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B. Mishra

Publications and source records attributed to B. Mishra.

At least 73 records · Page 4Linked to original sources
arXiv2023

Existence of non-exotic traversable wormholes in squared trace extended gravity theory

An extended gravity theory is used to explore the possibility of non-exotic matter traversable wormholes. In the extended gravity theory, additional terms linear and quadratic in the trace of the energy momentum tensor are considered in the Einstein-Hilbert action. Obviously, such an addition leads to violation of the energy-momentum tensor. The model parameters are constrained from the structure of the field equations. Non-exotic matter wormholes tend to satisfy the null energy conditions. We use two different traversable wormhole geometries namely an exponential and a power law shape functions to model the wormholes. From a detailed analysis of the energy conditions, it is found that, the existence of non-exotic matter traversable wormholes is not obvious in the model considered and its possibility may depend on the choice of the wormhole geometry. Also, we found that, non-exotic wormholes are possible within the given squared trace extended gravity theory for a narrow range of the chosen equation of state parameter.

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arXiv2023

Casimir wormhole with GUP correction in extended symmetric teleparallel gravity

Quantum mechanical concept such as the Casimir effect is explored to model traversable wormholes in an extended teleparallel gravity theory. The minimal length concept leading to the generalized uncertainty principle (GUP) is used to obtain the Casimir energy density. The effect of the GUP correction in the geometrical and physical properties of traversable Casimir wormholes are investigated. It is noted that the GUP correction has a substantial effect on the wormhole geometry and it modifies the energy condition. From a detailed calculation of the exotic matter content of the GUP corrected Casimir wormhole, it is shown that, a minimal amount of exotic matter is sufficient to support the stability of the wormhole.

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arXiv2023

Noether Symmetry Approach in Scalar-Torsion $f(T,ϕ)$ Gravity

The Noether Symmetry approach is applied to study an extended teleparallel $f(T,ϕ)$ gravity that contains the torsion scalar $T$ and the scalar field $ϕ$ in the context of an Friedmann-Lemaître-Robertson-Walker space-time. We investigate the Noether symmetry approach in $f(T,ϕ)$ gravity formalism with the specific form of $f(T,ϕ)$ and analyze how to demonstrate a nontrivial Noether vector. The Noether symmetry method is a helpful resource for generating models and finding out the exact solution of the Lagrangian. In this article, we go through how the Noether symmetry approach enables us to define the form of the function $f(T,ϕ)$ and obtain exact cosmological solutions. We also find the analytical cosmological solutions to the field equations, that is consistent with the Noether symmetry. Our results demonstrate that the obtained solutions enable an accelerated expansion of the Universe. We have also obtained the present value of the Hubble parameter, deceleration parameter, and effective equation of state parameter, which is fit in the range of current cosmological observations.

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arXiv2023

Accelerating cosmological models in $f(Q)$ gravity and the phase space analysis

The dynamical aspect of accelerating cosmological model has been studied in this paper in the context of modified symmetric teleparallel gravity, the $f(Q)$ gravity. Initially, we have derived the dynamical parameters for two well known forms of $f(Q)$ such as: (i) log-square-root form and (ii) exponential form. The equation of state (EoS) parameter for the dark energy in the $f(Q)$ gravity in both the models emerges into a dynamical quantity. At present model-I shows the quintessence behavior and behave like the $Λ$CDM at the late time whereas model-II shows phantom behaviour. Further, the dynamical system analysis has been performed to determine the cosmological behaviour of the models along with its stability behaviour. For both the models the critical points are obtained and analysed the stability at each critical points with phase portraits. The evolutionary behaviour of density parameters for the matter-dominated, radiation-dominated, and dark energy phases are also shown for both the models.

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arXiv2023

Global phase space analysis for a class of single scalar field bouncing solutions in general relativity

We carry out a compact phase space analysis of a non-canonical scalar field theory whose Lagrangian is of the form $F(X)-V(ϕ)$ within general relativity. In particular, we focus on a kinetic term of the form $F(X)=βX^m$ with power law potential $V_0 ϕ^n$ and exponential potential $V_0 e^{-λϕ/M_{Pl}}$ of the scalar field. The main aim of this work is to investigate the genericity of nonsingular bounce in these models and to investigate the cosmic future of the bouncing cosmologies when they are generic. A global dynamical system formulation that is particularly suitable for investigating nonsingular bouncing cosmologies is used to carry out the analysis. We show that when $F(X)=βX^m$ ($β<0$), nonsingular bounce is generic for a power law potential $V(ϕ) = V_0 ϕ^n$ only within the parameter range $\left\lbrace \frac{1}{2}<m<1,\,n<\frac{2m}{m-1}\right\rbrace$ and for an exponential potential $V(ϕ) = V_0 e^{-λϕ/M_{Pl}}$ only within the parameter range $\left\lbrace\frac{1}{2}<m\leq1\right\rbrace$. Except in these cases, nonsingular bounce in these models is not generic due to the non-existence of global past or future attractors. Our analysis serves to show the importance of a global phase space analysis to address important questions about nonsingular bouncing solutions, an idea that may and must be adopted for such solutions even in other theories.

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arXiv2023

Observational Constraints on Hybrid Scale Factor in f(Q,T) Gravity with Anisotropic Space-Time

In this paper, we present an accelerating cosmological model by constraining the free parameters using the cosmological datasets in an extended symmetric teleparallel gravity for the flat and anisotropic space-time. We employ a time variable deceleration parameter that behaves early deceleration and late time acceleration in the form of Hybrid Scale Factor (HSF). We obtain the present values of deceleration parameter and analyse the late time behavior of the Universe based on the best-fit values of free parameters. We derive the dynamical parameters of the model and obtain the equation of state parameter at present in the quintessence region; however at late time it approaches to $Λ$CDM. The energy conditions are also analysed to validate the modified gravity and we find that strong energy condition is violating. We establish the importance of hybrid scale factor in the late time cosmic phenomena issue.

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arXiv2023

Accelerating Cosmological Models in $f(T, B)$ Gravitational Theory

In this paper, we have explored the field equations of f(T, B) gravity as an extension of teleparallel gravity in an isotropic and homogeneous space time. In the basic formalism developed, the dynamical parameters are derived by incorporating the power law and exponential scale factor function. The models are showing accelerating behaviour and approaches to $Λ$CDM at late time.The present value of the equation of state parameter for both the cases are obtained to be in accordance with the range provided by cosmological observations. The geometrical parameters and the scalar field reconstruction are performed to assess the viability of a late time accelerating Universe. Further the stability of both the models are presented. It has been observed that both the models are parameters dependent. Since most of the geometrically modified theories of gravity are favouring the violation of strong energy condition, we have derived the energy conditions both for the power law and exponential model. In both the models, the violation of strong energy condition established.

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arXiv2023

Phantom cosmological model with observational constraints in $f(Q)$ gravity

In this paper, the cosmological model of the Universe has been presented in $f(Q)$ gravity and the parameters are constrained from the cosmological data sets. At the beginning, we have employed a well motivated form of $f(Q) = α+ βQ^{n}$, where $α$, $β$ and $n$ are model parameters. We have obtained the Hubble parameter in redshift with some algebraic manipulation from the considered form of $f(Q)$. Then we parameterize with the recent $Hubble$ data and $Pantheon + SHOES$ data using $\textit{MCMC}$ analysis. We validate our obtained model parameter values with $\textit{BAO}$ data set. A parametrization of the cosmographic parameters shows the early deceleration and late time acceleration with the transition at $z_t\approx0.75$. The $Om(z)$ diagnostics gives positive slope which shows that the model in the phantom phase. Also the current age of Universe has been obtained as, $t_{0} = 13.85~~Gyrs$. Based on the present analysis, it indicates that the $f(Q)$ gravity may provide an alternative to dark energy for addressing the current cosmic acceleration.

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arXiv2023

Evolutionary behaviour of cosmological parameters with dynamical system analysis in f (Q, T) gravity

We have investigated the accelerating behaviour of the universe in $f(Q,T)$ gravity in an isotropic and homogeneous space-time. We have initially derive the dynamical parameters in the general form of $f(Q,T)=αQ^{m}+βT$ [Xu et al., Eur. Phys. J. C, \textbf{79}, 708 (2019)] and then split it into two cases (i) one with $m=1$ and the (ii) other with $β=0$. In the first case, it reduces to the linear form of the functional $f(Q,T)$ and second case leads to the higher power of the nonmetricity $Q$. In an assumed form of the hyperbolic scale factor, the models are constructed and its evolutionary behaviours are studied. The geometrical parameters as well the equation of state parameter are obtained and found to be in the preferred range of the cosmological observations. Marginal variation has been noticed in the behaviour of $ω$ and $ω_{eff}$ at present time. The violation of strong energy conditions in both the cases are shown. The dynamical system analysis for the models has been performed.

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arXiv2023

On the uniqueness of $Λ$CDM-like evolution for homogeneous and isotropic cosmology in General Relativity

We address the question of the uniqueness of spatially flat $Λ$CDM-like evolution for FLRW cosmologies in General Relativity, i.e. whether any model other than the spatially flat $Λ$CDM can give rise to the same type of scale factor evolution. Firstly, we elaborate on what we exactly imply by a $Λ$CDM-like evolution or kinematic/cosmographic degeneracy with the $Λ$CDM model, using the lessons from the statefinder diagnostic. Then, we consider two models with interaction in the dark sector: coupled fluid-fluid model and coupled quintessence model. We enforce the \emph{kinematic} degeneracy with the spatially flat $Λ$CDM model via the cosmographic condition $j=1$ ($j$ being the jerk parameter), which in turn fixes the function of the interaction term that is a priori unspecified. We argue that in General Relativity this cosmographic condition is consistent only with spatial flatness. Employing a dynamical system approach, we show that the spatially flat coupled fluid-fluid interacting models kinematically degenerate with $Λ$CDM must necessarily be based on a phantom fluid, whereas the set of physically viable spatially flat coupled quintessence models with power law or exponential potential kinematically degenerate to $Λ$CDM is of measure zero. Our analysis establishes that coupled fluid-fluid models with non-phantom fluids or coupled quintessence models with power law and exponential potential can never reproduce a cosmological evolution similar to that of the $Λ$CDM. The astrophysical consequences of our findings are qualitatively discussed in light of observational cosmological tensions.

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arXiv2023

Constrained $f(Q,T)$ gravity accelerating cosmological model and its dynamical system analysis

In this paper, we have presented an accelerating cosmological model of the Universe in an extended symmetric teleparallel gravity or $f(Q,T)$ gravity. The parametric form of the Hubble parameter is, $H\left(z\right) =H_{0}\left[ α+\left( 1-α\right) \left( 1+z\right) ^{n}\right] ^{\frac{3}{2n}}$, where $H_0$ and $n$ are constants and for $n=3$, the $Λ$CDM scenario can be obtained. We have considered the logarithmic form of $f(Q,T)$ as, $f(Q,T)=-Q+β\log\left(\frac{Q}{Q_{0}}\right)+γT$, where $β$ and $γ$ are the free model parameters. Using the Hubble, Baryon Acoustic Oscillations (BAO), and Type Ia Supernovae (SNe Ia) datasets, the present value of the Hubble parameter and other free parameters are constrained. Further other cosmographic and dynamical parameters are presented using the obtained constrained values of the Hubble and free parameters. The model shows the quintessence behavior of the Universe at the present time. The present value of the EoS parameter is obtained as, $ω_{0}=-0.56$ for the $Hubble+BAO+SNe$ datasets. The energy conditions are presented and the violation of the strong energy condition has been shown. We have performed the dynamical system analysis to validate the stability of the model. From the evolutionary plot obtained through the dynamical system variables, the present value of density parameters have been obtained as $Ω_m\approx0.3$ and $Ω_{de}\approx0.7$.

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arXiv2023

Observationally constrained accelerating cosmological model with higher power of non-metricity and squared trace

In this paper, a cosmological model of the Universe is presented in $f(Q,T)$ gravity and the parameters are constrained by cosmological data sets. Initially, a generalised form of $f(Q,T)$ model is used as $f(Q,T)=-λ_{1} Q^{m}-λ_{2} T^2$, where $λ_{1}$, $λ_{2}$ and $m$ are model parameters. With some algebraic manipulation, the Hubble parameter is obtained in terms of redshift. Then, using MCMC analysis, the model parameters are constrained using the most current Hubble and Pantheon$^{+}$ data. The model parameters are also verified through the BAO data set. The model shows an early deceleration transitioning to an accelerating phase of the Universe. The $Om(z)$ diagnostics indicate a positive slope, favouring the model to be in a phantom field dominated phase.

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arXiv2023

Weyl type $f(Q,T)$ gravity observational constrained cosmological model

In this paper, we have studied the dynamical aspects of the cosmological model of the Universe in the Weyl type $f(Q,T)$ gravity, which is an extension of symmetric teleparallel gravity. The non-metricity scalar $Q$ has been expressed in standard Weyl form and can be determined by a vector field $w_μ$ and the trace of energy momentum tensor denoted as $T$. The logarithmic form of the Hubble parametrization has been incorporated and the best fit values of the free parameters have been determined using $32~CC$ sample points, $1701~Pantheon^{+}$ and $6~BAO$ data points. The present value of the $H_{0}\approx 70.2\pm 4.6$, $H_{0}\approx 68.69_{-0.59}^{+0.67}$ and $H_{0}\approx 69.26_{-0.53}^{+0.57}$ respectively for $CC~Sample$, $CC + Pantheon^{+}$ and $CC + Pantheon^{+} + BAO$ datasets. With the constrained values of the free parameters, the cosmographic parameters are constrained and the present value of each parameter has been noted. The deceleration parameter for $CC~Sample$, $CC + Pantheon^{+}$ and $CC + Pantheon^{+} + BAO$ datasets provides $-0.5221$, $-0.5477$ and $-0.5691$ respectively at present time. We have considered exponential and non-linear form of the Weyl type function $f(Q,T)$ to assess the dynamical behaviour of the model. The accelerating cosmological models show the quintessence behaviour at present time as we get the present EoS parameter values obtained as $ω\approx -0.7068$ and $ω\approx -0.6828$ for $CC~Sample$, $ω\approx -0.6991$ and $-0.6949$ for $CC + Pantheon^{+}$ and $ω\approx -0.7001$ and $-0.7084$ for $CC + Pantheon^{+} + BAO$ respectively.

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arXiv2023

Bouncing cosmological models in a functional form of F(R) gravity

We have investigated some bouncing cosmological models in an isotropic and homogeneous space time with the F(R) theory of gravity. Two functional forms of F(R) have been investigated with a bouncing scale factor. The dynamical parameters are derived and analysed along with the cosmographic parameters. The analysis in both the models show the occurrence of bouncing scenario. The violation of strong energy conditions in both models is also shown. In the stability point of view we have analysed the behaviour of F_R = dF/dR with respect to cosmic time and both the models exhibit stable behaviour.

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arXiv2023

Cosmology from non-minimal geometry-matter coupling

We construct a cosmological model from the inception of the Friedmann-Lemâitre-Robertson-Walker metric into the field equations of the f(R,L_m) gravity theory, with R being the Ricci scalar and L_m being the matter lagrangian density. The formalism is developed for a particular f(R,L_m) function, namely R/16π+(1+σR)L_{m}, with σbeing a constant that carries the geometry-matter coupling. Our solutions are remarkably capable of evading the Big-Bang singularity as well as predict the cosmic acceleration with no need for the cosmological constant, but simply as a consequence of the geometry-matter coupling terms in the Friedmann-like equations

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arXiv2023

Unimodular Gravity Traversable Wormholes

Wormholes are outstanding solutions of Einstein's General Relativity. They were worked out in the late 1980's by Morris and Thorne, who have figured out a recipe that wormholes must obey in order to be traversable, that is, safely crossed by travelers. A remarkable feature is that General Relativity Theory wormholes must be filled by {\it exotic matter}, which Morris and Thorne define as matter satisfying $-p_r>ρ$, in which $p_r$ is the radial pressure and $ρ$ is the energy density of the wormhole. In the present article, we introduce, for the first time in the literature, traversable wormhole solutions of Einstein's Unimodular Gravity Theory. Unimodular Gravity was proposed by Einstein himself as the theory for which the field equations are the traceless portion of General Relativity field equations. Later, Weinberg has shown that this approach elegantly yields the solution of the infamous cosmological constant problem. The wormhole solutions here presented satisfy the metric conditions of "traversability" and remarkably evade the exotic matter condition, so we can affirm that Unimodular Gravity wormholes can be filled by ordinary matter.

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arXiv2023

Noether Symmetries in $f(T,T_G)$ Cosmology

All degrees of freedom related to the torsion scalar can be explored by analysing, the $f(T,T_G)$ gravity formalism where, $T$ is a torsion scalar and $T_G$ is the teleparallel counterpart of the Gauss-Bonnet topological invariant term. The well-known Noether symmetry approach is a useful tool for selecting models that are motivated at a fundamental level and determining the exact solution to a given Lagrangian, hence we explore Noether symmetry approach in $f(T,T_G)$ gravity formalism with three different forms of $f(T,T_G)$ and study how to establish nontrivial Noether vector form for each one of them. We extend the analysis made in \cite{capozziello2016noether} for the form $f(T,T_{G})=b_{0}T_{G}^{k}+t_{0}T^{m}$ and discussed the symmetry for this model with linear teleparallel equivalent of the Gauss-Bonnet term, followed by the study of two models containing exponential form of the teleparallel equivalent of the Gauss-Bonnet term. We have shown that all three cases will allow us to obtain non-trivial Noether vector which will play an important role to obtain the exact solutions for the cosmological equations.

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arXiv2023

Evolution of Generalized Brans-Dicke parameter within a Superbounce scenario

We have studied a superbounce scenario in a set up of Brans-Dicke (BD) theory. The BD parameter is considered to be time dependent and is assumed to evolve with the Brans-Dicke scalar field. In the superbounce scenario, the model bounces at an epoch corresponding to a Big Crunch provided the ekpyrotic phase continues until that time. Within the given superbounce scenario, we investigate the evolution of the BD parameter for different equations of state. We chose an axially symmetric metric that has an axial symmetry along the x-axis. The metric is assumed to incorporate an anisotropic expansion effect. The effect of asymmetric expansion and the anisotropic parameter on the evolving and the non-evolving part of the BD parameter is investigated.

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