48 research outputs found
Accelerating dark energy models in bianchi Type-V space-time
Some new exact solutions of Einstein's field equations in a spatially
homogeneous and anisotropic Bianchi type-V space-time with minimally
interaction of perfect fluid and dark energy components have been obtained. To
prevail the deterministic solution we choose the scale factor , 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 , the quintessence model is reproducible with present and expected
future evolution of the universe. The other models (for ), we observe
the phantom scenario. The quintessence as well as phantom models approach to
isotropy at late time. For different values of , 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.Comment: 12 pages, 6 figure
String Cosmology in Anisotropic Bianchi-II Space-time
The present study deals with a spatially homogeneous and anisotropic
Bianchi-II cosmological model representing massive strings. The energy-momentum
tensor, as formulated by Letelier (1983), has been used to construct a massive
string cosmological model for which the expansion scalar 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 that
yields a constant value of deceleration parameter in Bianchi-II space-time. A
comparative study of accelerating and decelerating modes of the evolution of
universe has been carried out in the presence of string scenario. The study
reveals that massive strings dominate the early Universe. The strings
eventually disappear from the Universe for sufficiently large times, which is
in agreement with the current astronomical observations.Comment: 11 pages, 6 figures (To appear in Mod. Phys. Lett. A) In this
version, the cosmic string has been directed along z-direction and the
resultant field equations have been solved exactl
Two-Fluid Scenario for Dark Energy Models in an FRW Universe-Revisited
In this paper we study the evolution of the dark energy parameter within the
scope of a spatially homogeneous and isotropic Friedmann-Robertson-Walker (FRW)
model filled with barotropic fluid and dark energy by revisiting the recent
results (Amirhashchi et al. in Chin. Phys. Lett. 28:039801, 2011a). To prevail
the deterministic solution we select the scale factor which generates 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 consider the
two cases of an interacting and non-interacting two-fluid (barotropic and dark
energy) scenario and obtained general results. The cosmic jerk parameter in our
derived model is also found to be in good agreement with the recent data of
astrophysical observations under the suitable condition. The physical aspects
of the models and the stability of the corresponding solutions are also
discussed.Comment: 10 pages, 4 figures. arXiv admin note: substantial overlap with
arXiv:1011.394
Modified gravity in a viscous and non-isotropic background
We study the dynamical evolution of an model of gravity in a viscous
and anisotropic background which is given by a Bianchi type-I model of the
Universe. We find viable forms of gravity in which one is exactly the
Einsteinian model of gravity with a cosmological constant and other two are
power law models. We show that these two power law models are stable
with a suitable choice of parameters. We also examine three potentials which
exhibit the potential effect of models in the context of scalar tensor
theory. By solving different aspects of the model and finding the physical
quantities in the Jordan frame, we show that the equation of state parameter
satisfy the dominant energy condition. At last we show that the two power law
models behave like quintessence model at late times and also the shear
coefficient viscosity tends to zero at late times.Comment: 7 pages, 2 figure
A New Class of Bianchi Type-I Cosmological Models in Scalar-Tensor Theory of Gravitation and Late Time Acceleration
A new class of a spatially homogeneous and anisotropic Bianchi type-I
cosmological models of the universe for perfect fluid distribution within the
framework of scalar-tensor theory of gravitation proposed by Saez and Ballester
(Phys. Lett. 113:467, 1986) is investigated. To prevail the deterministic
solutions we choose the different scale factors which yield time-dependent
deceleration parameters (DP) representing models which generate a transition of
the universe from the early decelerated phase to the recent accelerating phase.
Three different physically viable models of the universe are obtained in which
their anisotropic solutions may enter to some isotropic inflationary era. The
modified Einstein's field equations are solved exactly and the models are found
to be in good concordance with recent observations. Some physical and geometric
properties of the models are also discussed.Comment: 16 pages, 8 figure
Bianchi type II models in the presence of perfect fluid and anisotropic dark energy
Spatially homogeneous but totally anisotropic and non-flat Bianchi type II
cosmological model has been studied in general relativity in the presence of
two minimally interacting fluids; a perfect fluid as the matter fluid and a
hypothetical anisotropic fluid as the dark energy fluid. The Einstein's field
equations have been solved by applying two kinematical ans\"{a}tze: we have
assumed the variation law for the mean Hubble parameter that yields a constant
value of deceleration parameter, and one of the components of the shear tensor
has been considered proportional to the mean Hubble parameter. We have
particularly dwelled on the accelerating models with non-divergent expansion
anisotropy as the Universe evolves. Yielding anisotropic pressure, the fluid we
consider in the context of dark energy, can produce results that can be
produced in the presence of isotropic fluid in accordance with the \Lambda CDM
cosmology. However, the derived model gives additional opportunities by being
able to allow kinematics that cannot be produced in the presence of fluids that
yield only isotropic pressure. We have obtained well behaving cases where the
anisotropy of the expansion and the anisotropy of the fluid converge to finite
values (include zero) in the late Universe. We have also showed that although
the metric we consider is totally anisotropic, the anisotropy of the dark
energy is constrained to be axially symmetric, as long as the overall energy
momentum tensor possesses zero shear stress.Comment: 15 pages; 5 figures; matches the version published in The European
Physical Journal Plu
Accelerating Bianchi Type-V Cosmology with Perfect Fluid and Heat Flow in Saez-Ballester Theory
In this paper we discuss the law of variation of scale factor which yields a time-dependent deceleration
parameter (DP) representing a new class of models that generate a transition of
universe from the early decelerated phase to the recent accelerating phase.
Exact solutions of Einstein's modified field equations with perfect fluid and
heat conduction are obtained within the framework of Saez-Ballester
scalar-tensor theory of gravitation and the model is found to be in good
agreement with recent observations. We find, for n = 3, k = 1, the present
value of DP in derived model as q_0 = -0.67 which is very near to the observed
value of DP at present epoch. We find that the time-dependent DP is sensible
for the present day Universe and give an earmark description of evolution of
universe. Some physical and geometric properties of the models are also
discussed.Comment: 12 pages, 5 figure
Lyra's Cosmology of Massive String in Anisotropic Bianchi-II Space-time
The paper deals with a spatially homogeneous and totally anisotropic Bianchi
II cosmological models representing massive strings in normal gauge for Lyra's
manifold. The modified Einstein's field equations have been solved by applying
variation law for Hubble's parameter. This law generates two type of solutions
for average scale factor, one is of power law type and other is of exponential
law type. The power law describes the dynamics of Universe from big bang to
present epoch while exponential law seems reasonable to project dynamics of
future Universe. It has been found that the displacement actor is a
decreasing function of time and it approaches to small positive value at late
time, which is collaborated with Halford (1970) as well as recent observations
of SN Ia. The study reveals that massive strings dominate in early Universe and
eventually disappear from Universe for sufficiently large time, which is in
agreement with the current astronomical observations.Comment: 12 pages, 5 figure