ϕ(r)$ which characterize its properties, so the shape function and redshift function play an important role in wormhole modeling. In this work, the wormhole solutions are determined for (i) $ϕ(r)=\frac{1}{r}$ and (ii) $ϕ(r)=c$ (constant) with $b(r)=\frac{r}{exp(r-r_0)}$ \citep{godani1}. Further, the regions respecting the energy conditions are investigated.

gr-qc↗

arXiv2019

Traversable Wormholes in $R+αR^n$ Gravity

In this work, the study of traversable wormholes in $f(R)$ gravity with the function $f(R)=R+αR^n$, where $α$ and $n$ are arbitrary constants, is taken into account. The shape function $b(r)=\frac{r}{\exp(r-r_0)}$, proposed by Samanta et al. \cite{godani1}, is considered. The energy conditions with respect to both constant and variable redshift functions are discussed and the existence of wormhole solutions without presence of exotic matter is investigated.

gr-qc↗

arXiv2019

Static Traversable Wormholes in $f(R, T)=R+2α\ln T$ Gravity

Traversable wormholes, studied by Morris and Thorne \cite{Morris1} in general relativity, are investigated in this research paper in $f(R,T)$ gravity by introducing a new form of non-linear $f(R,T)$ function. By using this novel function, the Einstein's field equations in $f(R,T)$ gravity are derived. To obtain the exact wormhole solutions, the relations $p_t=ωρ$ and $p_r=\sinh(r)p_t$, where $ρ$ is the energy density, $p_r$ is the radial pressure and $p_t$ is the tangential pressure, are used. Other than these relations, two forms of shape function defined in literature are used, and their suitability is examined by exploring the regions of validity of null, weak, strong and dominant energy conditions . Consequently, the radius of the throat or the spherical region, with satisfied energy conditions, is determined and the presence of exotic matter is minimized.

physics.gen-ph↗

arXiv2019

Wormhole Modeling Supported by Non-Exotic Matter

In the present paper, the modelling of traversale wormholes, proposed by Morris \& Thorne \cite{morris1}, is performed within the $f(R)$ gravity with particular viable case $f(R)=R-μR_c\Big(\frac{R}{R_c}\Big)^p$, where $μ, R_c>0$ and $0<p<1$. The energy conditions are analyzed using the shape function $b(r)=\frac{r\log(r+1)}{\log(r_0+1)}$ defined by Godani and Samanta \cite{godani} and geometric nature of wormholes is analyzed.

gr-qc↗

arXiv2019

Non violation of energy conditions in wormholes modelling

Morris \& Thorne \cite{morris1} proposed geometrical objects called traversable wormholes that act as bridges in connecting two spacetimes or two different points of the same spacetime. The geometrical properties of these wormholes depend upon the choice of the shape function. In literature, these are studied in modified gravities for different types of shape functions. In this paper, the traversable wormholes having shape function $b(r)=\frac{r_0\tanh(r)}{\tanh(r_0)}$ are explored in $f(R)$ gravity with $f(R)=R+αR^m-βR^{-n}$, where $α$, $β$, $m$ and $n$ are real constants. For different values of constants in function $f(R)$, the analysis is done in various cases. In each case, the energy conditions, equation of state parameter and anisotropic parameter are determined.

gr-qc↗

arXiv2019

Stability analysis for cosmological models in $f(R)$ gravity using dynamical system analysis

Modified gravity theories have received increased attention lately to understand the late time acceleration of the universe. This viewpoint essentially modifies the geometric components of the universe. Among numerous extension to Einstein's theory of gravity, theories which include higher order curvature invariant, and specifically the class of $f(R)$ theories, have received several acknowledgments. In our current work we try to understand the late time acceleration of the universe by modifying the geometry of the space and using dynamical system analysis. The use of this technique allows to understand the behavior of the universe under several circumstances. Apart from that we study the stability properties of the critical point and acceleration phase of the universe which could then be analyzed with observational data. We consider a particular model $f(R) = R - μR_{c}(R/R_{c})^{p}$ with $ 0 < p < 1, μ, R_{c} > 0$ for the study. As a first case we consider the matter and radiation component of the universe with an assumption of no interaction between them. Later, as a second case we take matter, radiation and dark energy (cosmological constant) where study on effects of linear, non-linear and no interaction between matter and dark energy is considered and results have been discussed in detail.

