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Pradhan A.,Hindu Post graduate College | Singh A.K.,Hindu Post graduate College
International Journal of Theoretical Physics | Year: 2011

The present study deals with a spatially homogeneous and anisotropic Bianchi type-I (B-I) cosmological models representing massive strings in normal gauge for Lyra's manifold by applying the variation law for generalized Hubble's parameter that yields a constant value of deceleration parameter. The variation law for Hubble's parameter generates two types of solutions for the average scale factor, one is of power-law type and other is of the exponential-law type. Using these two forms, Einstein's modified field equations are solved separately that correspond to expanding singular and non-singular models of the universe respectively. The energy-momentum tensor for such string as formulated by Letelier, P. S.: Phys. Rev. D 28, 2414 (1983) is used to construct massive string cosmological models for which we assume that the expansion (θ) in the model is proportional to the component σ1 1of the shear tensor σj i. This condition leads to A=(BC)m, where A, B and C are the metric coefficients and m is proportionality constant. Our models are in accelerating phase which is consistent to the recent observations. It has been found that the displacement vector β behaves like cosmological term Λ in the normal gauge treatment and the solutions are consistent with recent observations of SNe Ia. It has been found that massive strings dominate in the both decelerating and accelerating universes. The strings dominate in the early universe and eventually disappear from the universe for sufficiently large times. This is in consistent with the current observations. Some physical and geometric behaviour of these models are also discussed. © 2010 Springer Science+Business Media, LLC.


Pradhan A.,Hindu Post graduate College | Jotania K.,M. S. University of Baroda
International Journal of Theoretical Physics | Year: 2010

In this paper we have obtained some new exact solutions of Einstein's field equations in a spatially homogeneous and anisotropic Bianchi type-V space-time with perfect fluid distribution along with heat-conduction and decaying vacuum energy density Λ by applying the variation law for generalized Hubble's parameter that yields a constant value of deceleration parameter. We find that the constant value of deceleration parameter is reasonable for the present day universe. The variation law for Hubble's parameter generates two types of solutions for the average scale factor, one is of power-law type and other is of the exponential form. Using these two forms, Einstein's field equations are solved separately that correspond to expanding singular and non-singular models of the universe respectively. The cosmological constant Λ is found to be a decreasing function of time and positive which is corroborated by results from recent supernovae Ia observations. Expressions for lookback time-redshift, neoclassical tests (proper distance d(z)), luminosity distance red-shift and event horizon are derived and their significance are described in detail. The physical and geometric properties of spatially homogeneous and anisotropic cosmological models are discussed. © 2010 Springer Science+Business Media, LLC.


Amirhashchi H.,Islamic Azad University at Mahshahr | Pradhan A.,Hindu Post graduate College | Zainuddin H.,University Putra Malaysia
Research in Astronomy and Astrophysics | Year: 2013

We study the evolution of the dark energy parameter within the scope of a spatially non-flat and isotropic Friedmann-Robertson-Walker model filled with barotropic fluid and bulk viscous stresses. We have obtained cosmological solutions that do not have a Big Rip singularity, and concluded that in both non-interacting and interacting cases the non-flat open Universe crosses the phantom region. We find that during the evolution of the Universe, the equation of state for dark energy ωD changes from ω eff D > -1 to ωeff D < -1, which is consistent with recent observations. © 2013 National Astronomical Observatories of Chinese Academy of Sciences and IOP Publishing Ltd..


Pradhan A.,Hindu Post graduate College
Research in Astronomy and Astrophysics | Year: 2013

Motivated by the increasing evidence for the need of a geometry that resembles Bianchi morphology to explain the observed anisotropy in the WMAP data, we have discussed some features of Bianchi type VIo universes in the presence of a fluid that has an anisotropic equation of state (EoS) parameter in general relativity. We present two accelerating dark energy (DE) models with an anisotropic fluid in Bianchi type VI space-time. To ensure a deterministic solution, we choose the scale factor a(t) = √tnete, which yields a time-dependent deceleration parameter, representing a class of models which generate a transition of the universe from the early decelerating phase to the recent accelerating phase. Under suitable conditions, the anisotropic models approach an isotropic scenario. The EoS for DE ω is found to be time-dependent and its existing range for derived models is in good agreement with data from recent observations of type Ia supernovae (SNe Ia) (Knop et al. 2003), SNe Ia data combined with cosmic microwave background (CMB) anisotropy and galaxy clustering statistics (Tegmark et al. 2004a), as well as the latest combination of cosmological datasets coming from CMB anisotropies, luminosity distances of high redshift SNe Ia and galaxy clustering. For different values of n, we can generate a class of physically viable DE models. The cosmological constant Λ is found to be a positive decreasing function of time and it approaches a small positive value at late time (i.e. the present epoch), which is corroborated by results from recent SN Ia observations. We also observe that our solutions are stable. The physical and geometric aspects of both models are also discussed in detail. © 2013 National Astronomical Observatories of Chinese Academy of Sciences and IOP Publishing Ltd..


Pradhan A.,Hindu Post Graduate College | Jotania K.,M. S. University of Baroda
Indian Journal of Physics | Year: 2011

A class of new LRS Bianchi type-I cosmological models with a variable cosmological term is investigated in presence of perfect fluid. A procedure to generate new exact solutions to Einstein's field equations is applied to LRS Bianchi type-I space-time. Starting from some known solutions a class of new perfect fluid solutions of LRS Bianchi type-I are obtained. The cosmological constant Λ is found to be positive and a decreasing function of time which is supported by results from recent supernovae Ia observations. The physical and geometric properties of spatially homogeneous and anisotropic cosmological models are discussed. © 2011 IACS.


Pradhan A.,Hindu Post graduate College
Communications in Theoretical Physics | Year: 2011

The present study deals with a spatially homogeneous and anisotropic Bianchi-I cosmological models representing massive strings with magnetic field and decaying vacuum energy density Λ. The energy-momentum tensor, as formulated by Letelier (1983), has been used to construct massive string cosmological models for which we assume the expansion scalar in the models is proportional to one of the components of shear tensor. The Einstein's field equations have been solved by applying a variation law for generalized Hubble's parameter in Bianchi-I space-time. The variation law for Hubble's parameter generates two types of solutions for the average scale factor, one is of power-law type and other is of the exponential form. Using these two forms, Einstein's field equations are solved separately that correspond to expanding singular and non-singular models of the universe respectively. We have made a comparative study of accelerating and decelerating models in the presence of string scenario. The study reveals that massive strings dominate in the decelerating universe whereas strings dominate in the accelerating universe. The strings eventually disappear from the universe for sufficiently large times, which is in agreement with current astronomical observations. The cosmological constant Λ is found to be a positive decreasing function of time which is corroborated by results from recent supernovae Ia observations. The physical and geometric properties of the models have been also discussed in detail. © 2011 Chinese Physical Society and IOP Publishing Ltd.


Pradhan A.,Hindu Post graduate College | Amirhashchi H.,Islamic Azad University at Mahshahr
Modern Physics Letters A | Year: 2011

Some new exact solutions of Einstein's field equations in a spatially homogeneous and anisotropic Bianchi type-V spacetime with minimally interaction of perfect fluid and dark energy components have been obtained. To prevail the deterministic solution we choose the scale factor a(t) = √t net, which yields a time-dependent deceleration parameter (DP), representing a model which generates a transition of the universe from the early decelerating phase to the recent accelerating phase. We find that for n ≥ 1, the quintessence model is reproducible with present and expected future evolution of the universe. The other models (for n < 1), we observe the phantom scenario. The quintessence as well as phantom models approach to isotropy at late time. For different values of n, we can generate a class of physically viable DE models. The cosmic jerk parameter in our descended model is also found to be in good concordance with the recent data of astrophysical observations under appropriate condition. The physical and geometric properties of spatially homogeneous and anisotropic cosmological models are discussed. © 2011 World Scientific Publishing Company.


Ahmed N.,Taibah University | Ahmed N.,National Research Institute of Astronomy and Geophysics | Pradhan A.,Hindu Post graduate College
International Journal of Theoretical Physics | Year: 2014

A new class of cosmological models in f(R,T) modified theories of gravity proposed by Harko et al. (Phys. Rev. D 84:024020, 2011), where the gravitational Lagrangian is given by an arbitrary function of Ricci scalar R and the trace of the stress-energy tensor T, have been investigated for a specific choice of f(R,T)=f1(R)+f2(T) by considering time dependent deceleration parameter. The concept of time dependent deceleration parameter (DP) with some proper assumptions yield the average scale factor a(t) = sinh1\n(αt), where n and α are positive constants. For 01, the models of universe exhibit phase transition from early decelerating phase to present accelerating phase which is in good agreement with the results from recent astrophysical observations. Our intention is to reconstruct f(R,T) models inspired by this special law for the deceleration parameter in connection with the theories of modified gravity. In the present study we consider the cosmological constant Λ as a function of the trace of the stress energy-momentum-tensor, and dub such a model "Λ(T) gravity" where we have specified a certain form of Λ(T). Such models may display better uniformity with the cosmological observations. The statefinder diagnostic pair {r,s} parameter has been embraced to characterize different phases of the universe. We also discuss the physical consequences of the derived models. © 2013 Springer Science+Business Media New York.


Pradhan A.,Hindu Post Graduate College
Indian Journal of Physics | Year: 2014

Evolution of dark energy parameter within scope of a spatially homogeneous and isotropic Friedmann-Robertson-Walker model filled with perfect fluid and dark energy components is studied by generalizing recent results. The two sources are claimed to interact minimally so that their energy momentum tensors are conserved separately. The conception of time-dependent deceleration parameter with some suitable assumption yields an average scale factora = [sinh(at)]1/n, with a and n being positive arbitrary constants. For 0 < n ≤ 1, this generates a class of accelerating models while for n >1, the models of universe exhibit phase transition from early decelerating phase to present accelerating phase which is supported by results from recent astrophysical observations. It is observed that the transition red shift (zt) for our derived model with q0 = -0.73 is ∼0.32. This is in good agreement with cosmological observations in literature. Some physical and geometric properties of this model along with physical acceptability of cosmological solutions have been discussed in detail. © 2013 IACS.


Pradhan A.,Hindu Post graduate College | Amirhashchi H.,Islamic Azad University at Mahshahr
Astrophysics and Space Science | Year: 2011

A new dark energy model in anisotropic Bianchi type-III space-time with variable equation of state (EoS) parameter has been investigated in the present paper. To get the deterministic model, we consider that the expansion θ in the model is proportional to the eigen value σ 2 2 of the shear tensor σ j i. The EoS parameter ω is found to be time dependent and its existing range for this model is in good agreement with the recent observations of SNe Ia data (Knop et al. in Astrophys. J. 598:102, 2003) and SNe Ia data with CMBR anisotropy and galaxy clustering statistics (Tegmark et al. in Astrophys. J. 606:702, 2004). It has been suggested that the dark energy that explains the observed accelerating expansion of the universe may arise due to the contribution to the vacuum energy of the EoS in a time dependent background. Some physical aspects of dark energy model are also discussed. © 2010 Springer Science+Business Media B.V.

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