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1.
The evolution of a perfect fluid perturbation is considered in the radiation dominated period and in the dust epoch. In the investigation we make use of the general formalism developed in previous papers.It turns out that the evolution tendency is predicted by the state of the cosmic background. The radiation dominated universe does not stimulate growing processes of the perturbation, whereas the dust dominated universe causes a growing tendency of small perfect fluid formations. The results of this work are in accordance with these obtained by the present writers in a previous work. 相似文献
2.
Carlos E. Laciana 《Astrophysics and Space Science》1995,229(2):173-184
A cosmological model describing the different stages of the universe, i.e.: inflation, radiation dominated period and matter dominated (Friedmann-like) period is shown. The model consists of the usual gravitational lagrangian with a R
2 term, and, for the matter content, the lagrangian of a massive conformally coupled scalar field. The effect of backreaction is evaluated by means of an extremum condition on the entropy at each time. The differential equation, obtained when the lowest quantum order is considered, describes all the periods of evolution of the universe. For a range of values of, inflation is unstable and the universe can reach the following regime. 相似文献
3.
Evolution of the universe is discussed in the framework of f(R) theory of gravity. The deceleration parameter is used to interpret various phases of the universe. We investigate the future evolution of the flat FRW universe by using observationally viable f(R) models. A numerical technique is applied to solve the evolution equation in terms of Hubble parameter which is used to explore late time acceleration of the universe. Some novel and interesting results based on the choice of coupling parameters in gravitational action are obtained. We can conclude that the considered f(R) models imply unification of matter dominated epoch with present accelerating phase of the universe. 相似文献
4.
El-Nabulsi Ahmad?Rami 《Astrophysics and Space Science》2011,332(2):491-495
We investigate the late-time dynamics of a four-dimensional universe based on modified scalar field gravity in which the standard
Einstein-Hilbert action R is replaced by f(φ)R+f(R) where f(φ)=φ
2 and f(R)=AR
2+BR
μν
R
μν,(A,B)∈ℝ. We discussed two independent cases: in the first model, the scalar field potential is quartic and for this special form
it was shown that the universe is dominated by dark energy with equation of state parameter w≈−0.2 and is accelerated in time with a scale factor evolving like a(t)∝t
5/3 and B+3A≈0.036. When, B+3A→∞ which corresponds for the purely quadratic theory, the scale factor evolves like a(t)∝t
1/2 whereas when B+3A→0 which corresponds for the purely scalar tensor theory we found when a(t)∝t
1.98. In the second model, we choose an exponential potential and we conjecture that the scalar curvature and the Hubble parameter
vary respectively like
R=hH[(f)\dot]/f,h ? \mathbbRR=\eta H\dot{\phi}/\phi,\eta\in\mathbb{R} and
H=g[(f)\dot]c,(g,c) ? \mathbbRH=\gamma\dot{\phi}^{\chi},(\gamma,\chi)\in\mathbb{R}. It was shown that for some special values of χ, the universe is free from the initial singularity, accelerated in time, dominated by dark or phantom energy whereas the
model is independent of the quadratic gravity corrections. Additional consequences are discussed. 相似文献
5.
Exact solution of Einstein’s field equations is obtained for massive string cosmological model of Bianchi III space-time using
the technique given by Letelier (Phys. Rev. D 20:2414, 1983) in presence of perfect fluid and decaying vacuum energy density Λ. To get the deterministic solution of the field equations
the expansion θ in the model is considered as proportional to the eigen value s2 2\sigma^{2}_{~2} of the shear tensor sj i\sigma^{j}_{~i} and also the fluid obeys the barotropic equation of state. The vacuum energy density Λ is found to be positive and a decreasing
function of time which is supported by the results from recent supernovae Ia observations. It is also observed that in early
stage of the evolution of the universe string dominates over the particle whereas the universe is dominated by massive string
at the late time. Some physical and geometric properties of the model are also discussed. 相似文献
6.
K. S. Adhav 《Astrophysics and Space Science》2011,335(2):611-617
The Bianchi type-V cosmological model with variable modified Chaplygin gas having the equation of state p=Aρ−B/ρ
α
, where 0≤α≤1, A is a positive constant and B is a positive function of the average scale factor a(t) of the universe [i.e. B=B(a)] has been studied. While studying its role in accelerated phase of the universe, it is observed that the equation of state
of the variable modified Chaplygin gas interpolates from radiation dominated era to quintessence dominated era. The statefinder
diagnostic pair {r,s} is adopted to characterize different phases of the universe. 相似文献
7.