gr-qc↗

arXiv2019

Dark energy in spherically symmetric universe coupled with Brans-Dicke scalar field

The phenomenon of dark energy and its manifestations are studied in a spherically symmetric universe considering the Brans-Dicke scalar tensor theory. In the first model the dark energy behaves like a phantom type and in such a universe the existence of negative time is validated with an indication that our universe started its evolution before $t=0$. Dark energy prevalent in this universe is found to be more active at times when other types of energies remain passive. The second model universe begins with big bang. On the other hand the dark energy prevalent in the third model is found to be of the quintessence type. Here it is seen that the dark energy triggers the big bang and after that much of the dark energy reduces to dark matter. One peculiarity in such a model is that the scalar field is prevalent eternally, it never tends to zero.

gr-qc↗

arXiv2019

Dynamics of General Barotropic Stellar Fluid in the Framework of $R+2αT$ Gravity

Gravitational collapse of a spherically symmetric homogeneous perfect barotropic fluid with linear as well as polytropic type Equation of State (EoS) has been investigated in the framework of a linear model of $f(R,T)$ gravity. This modified gravity has the potential to explain the observed cosmic acceleration. The calculations have been done taking the transformed time coordinate $t \to \sqrt{\frac{ρ_0}{3}} t$, where $ρ_0$ is the initial density of the fluid. For linear EoS $p=ωρ$, the condition for being a true singularity, along with sufficient condition for the formation of apparent horizon covering the singularity has been derived. For a polytrope having the EoS $p=Kρ^{1+\frac{1}{n}}$, the scale factor ($A$) as a function of fluid density ($ρ$) has been obtained which is then used to study the dynamics of the fluid. Role of the polytropic index ($n$) and the constant of proportionality ($K$) in the dynamics of the fluid is also studied. A new type of exotic matter field having varied dependence of scale factor on the density, and having the potential to give rise to bouncing cosmology, provided it is the dominating fluid in the universe, is obtained in this domain and is investigated. Energy conditions are discussed.

gr-qc↗

arXiv2018

Gravitational Collapse in General Relativity and in $R^2$-gravity: A Comparative Study

We compare the gravitational collapse of homogeneous perfect fluid with various equations of state in the framework of General Relativity and in $R^2$ gravity. We make our calculations using dimensionless time with characteristic timescale $t_{g}\sim (Gρ)^{-1/2}$ where $ρ$ is a density of collapsing matter. The cases of matter, radiation and stiff matter are considered. We also account the possible existence of vacuum energy and its influence on gravitational collapse. In a case of $R^2$ gravity we have additional degree of freedom for initial conditions of collapse. For barotropic equation of state $p=wρ$ the result depends from the value of parameter $w$: for $w>1/3$ the collapse occurs slowly in comparison with General Relativity while for $w<1/3$ we have opposite situation. Vacuum energy as expected slows down the rate of collapse and for some critical density gravitational contraction may change to expansion. It is interesting to note that for General Relativity such expansion is impossible. We also consider the collapse in the presence of so-called phantom energy. For description of phantom energy we use Lagrangian in the form $-X-V$ (where $X$ and $V$ are the kinetic and potential energy of the field respectively) and consider the corresponding Klein-Gordon equation for phantom scalar field.

gr-qc↗

arXiv2018

Dark energy and its manifestations

In a four dimensional manifold formalism we study the evolutionary behavior as well as the ultimate fate of the universe, in the course of which the contribution of dark energy in these phases are investigated. At one stage we get a situation (a condition) where the dark energy contained dominates other types of energies available in this universe. In the model universes we obtain here the dark energy is found to be of $Λ$CDM and quintessence types-which bear testimony to being real universes. In one of the cases where the equation of state between the fluid pressure and density is of the type of the van der Waals equation, it is found that our universe may end in dust. And, also, it is seen that the behavior of the deceleration parameter is almost compatible with the recent observation.

gr-qc↗