arXiv subjects

P. K. Sahoo

Publications and source records attributed to P. K. Sahoo.

At least 181 records · Page 10Linked to original sources

arXiv2020

Bouncing scenario in $f(R,T)$ gravity

The present manuscript presents modeling of matter bounce in the framework of $f(R,T)$ gravity where $f(R,T) = R + 2 λT$. We start by defining a parametrization of scale factor which is non-vanishing. The geometrical parameters such as the Hubble parameter and deceleration parameter are derived, from which expressions of pressure, density and Equation of State (EoS) parameter and a qualitative understanding of the initial conditions of the universe at the bounce are ascertained. We found that the initial conditions of the universe are finite owing to the non-vanishing nature of the scale factor thus eliminates the initial singularity problem. Furthermore, we show the violation of energy conditions near the bouncing region and analyzed the stability of our model with respect to linear homogeneous perturbations in Friedmann-Lematre-Robertson-Walker (FLRW) spacetime. We found that our model and hence matter bounce scenarios in general are highly unstable at the bounce in the framework of $f(R,T)$ gravity but the perturbations decay out rapidly away from the bounce safeguarding its stability at late times.

gr-qc↗

arXiv2020

Thermodynamical aspects of relativistic hydrodynamics in f(R,G) gravity

Present investigation devoted to the dynamical study of Relativistic Hydrodynamics with some thermodynamical characteristics in $f(R,G)$ gravity towards spatially homogeneous isotropic cosmological model filled with isotropic fluid. We govern the features of the derived cosmological model by considering the power-law inflation for the average scale factor. The temperature and entropy density of the proposed model are positive definite. We also discuss the energy conditions to our solutions. The strong energy condition violated, which indicates the accelerated expansion of the proposed model.

physics.gen-ph↗

arXiv2020

Temporally Varying Universal Gravitational "Constant" and Speed of Light in Energy Momentum Squared Gravity

Energy Momentum Squared Gravity (EMSG) is a cosmological model where the scale factor is non vanishing at all times and hence does not favor big bang cosmology. However, the profile of density in the radiation dominated universe shows that EMSG supports inflationary cosmology. Inflationary cosmological models are successful in providing convincing answers to major cosmological issues like horizon problem, flatness problem and small value of cosmological constant but hitherto no model of inflation has been observationally confirmed. Owing to this, Varying Speed of Light (VSL) were introduced which are a class of cosmological models which disfavor inflation and propose an alternative route to solve these cosmological issues by just allowing the speed of light (and Newtonian Gravitational constant) to vary. VSL theories were motivated to address the shortcomings of inflation but do not address the shortcomings related to the initial big bang singularity. In this spirit, we present here a novel cosmological model which is free from both the "initial big bang singularity" and "inflation" by incorporating a mutually varying speed of light $c(t)$ and Newtonian gravitational constant $G(t)$ in the framework of EMSG. We report that in EMSG, for a dust universe ($ω=0$), cosmological models for a time varying $c(t)$ and $G(t)$ and constant $c$ and $G$ are indistinguishable, whereas for a radiation dominated universe ($ω= 1/3$), a mutually varying $c(t)$ and $G(t)$ provides an exiting alternative to inflationary cosmology which is also free from initial big bang singularity. We further report that for an ansatz of scale factor representing a bouncing cosmological model, the VSL theory can be applied to a quadratic $T$ gravity model to get rid of "inflation" and "big bang singularity" and concurrently solve the above mentioned cosmological enigmas.

gr-qc↗

arXiv2019

Dark Energy and Modified Scale Covariant Theory of Gravitation

Taking up four model universes we study the behaviour and contribution of dark energy to the accelerated expansion of the universe, in the modified scale covariant theory of gravitation. Here, it is seen that though this modified theory may be a cause of the accelerated expansion it cannot totally outcast the contribution of dark energy in causing the accelerated expansion. In one case the dark energy is found to be the sole cause of the accelerated expansion. The dark energy contained in these models come out to be of the $Λ$CDM type and quintessence type comparable to the modern observations. Some of the models originated with a big bang, the dark energy being prevalent inside the universe before the evolution of this era. One of the models predicts big rip singularity, though at a very distant future. It is interestingly found that the interaction between the dark energy and the other part of the universe containing different matters is enticed and enhanced by the gauge function $ϕ(t)$ here.

gr-qc↗

arXiv2019

Revisiting The Coincidence Problem in $f(R)$ Gravitation

The energy densities of dark matter (DM) and dark energy (DE) are of the same order at the present epoch despite the fact that both these quantities have contrasting characteristics and are presumed to have evolved distinctively with cosmic evolution. This is a major issue in standard $Λ$CDM cosmology and is termed "The Coincidence Problem" which hitherto cannot be explained by any fundamental theory. In this spirit, Bisabr \cite{bisabr} reported a cosmological scenario in $f(R)$ gravity where DM and DE interact and exchange energy with each other and therefore evolve dependently. We investigate the efficiency and model independancy of the technique reported in Bisabr \cite{bisabr} in addressing the Coincidence problem with the help of two $f(R)$ gravity models with model parameters constrained from various observations. Our result confirm the idea that not all scalar-tensor gravity theories and models can circumvent the Coincidence Problem and any cosmological scenario with interacting fluids is highly model dependent and hence alternate model independent theories and ideas should be nominated to solve this mystery.

gr-qc↗

arXiv2019

Observational constraint on interacting Tsallis holographic dark energy in logarithmic Brans-Dicke theory

In this paper, we investigate the dark energy phenomenon by studying the Tsallis holographic dark energy within the framework of Brans-Dicke (BD) scalar-tensor theory of gravity [Phys. Rev. \textbf{124}, 925 (1961)]. In this context, we choose the BD scalar field $ϕ$ as a logarithmic function of the average scale factor $a(t)$ and Hubble horizon as the IR cutoff ($L=H^{-1}$). We reconstruct two cases of non-interacting and interacting fluid (dark sectors of cosmos) scenario. The physical behavior of the models are discussed with the help of graphical representation to explore the accelerated expansion of the universe. Moreover, the stability of the models are checked through squared sound speed $v_s^2$. The well-known cosmological plane i.e., $ω_{de}-ω^{\prime}_{de}$ is constructed for our models. We also include comparison of our findings of these dynamical parameters with observational constraints. It is also quite interesting to mention here that the results of deceleration, equation of state parameters and $ω_{de}-ω^{\prime}_{de}$ plane coincide with the modern observational data.

gr-qc↗

arXiv2019

Energy-Momentum Distribution in General Relativity for a Phantom Black Hole Metric

We use the Møller and Landau-Lifshitz energy-momentum definitions in General Relativity (GR) to evaluate the energy-momentum distribution of the phantom black hole space-time. The phantom black hole model was applied to the supermassive black hole at the Galactic Centre. We obtain that in both pseudotensorial prescriptions the energy distribution depends on the mass $M$ of the black hole, the phantom constant $p$ and the radial coordinate $r$. Further, all the calculated momenta are found to be zero. The limiting cases $r\rightarrow 0$, $r\rightarrow \infty $ and $r\rightarrow -\infty $ have also been the subject of the study.

gr-qc↗

arXiv2019

Wormholes in exponential $f(R,T)$ gravity

Alternative gravity is nowadays an extremely important tool to address some persistent observational issues, such as the dark sector of the universe. They can also be applied to stellar astrophysics, leading to outcomes one step ahead of those obtained through General Relativity. In the present article we test a novel $f(R,T)$ gravity model within the physics and geometry of wormholes. The $f(R,T)$ gravity is a reputed alternative gravity theory in which the Ricci scalar $R$ in the Einstein-Hilbert gravitational lagrangian is replaced by a general function of $R$ and $T$, namely $f(R,T)$, with $T$ representing the trace of the energy-momentum tensor. We propose, for the first time in the literature, an exponential form for the dependence of the theory on $T$. We derive the field equations as well as the non-continuity equation and solve those to wormhole metric and energy-momentum tensor. The importance of applying alternative gravity to wormholes is that through these theories it might be possible to obtain wormhole solutions satisfying the energy conditions, departing from General Relativity well-known outcomes. In this article, we indeed show that it is possible to obtain wormhole solutions satisfying the energy conditions in the exponential $f(R,T)$ gravity. Naturally, there is still a lot to do with this model, as cosmological, galactical and stellar astrophysics applications, and the reader is strongly encouraged to do so, but, anyhow, one can see the present outcomes as a good indicative for the theory.

gr-qc↗

arXiv2019

Phantom fluid wormhole in $f(R,T)$ gravity

Wormholes (WHs) are considered as hypothetical shortcuts or tunnels in spacetime. In general relativity (GR), the fundamental ingredient of WH geometry is the presence of exotic matter at the throat, which is responsible for the violation of null energy condition (NEC). However, the modified gravity theories has shown to be able to provide WH solutions satisfying energy conditions (ECs). In this paper, we study the static spherically symmetric WH solutions in modified $f(R,T)$ gravity for a phantom fluid case. The exact solutions of this model are obtained through the equation of state (EoS), $p=ωρ$, associated with phantom dark energy (DE) $ω<-1$. We find the existence of spherically symmetric WH solution supported by phantom energy distribution. The shape function of the WH is obtained in this model obeys all the WH metric conditions. In modified gravity scenario the phantom fluid WH violates the NEC in radial case, unlike in the tangential case. Furthermore, using the "volume integral quantifier" (VIQ) method, the total amount of EC violating matter in spacetime is discussed briefly.

gr-qc↗

arXiv2019

A cosmological scenario from the Starobinsky model within the $f(R,T)$ formalism

In this paper we derive a novel cosmological model from the $f(R,T)$ theory of gravitation, for which $R$ is the Ricci scalar and $T$ is the trace of the energy-momentum tensor. We consider the functional form $f(R,T)=f(R)+f(T)$, with $f(R)$ being the Starobinksy model, named $R+αR^{2}$, and $f(T)=2γT$, with $α$ and $γ$ being constants. We show that a hybrid expansion law form for the scale factor is a solution for the derived Friedmann-like equations. In this way, the model is able to predict both the decelerated and the accelerated regimes of expansion of the universe, with the transition redshift between these stages being in accordance with recent observations. We also apply the energy conditions to our material content solutions. Such an application makes us able to obtain the range of acceptability for the free parameters of the model, named $α$ and $γ$.

gr-qc↗

arXiv2019

Phantom energy dominated universe as a transient stage in $f(R)$ cosmology

The $f(R)$ theories of gravity are the most popular, simple and well succeeded extension of Einstein's General Relativity. They can account for some observational issues of standard cosmology with no need for evoking the dark sector of the universe. In the present letter we will investigate LRS Bianchi type-I space-time in $f(R)$ gravity theory within the phantom energy dominated era. We show that in this formalism the phantom energy dominated universe is a transient stage and in the further stage of the universe dynamics, it is dominated, once again, by dark energy. Such an important feature is obtained from the model, rather than imposed to it, and may have a relation to loop quantum cosmology.

gr-qc↗

arXiv2018

$f(R,T)=f(R)+λT$ gravity models as alternatives to cosmic acceleration

This article presents cosmological models that arise in a subclass of $f(R,T)=f(R)+f(T)$ gravity models, with different $f(R)$ functions and fixed $T$-dependence. That is, the gravitational lagrangian is considered as $f(R,T)=f(R)+λT$, with constant $λ$. Here $R$ and $T$ represent the Ricci scalar and trace of the stress-energy tensor, respectively. The modified gravitational field equations are obtained through the metric formalism for the Friedmann-Lemaître-Robertson-Walker metric with signature $(+,-,-,-)$. We work with $f(R)=R+αR^2-\frac{μ^4}{R}$, $f(R)=R+k\ln(γR)$ and $f(R)=R+me^{[-nR]}$, with $α, μ, k, γ, m$ and $n$ all free parameters, which lead to three different cosmological models for our Universe. For the choice of $λ=0$, this reduces to widely discussed $f(R)$ gravity models. This manuscript clearly describes the effects of adding the trace of the energy-momentum tensor in the $f(R)$ lagrangian. The exact solution of the modified field equations are obtained under the hybrid expansion law. Also we present the Om diagnostic analysis for the discussed models.

gr-qc↗

arXiv2018

A periodic varying deceleration parameter in $f(R,T)$ gravity

The phenomenon of accelerated expansion of the present universe and a cosmic transit aspect is explored in the framework of a modified gravity theory known as $f(R,T)$ gravity (where $R$ is the Ricci scalar and $T$ is the trace of the energy momentum tensor of the matter content). The cosmic transit phenomenon signifies a signature flipping behaviour of the deceleration parameter. We employ a periodic varying deceleration parameter and obtained the exact solution of field equations. The dynamical features of the model including the oscillatory behaviour of the EOS parameter are studied. We have also explored the obvious violation of energy momentum conservation in $f(R,T)$ gravity. The periodic behaviour of energy conditions for the model are also discussed with a wide range of the free parameters.

