arXiv subjects

Nisha Godani

Publications and source records attributed to Nisha Godani.

15 recordsLinked to original sources

arXiv2022

Non-Local Gravity Wormholes

We consider Non-local Gravity in view to obtain stable and traversable wormhole solutions. In particular, the class of Non-local Integral Kernel Theories of Gravity, with the inverse d'Alembert operator in the gravitational action, is taken into account. We obtain constraints for the null energy condition and derive the field equations. Two special cases for the related Klein-Gordon equation are assumed: one where the function in the gravitational action has a linear form and another one with exponential form. In each case, we take into account two forms for scalar fields and derive the shape functions. Asymptotic flatness and flaring-out conditions are checked. Energy conditions and dynamics of the solutions are examined at the throat. The main result is that non-local gravity contributions allow stability and traversability of the wormhole without considering any exotic matter.

gr-qc↗

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.

gr-qc↗

arXiv2021

FRW Cosmology in $f(Q,T)$ Gravity

In this paper, we considered the study of Friedmann-Robertson-Walker (FRW) model in the framework of $f(Q,T)$ gravity, an extension of symmetric teleparallel gravity, recently defined by Y. Xu et al. \cite{Xu}. The non-linear model $f(Q,T)=-αQ-βT^2$, where $α>0$ and $β>0$ are constants, is taken into account. The equation of state of perfect fluid is assumed and 31 points of Hubble data are used to constrain the value of model parameter. To explore the evolution of the universe, the numerical solutions of cosmological implications such as Hubble parameter, deceleration parameter, apparent magnitude and luminosity distance are determined and the energy conditions are examined. The theoretical results of Hubble parameter are compared with $Λ$CDM model. Further, 57 Supernova data (42 from Supernova cosmology project and 15 from Calán/ Tolono supernova survey) are also used to have consistent results of apparent magnitude and luminosity distance.

gr-qc↗

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

Physical Parameters for Stable $f(R)$ Models

Nojiri \& Odintsov \cite{noj1} and Hu \& Sawicki \cite{hu} have studied non-linear functions in modified gravity that explain the cosmic acceleration without cosmological constant, fulfil the conditions of local gravity \& stability and pass the solar system tests. In this paper, FRW model, a best fitted and fruitful mathematical model of the physical universe \cite{rob, hub, alph, pen} is studied in the context of these non-linear functions. The cosmological implications such as Hubble parameter, deceleration parameter, jerk parameter, matter density and the effective equation of state parameter of the universe are plotted with respect to redshift. Subsequently, the age of the universe is predicted in $f(R)$ gravity. All are found to represent the features of present phase of the universe.

gr-qc↗

arXiv2019

Validation of Energy Conditions in Wormhole Geometry within Viable $f(R)$ Gravity

In this work, wormholes, tunnel like structures introduced by Morris \& Thorne \cite{Morris95}, are explored within the framework of $f(R)$ gravity. Using the shape function $b(r)=r_0\big(\frac{r}{r_0}\big)^γ$, where $0<γ<1$, and the equation of state $p_r=ωρ$, the $f(R)$ function is derived and the field equations are solved. Then null, weak, strong and dominated energy conditions are analyzed and spherical regions satisfying these energy conditions are determined. Furthermore, we calculated the range of the radius of the throat of the wormhole, where the energy conditions are satisfied.

gr-qc↗

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↗

arXiv2018

Traversable Wormholes and Energy Conditions with Two Different Shape Functions in $f(R)$ Gravity

Traversable wormholes, tunnel like structures introduced by Morris \& Thorne \cite{morris1}, have a significant role in connection of two different space-times or two different parts of the same space-time. The characteristics of these wormholes depend upon the redshift and shape functions which are defined in terms of radial coordinate. In literature, several shape functions are defined and wormholes are studied in $f(R)$ gravity with respect to these shape functions \cite{lobo, saiedi, baha}. In this paper, two shape functions (i) $b(r)=\dfrac{{r_0} \log (r+1)}{\log ({r_0}+1)}$ and (ii) $b(r)=r_0(\frac{r}{r_0})^γ$, $0<γ<1$ are considered. The first shape function is newly defined, however the second one is collected from the literature\cite{cataldo}. The wormholes are investigated for each type of shape function in $f(R)$ gravity with $f(R)=R+αR^m-βR^{-n}$, where $m$, $n$, $α$, and $β$ are real constants. Varying parameters $α$ or $β$, $f(R)$ model is studied in five subcases for each type of shape function. In each case, the energy density, radial \& tangential pressures, energy conditions that include null energy condition, weak energy condition, strong energy condition \& dominated energy condition, and anisotropic parameter are computed. The energy density is found to be positive and all energy conditions are obtained to be violated which supports the existence of wormholes. Also, the equation of state parameter is obtained to possess values less than -1, that shows the presence of the phantom fluid and leads towards the expansion of the universe.

gr-qc↗