Alternative cosmologies, based on extensions of General Relativity, predict modified thermal histories in the early universe during the pre Big Bang Nucleosynthesis (BBN) era. When the expansion rate is enhanced with respect to the standard case, thermal relics typically decouple with larger relic abundances. In this paper, we study the dynamical evolution of an f(R) model of gravity in a homogeneous and anisotropic background which is given by a Bianchi type-I model of the universe filled with dark matter, which is described by a perfect fluid with a barotropic equation of state. As an example of a consistent analysis of modified gravity, we apply the formalism to a simple background solution of R+βR n gravity. Our analysis shows that f(R) cosmology allows dark matter masses lesser than 100 GeV, in the regime ρ c ?ρ m . We finally discuss how these limits apply to some specific realizations of standard cosmologies: an f(R) gravity model, Einstein frame model. 相似文献
8.
S. M. Gong 《Astrophysics and Space Science》1989,158(1):1-7
The maximum volume of the closed Friedmann universe is further investigated and is shown to be 22
R
3
(t), instead of 2
R
3
(t) as found previously. This discrepancy comes from the incomplete use of the volume formula of 3-dimensional spherical space in the astronomical literature. Mathematically, there exists the maximum volume at any cosmic timet in a 3-dimensional spherical case. However, the Friedmann closed universe in expansion reaches its maximum volume only at the timet
m
of the maximum scale factorR(t
m
). The particle horizon has no limitation for the farthest objects in the closed Friedmann universe if the proper distance of objects is compared with the particle horizon as it should be. It will lead to absurdity if the luminosity distance of objects is compared with the proper distance of the particle horizon. 相似文献
9.
Arbab I. Arbab 《Astrophysics and Space Science》2004,291(2):143-149
We have shown that the phenomenological models with a cosmological constant of the type Λ=β(
) and Λ=3αH
2, where R is the scale factor of the universe and H is the Hubble constant, are equivalent to a quintessence model with a scalar (φ) potential of the formV∝φ-n, n= constant. The equation of state of the cosmic fluid is described by these parameters (α, β, n) only. The equation of state of the cosmic fluid (dark energy) can be determined by any of these parameters. The actual amount of dark energy will define the equation of state of the cosmic fluid.All of the three forms can give rise to cosmic acceleration depending the amount of dark energy in the universe. 相似文献
10.
We consider a collapsing sphere and discuss its evolution under the vanishing expansion scalar in the framework of f(R) gravity. The fluid is assumed to be locally anisotropic which evolves adiabatically. To study the dynamics of the collapsing
fluid, Newtonian and post Newtonian regimes are taken into account. The field equations are investigated for a well-known
f(R) model of the form R+δR
2 admitting Schwarzschild solution. The perturbation scheme is used on the dynamical equations to explore the instability conditions
of expansionfree fluid evolution. We conclude that instability conditions depend upon pressure anisotropy, energy density
and some constraints arising from this theory. 相似文献
11.
12.
The effect of time dependent bulk viscosity on the evolution of Friedmann models with zero curvature in Brans-Dicke theory
is studied. The solutions of the field equations with ‘gamma-law’ equation of state p = (γ-1) ρ, where γ varies continuously
as the Universe expands, are obtained by using the power-law relation φ = bR
n
, which lead to models with constant deceleration parameter. We obtain solutions for the inflationary period and radiation
dominated era of the universe. The physical properties of cosmological solutions are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
M. R. Setare 《Astrophysics and Space Science》2010,326(1):27-31
In this paper, we study a cosmological application of the new agegraphic dark energy density in the f(R) gravity framework. We employ the new agegraphic model of dark energy to obtain the equation of state for the new agegraphic
energy density in a spatially flat universe. Our calculations show, taking n<0, that it is possible to have w
Λ crossing −1. This implies that one can generate a phantom-like equation of state from a new agegraphic dark energy model
in a flat universe in the modified gravity cosmology framework. Also, we develop a reconstruction scheme for the modified
gravity with f(R) action. 相似文献
14.
In this study, we consider a flat Friedmann-Robertson-Walker (FRW) universe in the context of Palatini f(R) theory of gravity. