arXiv subjects

P. K. Sahoo

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

At least 163 records · Page 9Linked to original sources

arXiv2020

Energy conditions in $f(Q,T)$ gravity

The recently proposed $f(Q, T)$ gravity (Xu et al. Eur. Phys. J. C \textbf{79} (2019) 708) is an extension of the symmetric teleparallel gravity. The gravitational action $L$ is given by an arbitrary function $f$ of the non-metricity $Q$ and the trace of the matter-energy momentum tensor $T$. In this paper, we examined the essence of some well prompted forms of $f(Q,T)$ gravity models i.e. $f(Q,T)= mQ+bT$ and $f(Q,T)= m Q^{n+1}+b T$ where $m$, $b$, and $n$ are model parameters. We have used the proposed deceleration parameter, which predicts both decelerated and accelerated phases of the Universe, with the transition redshift by recent observations and obtains energy density ($ρ$) and pressure ($p$) to study the various energy conditions for cosmological models. The equation of state parameter ($ω\simeq -1$) in the present model also supports the accelerating behavior of the Universe. In both, the models, the null, weak, and dominant energy conditions are obeyed with violating strong energy conditions as per the present accelerated expansion.

gr-qc↗

arXiv2020

A Complete Cosmological Scenario in Teleparallel Gravity

Teleparallel gravity is a modified theory of gravity in which the Ricci scalar $R$ of the Lagrangian replaced by the general function of torsion scalar $T$ in action. With that, cosmology in teleparallel gravity becomes profoundly simplified because it is second-order theory. The article present a complete cosmological scenario in $f(T)$ gravity with $f(T)=T+β(-T)^α$, where $α,$ and $β$ are model parameters. We present the profiles of energy density, pressure, and equation of state (EoS) parameter. Next to this, we employ statefinder diagnostics to check deviation from the $Λ$CDM model as well as the nature of dark energy. Finally, we discuss the energy conditions to check the consistency of our model and observe that SEC violates in the present model supporting the acceleration of the Universe as per present observation.

physics.gen-ph↗

arXiv2020

Energy Conditions in $f(Q)$ gravity

A complete theory of gravity impels us to go beyond Einstein's General Relativity. One promising approach lies in a new class of teleparallel theory of gravity named $f(Q)$, where the nonmetricity $Q$ is responsible for the gravitational interaction. The important roles any of these alternative theories should obey are the energy condition constraints. Such constraints establish the compatibility of a given theory with the causal and geodesic structure of space-time. In this work, we present a complete test of energy conditions for $f(Q)$ gravity models. The energy conditions allowed us to fix our free parameters, restricting the families of $f(Q)$ models compatible with the accelerated expansion our Universe passes through. Our results straight the viability of $f(Q)$ theory, leading us close to the dawn of a complete theory for gravitation.

gr-qc↗

arXiv2020

Effective equation of state in modified gravity and observational constraints

In this article, the bulk viscosity is introduced in a modified gravity model. The gravitational action has a general $f(R,T)$ form, where $R$ and $ T $ are the curvature scalar and the trace of energy momentum tensor respectively. An effective equation of state (EoS) has been investigated in the cosmological evolution with bulk viscosity. In the present scenario, the Hubble parameter which has a scaling relation with the redshift can be obtained generically. The role of deceleration parameter $q$ and equation of state parameter $ω$ is discussed to explain the late-time accelerating expansion of the universe. The statefinder parameters and Om diagnostic analysis are discussed for our obtained model to distinguish from other dark energy models together with the analysis of energy conditions and velocity of sound for the model. We have also numerically investigated the model by detailed maximum likelihood analysis of $580$ Type Ia supernovae from Union $ 2.1$ compilation datasets and updated $57$ Hubble datasets ($31$ data points from differential age method and $26$ points from BAO and other methods). It is with efforts found that the present model is in good agreement with observations.

gr-qc↗

arXiv2020

Late-Time Viscous Cosmology in $f(R,T)$ Gravity

The article communicates an alternative route to suffice the late-time acceleration considering a bulk viscous fluid with viscosity coefficient $ζ=ζ_{0}+ ζ_{1} H + ζ_{2} H^{2}$, where $ζ_{0}, ζ_{1}, ζ_{2}$ are constants in the framework of $f(R,T)$ modified gravity. We presume the $f(R,T)$ functional form to be $f=R+2αT$ where $α$ is a constant. We then solve the field equations for the Hubble Parameter and study the cosmological dynamics of kinematic variables such as deceleration, jerk, snap and lerk parameters as a function of cosmic time. We observe the deceleration parameter to be highly sensitive to $α$ and undergoes a signature flipping at around $t\sim 10$ Gyrs for $α=-0.179$ which is favored by observations. The EoS parameter for our model assumes values close to $-1$ at $t_{0}=13.7$Gyrs which is in remarkable agreement with the latest Planck measurements. Next, we study the evolution of energy conditions and find that our model violate the Strong Energy Condition in order to explain the late-time cosmic acceleration. To understand the nature of dark energy mimicked by the bulk viscous baryonic fluid, we perform some geometrical diagnostics like the $\{r,s\}$ and $\{r,q\}$ plane. We found the model to mimic the nature of a Chaplygin gas type dark energy model at early times while a Quintessence type in distant future. Finally, we study the violation of continuity equation for our model and show that in order to explain the cosmic acceleration at the present epoch, energy-momentum must violate.

gr-qc↗

arXiv2020

$f(Q,T)$ gravity models with observational constraints

The paper presents late time cosmology in $f(Q,T)$ gravity where the dark energy is purely geometric in nature. We start by employing a well motivated $f(Q,T)$ gravity model, $f(Q,T)=mQ^{n}+bT$ where $m,n$ and $b$ are model parameters. Additionally we also assume the universe to be dominated by pressure-less matter which yields a power law type scale factor of the form $% a(t)=c_{2}(At+c_{1})^{\frac{1}{A}}$, where $A=\dfrac{3(8π+b)}{n(16π+3b)% }$ and $c_{1}$ \& $c_{2}$ are just integration constants. To investigate the cosmological viability of the model, constraints on the model parameters were imposed from the updated 57 points of Hubble data sets and 580 points of union 2.1 compilation supernovae data sets. We have thoroughly investigated the nature of geometrical dark energy mimicked by the parametrization of $f(Q,T)=mQ^{n}+bT$ with the assistance of statefinder diagnostic in $\{s,r\}$ and $\{q,r\}$ planes and also performed the $Om$ -diagnostic analysis. The present analysis makes it clear-cut that $f(Q,T)$ gravity can be promising in addressing the current cosmic acceleration and therefore a suitable alternative to the dark energy problem. Further studies in other cosmological areas are therefore encouraging to further investigate the viability of $f(Q,T)$ gravity.

gr-qc↗

arXiv2020

Inflation in $f(R,T)$ Gravity

The article presents modeling of inflationary scenarios for the first time in the $f(R,T)$ theory of gravity. We assume the $f(R,T)$ functional from to be $R + ηT$, where $R$ denotes the Ricci scalar, $T$ the trace of the energy-momentum tensor and $η$ the model parameter (constant). We first investigated an inflationary scenario where the inflation is driven purely due to geometric effects outside of GR. We found the inflation observables to be independent of the number of e-foldings in this setup. The computed value of the spectral index is consistent with latest Planck 2018 dataset while the scalar to tensor ratio is a bit higher. We then proceeded to analyze the behavior of an inflation driven by $f(R,T)$ gravity coupled with a real scalar field. By taking the slow-roll approximation, we generated interesting scenarios where a Klein Gordon potential leads to observationally consistent inflation observables. Our results makes it clear-cut that in addition to the Ricci scalar and scalar fields, the trace of energy momentum tensor also play a major role in driving inflationary scenarios.

gr-qc↗

arXiv2020

Constraining f(R,T) Gravity From The Dark Energy Density Parameter $Ω_Λ$

$f(R,T)$ gravity is a widely used extended theory of gravity introduced in \cite{9} which is a straightforward generalization of $f(R)$ gravity. The action in this extended theory of gravity incorporates well motivated functional forms of the Ricci scalar $R$ and trace of energy momentum tensor $T$. The present manuscript aims at constraining the most widely used $f(R,T)$ gravity model of the form $f(R+2λT)$ to understand its coherency and applicability in cosmology. We communicate here a novel method to find an lower bound on the model parameter $λ\gtrsim -1.9 \times 10^{-8}$ through the equation relating the cosmological constant ($Λ$) and the critical density of the universe ($ρ_{cr}$).

gr-qc↗

arXiv2020

Redshift Drift in $f(R,T)$ Gravity

Redshift drift refers to the phenomena that redshift of cosmic objects is a function of time. Measurement of redshift drift is of fundamental importance in physical cosmology and can be utilized to distinguish different cosmological models. Redshift drift can be expressed in two distinct methods. The first method is related to cosmography, where the Redshift drift is given as a series expansion of cosmological parameters, while the second method is written as a function of Hubble parameter and its time derivatives which ultimately involve field equations of a chosen theory of gravity. By equating corresponding terms from both the series, the model parameter(s) of any modified theory of gravity can be constrained. The present note aims at constraining the model parameter $ζ$ of $f(R,T)$ gravity theory where $f(R,T)= R + ζT$. By equating linear terms in redshift $z$ from both the series, we constrain $ζ$ in the range $-0.51 κ^{2} \lesssim ζ\lesssim -0.47 κ^{2}$, where $κ^{2}=\frac{8 πG}{c^{4}}$.

gr-qc↗

arXiv2020

Localization of Energy and Momentum in an Asymptotically Reissner-Nordström Non-singular Black Hole Space-time Geometry

The space-time geometry exterior to a new four-dimensional, spherically symmetric and charged black hole solution that, through a coupling of general relativity with a non-linear electrodynamics, is everywhere non-singular, for small $r$ it behaves as a de Sitter metric, and asymptotically it behaves as the Reissner-Nordström metric, is considered in order to study the energy-momentum localization. For the calculation of the energy and momentum distributions, the Einstein, Landau-Lifshitz, Weinberg and Møller energy-momentum complexes have been applied. The results obtained show that in all prescriptions the energy depends on the mass $M$ of the black hole, the charge $q$, two parameters $% a\in \mathbb{Z}^+$ and $γ\in \mathbb{R}^+$, and on the radial coordinate $r$. The calculations performed in each prescription show that all the momenta vanish. Additionally, some limiting and particular cases for $r$ and $q$ are studied, and a possible connection with strong gravitational lensing and micro lensing is attempted.

gr-qc↗

arXiv2020

Wormhole model with a hybrid shape function in f(R,T) gravity

In the present article we propose a new hybrid shape function for wormhole (WH)s in the modified $f(R,T)$ gravity. The proposed shape function satisfied the conditions of WH geometry. Geometrical behavior of WH solutions are discussed in both anisotropic and isotropic cases respectively. Also, the stability of this model is obtained by determining the equilibrium condition. The radial null energy condition and weak energy condition are validated in the proposed shape function indicating the absence of exotic matter in modified $f(R,T)$ gravity.

gr-qc↗

arXiv2020

Accelerating Universe in Hybrid and Logarithmic Teleparallel Gravity

Teleparallel gravity is a modified theory of gravity for which the Ricci scalar $R$ of the underlying geometry in the action is replaced by an arbitrary functional form of torsion scalar $T$. In doing so, cosmology in $% f(T)$ gravity becomes greatly simplified owing to the fact that $T$ contains only the first derivatives of the vierbeins. The article exploits this appealing nature of $f(T)$ gravity and present cosmological scenarios from hybrid and logarithmic teleparallel gravity models of the form $% f=e^{mT}T^n $ and $f=D\log(bT)$ respectively, where $m$, $n$, $D$ and $b$ are free parameters constrained to suffice the late time acceleration. We employ a well motivated parametrization of the deceleration parameter having just one degree of freedom constrained with a $χ^{2}$ test from 57 data points of Hubble data set in the redshift range $0.07<z<2.36$, to obtain the expressions of pressure, density and EoS parameter for both the teleparallel gravity models and study their temporal evolution. We find the deceleration parameter to experience a signature flipping for the $χ^{2}$ value of the free parameter at $z_{tr}\simeq0.6$ which is consistent with latest Planck measurements. Next, we present few geometric diagnostics of this parametrization to understand the nature of dark energy and its deviation from the $Λ$CDM cosmology. Finally, we study the energy conditions to check the consistency of the parameter spaces for both the teleparallel gravity models. We find the SEC to violate for both the models which is an essential recipe to obtain an accelerating universe.

physics.gen-ph↗

arXiv2020

Big Bang Nucleosynthesis and Entropy Evolution in $f(R,T)$ Gravity

The present article is devoted to constrain the model parameter $χ$ for the $f(R,T)= R + χT$ gravity model by employing the constraints coming from big bang nucleosynthesis. We solve the field equations and constrain $χ$ in the range $-0.14 κ^{2} \leq χ\leq 0.84 κ^{2}$ (where $κ^{2} = \frac{8 πG}{c^{4}}$) from the primordial abundances of light elements such as helium-4, deuterium and lithium-7. We found the abundances of helium-4 and deuterium agrees with theoretical predictions, however the lithium problem persists for the $f(R,T)$ gravity model. We also investigate the evolution of entropy for the constrained parameter space of $χ$ for the radiation and dust universe. We report that entropy is constant when $χ= 0$ for the radiation dominated universe, whereas for the dust universe, entropy increases with time. We finally use the constraints to show that $χ$ has negligible influence on the cold dark matter annihilation cross section.

