Browsing by Author "R. Chaubey"

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A new class of Bianchi cosmological models in f(R,T) gravity
(Kluwer Academic Publishers, 2013) R. Chaubey; A.K. Shukla
The new class of cosmological model of the early Universe is considered with f(R,T) modified theories of gravity (Harko et al. in Phys. Rev. D 84:024020, 2011). The exact solutions to the corresponding field equations are obtained in quadrature form. The cosmological parameters have been discussed in detail. We have also discussed the well-known astrophysical phenomena, namely the Hubble parameter H(z), luminosity distance (dL) and distance modulus μ(z) with redshift. © 2012 Springer Science+Business Media B.V.
A New Class of Bianchi Cosmological Models in Lyra's Geometry
(Springer Science and Business Media, LLC, 2013) R. Chaubey; A.K. Shukla
The new class of cosmological model of the early Universe filled with perfect fluid in Lyra's geometry has been considered. We obtain two classes of exact solutions of the field equations in Lyra's geometry with a time-dependent displacement vector. The exact solutions to the corresponding field equations are obtained in quadrature form. The cosmological parameters have been discussed in detail and it is also shown that the solutions tend asymptotically to isotropic Friedmann-Robertson-Walker cosmological model. We have also discussed the well-known astrophysical phenomena, namely the Hubble parameter H(z), luminosity distance dL and distance modulus μ(z) with redshift. © 2012 Springer Science+Business Media New York.
An interacting two-fluid scenario for dark energy in anisotropic cosmological model
(2013) T. Singh; R. Chaubey
The open anisotropic cosmological model of the early Universe is considered. We study the evolution of the dark energy parameter within anisotropic Bianchi type-V cosmological model filled with barotropic fluid and dark energy. The solutions have been obtained for power law and exponential forms of the expansion parameter. (They correspond to constant deceleration parameter in general relativity.) For large time (i.e., t → ∞), the models tend asymptotically to an isotropic Friedmann-Robertson-Walker cosmological model under certain conditions. © 2013 Published by NRC Research Press.
Aspects of non-interacting and interacting universe: a complete scenario
(Canadian Science Publishing, 2023) Rakesh Raushan; A.K. Shukla; R. Chaubey
The evolution of the dark energy within the scope of a spatially homogeneous and isotropic Friedmann–Robertson–Walker model filled with barotropic fluid and dark energy is examined. We have considered the two cases of an interacting and non-interacting two-fluid (barotropic and dark energy) scenario and obtained the exact solutions to the corresponding field equations. In an interacting model, we have considered linear as well as quadratic interaction between dark sectors of the universe. We have also given the quadrature form for general interaction coupling parameter via Q = αρnm, n ≥ 1 and discussed the evolution of energy densities and effective equation of state with cosmic time for flat, closed, and open universes. It is also examined that the matter energy density is dominated in early phase of the universe, whereas the dark energy density is dominated in later phase of the universe for both scenarios, which is in concurrence with current observations. © 2022 The Author(s).
Aspects of nonflat FRW bouncing models with quadratic equation of state
(World Scientific Publishing Co. Pte Ltd, 2018) Ashutosh Singh; Rakesh Raushan; R. Chaubey; T. Singh
In this paper, we investigate the possibility of a nonsingular model of universe in the framework of general relativity in nonflat FRW geometries with quadratic equation of state and bulk viscosity. We study whether a nonsingular bounce requires violation of energy conditions. We discuss the thermodynamical aspects of the resulting models with equilibrium description. In particular, we discuss the validity of the generalized second law of thermodynamics for resulting cosmologies. © 2018 World Scientific Publishing Company.
Bianchi type I cosmological model with a cosmological constant (Λ) in creation-field cosmology
(Maik Nauka Publishing / Springer SBM, 2015) R. Chaubey; A.K. Shukla; A. Singh; T. Singh
Solutions of Einstein’s field equations with a cosmological constant Λ in the presence of a creation field have been obtained for Bianchi type I cosmological models in the Hoyle-Narlikar creation field theory. The cosmological parameters are calculated. We examine the nature of the creation field and the cosmological constant which dominate at the early stage of the universe, but they do not survive for a long time and finally tend to zero at large cosmic times. © 2015, Pleiades Publishing, Ltd.
Bianchi type-V bulk viscous cosmological models with particle creation in Brans-Dicke theory
(2012) R. Chaubey
The Bianchi type-V cosmological model with viscous fluid and creation particle in Brans-Dicke theory has been considered. The present paper deals with Bianchi type-V cosmological model with bulk viscosity and particle creation described by full causal thermodynamics in Brans-Dicke theory. We have discussed two types of solutions of the average scale factor for a Bianchi type-V model by using a variation law of Hubble's parameter, which yields a constant value of the deceleration parameter. The exact solutions to the corresponding field equations are obtained in quadrature form. The solutions to the Einstein field equations are obtained for power law and exponential form. The cosmological parameters have been discussed in detail. © 2012 Springer Science+Business Media B.V.
