共查询到20条相似文献,搜索用时 31 毫秒
1.
S. D. Katore K. S. Adhav A. Y. Shaikh M. M. Sancheti 《Astrophysics and Space Science》2011,333(1):333-341
We consider a self-consistent system of Plane symmetric cosmology and binary mixture of perfect fluid and dark energy. The
perfect fluid is taken to be one obeying the usual equation of state p=γρ with γ∈[0,1]. The dark energy is considered to be either the quintessence or Chaplygin gas. Exact solutions to the corresponding
Einstein’s field equations are obtained as a quadrature. The cases of Zeldovich Universe, Dust Universe and Radiation Universe
and models with power-law and exponential expansion have discussed in detail. For large t, the models tend to be isotropic. 相似文献
2.
Kantowski-Sachs cosmological model in the presence of magnetized anisotropic dark energy is investigated. The energy-momentum
tensor consists of anisotropic fluid with anisotropic EoS p=ωρ and a uniform magnetic field of energy density ρ
B
. We obtain exact solutions to the field equations using the condition that expansion is proportional to the shear scalar.
The physical behavior of the model is discussed with and without magnetic field. We conclude that universe model as well as
anisotropic fluid does not approach isotropy through the evolution of the universe. 相似文献
3.
We consider a self-consistent system of Bianchi type-V cosmology and binary mixture of perfect fluid and dark energy. The perfect fluid is taken to be one obeying the usual equation of state p=γ ρ with γ∈[0,1]. The dark energy is considered to be either the quintessence or Chaplygin gas. Exact solutions to the corresponding Einstein equations are obtained as a quadrature. The cases of disordered radiation and models with power-law and exponential expansion have discussed in detail. For large t, the models tend to be isotropic. 相似文献
4.
A spatially homogeneous and isotropic Robertson-Walker model with zero-curvature of the universe is studied in Saez-Ballester
scalar-tensor theory. Exact solutions of the field equations are obtained for two different early phases of the universe viz.
the inflationary and the radiation-dominated phases by using gamma-law equation of state p=(γ-1)ρ in the presence of perfect fluid. The γ-index describing the material content varies continuously with cosmic time
so that in the course of its evolution, the universe goes through a transition from an inflationary phase to a radiation-dominated
phase. The coupling parameterω is allowed to depend on the cosmic time. The nature of scalar field and other physical significance
have also been discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
Dr. Bijan Saha PhD 《Astrophysics and Space Science》2006,302(1-4):83-91
We consider a self-consistent system of Bianchi type-I (BI) gravitational field and a binary mixture of perfect fluid and
dark energy given by a cosmological constant. The perfect fluid is chosen to be the one obeying either the usual equation
of state, i.e., p = ζ, with ζ ∊ [0, 1] or a van der Waals equation of state. Role of the Λ term in the evolution of the BI Universe has been
studied. 相似文献
6.
The field equations of Barber's (1982) second self-creation theory of gravitation are solved for 5D Friedmann-Robertson-Walker
space time using perfect fluid energy momentum tensor. By assuming an equation of state p= ε ρ, (0 ≤ ε ≤ 1), the solutions of the field equations, in different scenarios, in Barber's second self-creation theory
are presented and discussed. Some properties of these models are also discussed. 相似文献
7.
The properties of locally rotationally symmetric Bianchi type-II perfect fluid space-times are analyzed in Barber’s second
self-creation theory by using a special law of variation for Hubble’s parameter that yields a constant value of deceleration
parameter. By assuming the equation of state p=γ
ρ, many new solutions are obtained for different era—Zel’dovich, radiation, vacuum and vacuum energy dominated. The solutions
with power-law and exponential expansion are discussed. A detailed study of geometrical and physical parameters is carried
out. The nature of singularity is also clarified in each case. 相似文献
8.
In this paper we study the evolution of the dark energy parameter within the scope of a spatially homogeneous and isotropic
FRW universe filled with barotropic fluid and dark energy. The scale factor is considered as a power law function of time
which yields a constant deceleration parameter. We consider the case when the dark energy is minimally coupled to the perfect
fluid as well as direct interaction with it. The cosmic jerk parameter in our derived models is consistent with the recent
data of astrophysical observations. It is concluded that in non-interacting case, all the three open, close and flat universes
cross the phantom region whereas in interacting case only open and flat universes cross the phantom region. We find that during
the evolution of the universe, the equation of state (EoS) for dark energy ω
D
changes from ω
D
>−1 to ω
D
<−1, which is consistent with recent observations. 相似文献
9.
José Antonio Belinchón 《Astrophysics and Space Science》2002,281(4):765-775
Various models are considered with metric type flat FRW i.e. with k = 0 whose energy-momentum tensor is described by a perfect fluid whose generic equation of state is p = ωρ and taking into account the conservation principle div(T
i
j
) = 0, but considering some of the‘constants’ as variable. A set of solutions through dimensional analysis is trivially found.
The numeric calculations carried out show that the results obtained are not discordant with those presently observed for cosmological
parameters together with the electromagnetic and quantum quantities.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
In this paper, the problem of spatially homogeneous and an isotropic Bianchi type-1 space time with perfect fluid distribution
is considered in Barber's second theory of gravitation. To obtain determinate solutions, we have assumed the equation of statep= γρ, 0 ≤ γ ≤ 1. It is observed that the general fluid distribution degenerates isotropic vacuum model whenγ = 1 and Λ < 0.
Further it is observed that the vacuum model obtained in case of γ = 0, ρ = 0 andΛ = 0, reduces to well known Kasner model
in Einstein's theory. Some physical and geometrical aspects of the models together with singularities in the models are also
discussed
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
Anirudh Pradhan Rekha Jaiswal Kanti Jotania Rajeev Kumar Khare 《Astrophysics and Space Science》2012,337(1):401-413
We present two dark energy (DE) models with an anisotropic fluid in Bianchi type-VI
0 space-time by considering time dependent deceleration parameter (DP). The equation of state (EoS) for dark energy ω is found to be time dependent and its existing range for derived models is in good agreement with the recent observations.
Under the suitable condition, the anisotropic models approach to isotropic scenario. We also find that during the evolution
of the universe, the EoS parameter for DE changes from ω>−1 to ω=−1 in first model whereas from ω>−1 to ω<−1 in second model which is consistent with recent observations. 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 type Ia supernovae observations. The cosmic jerk parameter in our derived models is also found to be in good agreement
with the recent data of astrophysical observations. The physical and geometric aspects of both the models are also discussed
in detail. 相似文献
12.
K. Karami M. Jamil M. Roos S. Ghaffari A. Abdolmaleki 《Astrophysics and Space Science》2012,340(1):175-184
We study the entropy-corrected version of the new agegraphic dark energy (NADE) model and dark matter in a spatially non-flat Universe and in the framework of Hořava-Lifshitz cosmology. For the two cases containing noninteracting and interacting entropy-corrected NADE (ECNADE) models, we derive the exact differential equation that determines the evolution of the ECNADE density parameter. Also the deceleration parameter is obtained. Furthermore, using a parametrization of the equation of state parameter of the ECNADE model as ω Λ(z)=ω 0+ω 1 z, we obtain both ω 0 and ω 1. We find that in the presence of interaction, the equation of state parameter ω 0 of this model can cross the phantom divide line which is compatible with the observation. 相似文献
13.
Naveen Bijalwan 《Astrophysics and Space Science》2011,334(1):139-143
A most general exact solution to the Einstein-Maxwell equations for static charged perfect fluid is sought in terms of pressure.
Subsequently, metrics (e
λ
and e
υ
), matter density and electric intensity are expressible in terms of pressure. Consequently, Pressure is found to be an invertible
arbitrary function of ω(=c
1+c
2
r
2), where c
1 and c
2(≠0) are arbitrary constants, and r is the radius of star, i.e. p=p(ω). We present a general solution for charged pressure fluid in terms for ω. We list and discuss some old and new solutions which fall in this category. 相似文献
14.
In this paper, we investigate the dynamics of Born–Infeld (B–I) phantom model in the ω–ω′ plane, which is defined by the equation of state parameter for the dark energy and its derivative with respect to N (the logarithm of the scale factor a). We find the scalar field equation of motion in ω–ω′ plane, and show mathematically the property of attractor solutions which correspond to ω
φ
∼−1, Ω
φ
=1, which avoid the “Big rip” problem and meets the current observations well.
相似文献
15.
Time-dependent wormhole solutions are found which evolve in a cosmological background. Solutions are presented both for GR
and Brans-Dicke field equations. Conditions are derived for the supporting matter to be non-exotic. The traceless energy-momentum
tensor needed to support the geometry is in the form of an anisotropic fluid. Far from the wormhole, the equation of state
rapidly approaches that of an isotropic perfect fluid with p = 1/3 ρ. For the BD wormholes we obtain ρ = 0everywhere, except for the π = const. limit, in which case the GR results are
reproduced.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
17.
José Antonio Belinchón 《Astrophysics and Space Science》2009,323(2):185-195
We study a perfect fluid Bianchi II models with time varying constants under the self-similarity approach. In the first of
the studied model, we consider that only vary G and Λ. The obtained solution is more general that the obtained one for the classical solution since it is valid for an equation
of state ω∈(−1,∞) while in the classical solution ω∈(−1/3,1). Taking into account the current observations, we conclude that G must be a growing time function while Λ is a positive decreasing function. In the second of the studied models we consider
a variable speed of light (VSL). We obtain a similar solution as in the first model arriving to the conclusions that c must be a growing time function if Λ is a positive decreasing function. 相似文献
18.
Emilio Santos 《Astrophysics and Space Science》2011,332(2):423-435
It is shown that quantum vacuum fluctuations give rise to a curvature of space-time equivalent to a cosmological constant,
that is a homogeneous energy density ρ and pressure p fulfilling −p=ρ>0. The fact that the fluctuations produce curvature, even if the vacuum expectation of the energy vanishes, is a consequence
of the non-linear character of the Einstein equation. A calculation is made, involving plausible hypotheses within quantized
gravity, which establishes a relation between the two-point correlation of the vacuum fluctuations and the space-time curvature.
Arguments are given which suggest that the density ρ might be of order the “dark energy” density currently assumed to explain the observed accelerated expansion of the universe. 相似文献
19.
José Antonio Belinchón 《Astrophysics and Space Science》2009,323(3):307-315
We study a massive cosmic strings with BII symmetries cosmological models in two contexts. The first of them is the standard
one with a barotropic equation of state. In the second one we explore the possibility of taking into account variable “constants”
(G and Λ). Both models are studied under the self-similar hypothesis. We put special emphasis in calculating the numerical values
for the equations of state. We find that for ω∈(0,1], G, is a growing time function while Λ, behaves as positive decreasing time function. If ω=0, both “constants”, G and Λ, behave as true constants. 相似文献
20.
Mahesh Kumar Yadav Anirudh Pradhan Sheel Kumar Singh 《Astrophysics and Space Science》2007,311(4):423-429
Some Bianchi type-I viscous fluid string cosmological models with magnetic field are investigated. The viscosity coefficient
of bulk viscous fluid is assumed to be a power function of mass density ξ(t)=ξ
0
ρ
m
, where ξ
0 and m are constants. To get a determinate model, we assume conditions ρ=(1+ω)λ, where ρ is rest energy density, ω a positive constant and λ the string tension density and expansion θ is proportional to eigen value σ
11 of the shear tensor σ
j
i
. The behaviour of the models from physical and geometrical aspects in presence and absence of magnetic field is discussed.
相似文献