physics.gen-ph↗

arXiv2020

Viability of the $R+e^T$ cosmology

We propose a new theoretical approach for a cosmological model, which starts from an exponential of the trace of the energy-momentum tensor-dependence on the gravitational action, to be summed to the Ricci scalar. We derive the referred field equations and Friedmann-like equations. We derive the scale factor, Hubble parameter and deceleration parameter, in terms of both time and redshift. In possession of those parameters in terms of the redshift, we confront their predictions with the observational Hubble dataset and the outcomes are pretty satisfactory so that the model can be seen as a new alternative to the cosmological constant problem. We also present the statefinder diagnostic and discuss profoundly the dynamical behaviour of the model.

gr-qc↗

arXiv2020

Comprehensive Analysis of a Non-Singular Bounce in $f(R,T)$ Gravitation

The present paper is devoted to the study of bouncing cosmology in $f(R,T)$ modified gravity where we presume $f(R,T) = R + 2 λT$, with $R$ the Ricci scalar, $T$ the trace of energy momentum tensor and $λ$ the model parameter. We present here a novel parametrization of Hubble parameter which is apt in representing a successful bouncing scenario undergoing no singularity. We proceed to present a complete analysis of the proposed bouncing model by studying the evolution of primordial curvature perturbations, energy conditions and stability against linear homogeneous perturbations in flat space-time. We also delineate bouncing cosmology for the proposed model by employing Quintom matter. The present article further communicate for the first time that violation of energy-momentum materialize for both the contracting and expanding universes except for the bouncing epoch with energy flow directed away and into the matter fields for the contracting and expanding universe respectively. We further present a thorough investigation about the feasibility of the proposed bouncing scenario against first and generalized second law of thermodynamics. We found that the proposed bouncing scenario obeys the laws of thermodynamics for the constrained parameter space of $λ$. The manuscript conclude after investigating the viability of the proposed bouncing model in non minimal $f(R,T)$ gravity where $f(R,T) = R + χR T$.

gr-qc↗

arXiv2020

Baryogenesis in $f(Q,\mathcal{T})$ Gravity

The article communicates exploration of gravitational baryogenesis in presence of $f(Q,\mathcal{ T})$ gravity where $Q$ denote the nonmetricity and $\mathcal{ T}$ the trace of the energy momentum tensor. We study various baryogenesis interactions proportional to $\dot{Q}$ and $\dot{Q}f_{Q}$ for the $f(Q,\mathcal{ T})$ gravity model $f(Q,\mathcal{ T})=αQ ^{n+1} + β\mathcal{ T}$, where $α$, $β$ and $n$ are model parameters. Additionally we report the viable parameter spaces for which an observationally consistent baryon-to-entropy can be generated. Our results indicate that $f(Q,\mathcal{ T})$ gravity can contribute significantly and consistently to the phenomenon of gravitational baryognesis.

physics.gen-ph↗

arXiv2020

Existence of bulk viscous universe in $f(R,T)$ gravity and confrontation with observational data

In this paper we have investigated a bulk viscous universe in $f(R,T)$ gravity where $R$ and $T$ are the Ricci scalar and trace of energy momentum tensor respectively. We have obtained explicit solutions of field equations in modified gravity by considering the power law form of scale factor. The Hubble parameter and deceleration parameter are derived in terms of cosmic time and redshift both. We have estimated the present values of these parameters with observational Hubble data and SN Ia data sets. At 1$σ$ level, the estimated values of $q_{0}$ and $m$ are obtained as $q_{0}=-0.30 \pm 0.05$ \& $ m = 0.70 \pm 0.02 $ where $q_{0}$ is the present value of deceleration parameter and $m$ is the model parameter. The energy conditions and Om(z) analysis for the anisotropic LRS Bianchi type I model are also discussed.

physics.gen-ph↗

arXiv2020

Gravitational Baryogenesis in Non-Minimal Coupled $f(R,T)$ Gravity

Baryogenesis refers to the theoretical process that occurred in the early history of the universe producing excess of matter over antimatter. Hitherto, fundamental physics which can give rise to such events is unknown. Many theories have emerged which may be able to suffice this conundrum, although concrete understanding is obscure. Gravitational baryogenesis is one such theory introduced by H. Davoudiasl, et al., Phys. Rev. Lett. \textbf{93} (2004) 201301. In this theory, for a CP violating interaction proportional to $\partial_μR$, where R denote Ricci scalar, gravitational baryogenesis is inevitable. In this paper, we have investigated the consequences of CP violating interactions proportional to $\partial_μR$, $\partial_μT$ and $\partial_μf(R,T)$ in the framework of non-minimal f(R,T) gravity. We found that for interactions proportional to $\partial_μR$ and $\partial_μf(R,T)$, baryogenesis can be realized with suitable parameter spaces producing a net baryon asymmetry factor consistent with observations, whereas for interaction proportional to $\partial_μT$, results in a baryon asymmetry incompatible with observations.rther, we obtained baryon to entropy ratio in each case and put constraints on parameters spaces of our model.

physics.gen-ph↗