Bianchi Type-V Cosmological Model with Two Interacting Scalar Fields
(Springer Science and Business Media, LLC, 2013) R. Chaubey
The open anisotropic cosmological model of the early Universe is considered. Two interacting scalar fields with special form of potential energy are a source of matter fields. Analytic solutions for inflationary and scalaron stages are found. The exact solutions to the corresponding field equations are obtained in quadrature form. The cosmological parameters have been discussed in detail and it is also shown that the solutions tend asymptotically to isotropic Friedmann-Robertson-Walker cosmological model. © 2012 Springer Science+Business Media New York.
Bounce conditions for FRW models in modified gravity theories
(Springer Verlag, 2015) T. Singh; R. Chaubey; Ashutosh Singh
In this paper, we have derived the minimal conditions for a bounce to occur in FRW cosmologies for the theories like Hoyle-Narlikar creation field theory, Lyra geometry, Brans-Dicke theory, general class of Scalar-Tensor theories, Einstein’s theory with variable cosmological term, with bulk viscosity and Finslerian cosmology. We derive the model-independent minimal necessary conditions for non-singular bounce and show that there is an open temporal region surrounding the bounce over which the strong energy condition (SEC) must be violated. Null energy condition is also violated in some of the modified gravity theories. © 2015, Società Italiana di Fisica and Springer-Verlag Berlin Heidelberg.
Bounce conditions in Kantowski-Sachs and Bianchi cosmologies in modified gravity theories
(World Scientific Publishing Co. Pte Ltd, 2015) T. Singh; R. Chaubey; Ashutosh Singh
Many cosmological scenarios envisage either a bounce of the universe at early times, or a collapse locally to form a black hole which re-expands into a new expanding universe region. Energy conditions preclude this phenomena for ordinary matter in general relativistic universe, but scalar or other types of fields can violate some of these conditions, and so can, possibly, provide conditions for a bouncing universe. In this paper, we have investigated the necessary conditions for a bounce in Kantowski-Sachs (KS) and Bianchi type models in some modified theories of gravity like Hoyle-Narlikar creation field theory, Lyra geometry, general class of scalar-tensor theories, Einstein's theory with variable cosmological term. © 2015 World Scientific Publishing Company.
Bouncing cosmologies in Brans-Dicke theory
(Canadian Science Publishing, 2016) T. Singh; R. Chaubey; Ashutosh Singh
In this paper it is shown that in Brans-Dicke theory, if one considers a non-minimal coupling between the matter and scalar field, can give rise to a bouncing universe (i.e., an expanding universe preceded by a contracting universe). Two examples of such universes have been considered in a spatially flat Friedmann-Robertson-Walker universe and their physical properties have been studied. © 2016 Published by NRC Research Press.
Bouncing cosmologies with viscous fluids
(Springer Netherlands, 2016) T. Singh; R. Chaubey; Ashutosh Singh
The bounce in viscous fluid cosmology with inhomogeneous viscous fluids in Friedman-Robertson-Walker (FRW) space-time has been investigated. Different forms for the scale factor have been considered. The general features of the fluids which realize them and the possibility to have an acceleration after the bounce have been discussed. © 2016, Springer Science+Business Media Dordrecht.
Chameleon scalar field in LRS Bianchi type I cosmological model
(Maik Nauka Publishing / Springer SBM, 2016) R. Chaubey; Ashutosh Singh; Rakesh Raushan
We consider an anisotropic cosmological model with cold dark matter and a scalar field, where one component of the scale factor is taken in the framework of (i) a logamediate scenario (ii) an intermediate scenario, and (iii) an emergent scenario. In all cases we find expressions for the Chameleon field, Chameleon potential, the statefinder diagnostic pair, i.e., the {r, s} parameters, and the slow-roll parameters. All physical parameters are calculated and discussed in all three cases. It is also shown how the Chameleon field is directly affected by the role of curvature of space time. At large times (t→∞) the models tend asymptotically to an isotropic Friedmann–Robertson–Walker cosmological model. © 2016, Pleiades Publishing, Ltd.
Cosmic evolution of holographic dark energy in f (Q, T) gravity
(World Scientific, 2023) S. Mandal; A. Singh; R. Chaubey
In this paper, we investigate the dynamic evolution of universe in the models of holographic dark energy with f(Q,T) gravity framework where, Q is the non-metricity scalar and T is the energy-momentum tensor trace. We have considered f(Q,T) = αQ + βT framework and investigated the evolution of cosmological quantities like energy density, equation of state (EoS) parameter and classical stability parameter with redshift. We parameterize the deceleration parameter and confront the Hubble parameter with the observational data. We investigate for the late-time accelerated expansion of universe and discuss the stability of the model by using adiabatic sound speed squared parameter. A comparison among these derived models suggest that the Renyi holographic dark energy model with GO cutoff satisfies the observational constraint of Planck+SNe+BAO on EoS parameter at present time as compared to other models. Diagnostic tools such as Statefinders and Om diagnostic have been used to classify the dark energy evolution regions. © 2023 World Scientific Publishing Company.
Dark Energy with Generalized Equation of State in Bianchi Type-I Cosmological Model
(2012) T. Singh; R. Chaubey
Dark energy with the usually used equation of state p=γρ, where γ=const < 0 is hydrodynamically unstable. To overcome this drawback we consider the cosmology of a perfect fluid with a linear equation of state of a more general form p=α(ρ-ρ0), where the constants α and ρ0 are free parameters. The anisotropic Bianchi type-I cosmological model filled with dark energy has been considered. A generalized equation of state for the dark energy component of the universe has been used. The exact solutions to the corresponding Einstein field equations and the statefinder diagnostic pair i. e. {r,s} parameters have been obtained in three interesting cases (i) when ρΛ > 0 and A > 0 (ii) when ρΛ > 0 and A > 0 and (iii) when ρΛ < 0 and A > 0 at the singularities i. e. t→0 and t→±∞. © 2011 Springer Science+Business Media, LLC.
Dynamical analysis of anisotropic cosmological model with quadratic dark sector coupling
(World Scientific Publishing Co. Pte Ltd, 2019) Rakesh Raushan; R. Chaubey
The present work deals with the dynamical evolution of LRS Bianchi type I (LRS BI) cosmological model with quadratic dark sector coupling. We investigate the phase-plane analysis of LRS BI model with dark energy, when dark energy is modeled as exponential quintessence, and is coupled to dark matter via energy exchange. The evolution of cosmological solutions are studied by using dynamical systems method. Stability and viability of the models are discussed for four different choices of quadratic dark sector coupling parameters. In each model, we have obtained a late-time accelerating fixed point (future attractor), which is not a scaling solution. © 2019 World Scientific Publishing Company.
Dynamical analysis of anisotropic cosmological model with quintessence
(Springer Netherlands, 2016) R. Chaubey; Rakesh Raushan
The present work is a phase-plane analysis of LRS Bianchi type I cosmological model with a scalar field and exponential potential. The evolution equations are reduced to an autonomous system of ordinary equations by suitable transformation of variables. We also analyse the evolution of the effective equation of state parameter for different values of curvature. The nature of critical points is analysed and stable attractors are examined from the point of view of cosmology. © 2016, Springer Science+Business Media Dordrecht.
Finsler–Randers Cosmological Models in Modified Gravity Theories
(Springer, 2019) R. Chaubey; B. Tiwari; Anjani Kumar Shukla; Manoj Kumar
Finsler–Randers (FR) cosmological models in modified gravity theories have been investigated in this paper.The de Sitter and power law solutions are assumed for the scale factor in the corresponding cosmological models.All energy conditions have been discussed in detail for each scenario. Behaviour of FR cosmological models in modified theories of gravity like Einstein theory, Hoyle–Narlikar creation field theory, lyra geometry and general class of scalar–tensor theories have also been investigated. © 2018, The National Academy of Sciences, India.
Finsler–Randers cosmology in the framework of a particle creation mechanism: a dynamical systems perspective
(Springer Science and Business Media Deutschland GmbH, 2020) Rakesh Raushan; R. Chaubey
The present work deals with the dynamical evolution of Finsler–Randers (FR) cosmological model in the framework of particle creation mechanism. The particle creation mechanism presented the real non-equilibrium thermodynamics of the Universe. We investigate the phase plane analysis of the model. The evolution of cosmological solutions is studied by using dynamical system techniques. We find one additional critical point in FR cosmological model than Riemannian geometry. Stability and viability of the model are discussed. We find the solution of FR cosmological model which provide an acceleration phase of the Universe under some restriction. It is also shown that the evolution of the model at additional critical point is de Sitter Universe and can have stable late time attractor. © 2020, Società Italiana di Fisica (SIF) and Springer-Verlag GmbH Germany, part of Springer Nature.
General class of anisotropic cosmological models with dilaton and magnetic fields
(National Research Council of Canada, 2014) R. Chaubey
The Einstein-Maxwell dilaton field equations are considered for four-dimensional anisotropic Bianchi models. The general solutions have been obtained and their properties have been discussed. The exact solutions to the corresponding field equations are obtained for two different physical viable cosmologies. The cosmological parameters have been discussed in detail and it is also shown that the solutions tend asymptotically to isotropic Friedmann-Robertson-Walker cosmological model. © 2013 Published by NRC Research Press.