gr-qc↗

arXiv2018

Phantom fluid supporting traversable wormholes in alternative gravity with extra material terms

Wormholes are tunnels connecting different regions in space-time. They were obtained originally as a solution for Einstein's General Relativity theory and according to this theory they need to be filled by an exotic kind of anisotropic matter. In the present sense, by "exotic matter" we mean matter that does not satisfy the energy conditions. In this article we propose the modelling of wormholes within an alternative gravity theory that proposes an extra material (rather than geometrical) term in its gravitational action. Our solutions are obtained from well-known particular cases of the wormhole metric potentials, named redshift and shape functions, and yield the wormholes to be filled by a phantom fluid, that is, a fluid with equation of state parameter $ω<-1$. In possession of the solutions for the wormhole material content, we also apply the energy conditions to them. The features of those are carefully discussed.

gr-qc↗

arXiv2018

Wormholes in $R^2$-gravity within the $f(R,T)$ formalism

We propose, as a novelty in the literature, the modelling of wormholes within the particular case of the $f(R,T)$ gravity, namely $f(R,T)=R+αR^{2}+λT$, with $R$ and $T$ being the Ricci scalar and trace of the energy-momentum tensor, respectively, while $α$ and $λ$ are constants. Although such a functional form application can be found in the literature, those concern to compact astrophysical objects, such that no wormhole analysis has been done so far. The quadratic geometric and linear material corrections of this theory make the matter content of the wormhole to remarkably be able to obey the energy conditions.

gr-qc↗

arXiv2018

A study of electron and thermal transport in layered Titanium disulphide single crystals

We present a detailed study of thermal and electrical transport behavior of single crystal Titanium disulphide flakes, which belongs to the two dimensional, transition metal dichalcogenide class of materials. In-plane Seebeck effect measurements revealed a typical metal-like linear temperature dependence in the range of 85 - 285 K. Electrical transport measurements with in-plane current geometry exhibited a nearly T^2 dependence of resistivity in the range of 10 - 300 K. However, transport measurements along the out-of-plane current geometry showed a transition in temperature dependence of resistivity from T^2 to T^5 beyond 200 K. Interestingly, Au ion-irradiated TiS2 samples showed a similar T 5 dependence of resistivity beyond 200 K, even in the current-in-plane geometry. Micro- Raman measurements were performed to study the phonon modes in both pristine and ion-irradiated TiS2 crystals.

cond-mat.mes-hall↗

arXiv2018

Non-exotic matter wormholes in a trace of the energy-momentum tensor squared gravity

Wormholes are tunnels connecting two different points in space-time. In Einstein's General Relativity theory, wormholes are expected to be filled by exotic matter, i.e., matter that does not satisfy the energy conditions and may have negative density. We propose, in this paper, the achievement of wormhole solutions with no need for exotic matter. In order to achieve so, we consider quadratic terms in the trace of the energy-momentum tensor as corrections to the effective energy-momentum tensor of the underlined theory of gravity. We show that by following this formalism, it is possible, indeed, to obtain non-exotic matter wormhole solutions.

gr-qc↗

arXiv2017

Magnetized strange quark matter in $f(R,T)$ gravity with bilinear and special form of time varying deceleration parameter

In this paper, we have studied homogeneous and anisotropic, locally rotationally symmetric (LRS) Bianchi type-I universe model with magnetized strange quark matter (MSQM) distribution and cosmological constant $Λ$ in $f(R,T)$ gravity. The exact solutions of the field equations are obtained under bilinear and special form of time varying deceleration parameter (DP). Firstly we have considered two specific form of bilinear DP with a single parameter of the form: $q=\frac{α(1-t)}{1+t}$ and $q=-\frac{αt}{1+t}$, which leads to a function with constant or linear nature with respect to the choice of constant $α$. Second one is the special form of the DP as $q=-1+\fracβ{1+a^β}$. From obtained results, we could say that in the early universe the magnetic flux has more effects and it gradually reducing its effects in later. For $t\rightarrow \infty$, we get $p\rightarrow -B_c$ and $ρ\rightarrow B_c$. So the strange quark matter along with magnetic epoch gives an idea of accelerated expansion of the universe as per the observations of the type Ia Supernovae.

gr-qc↗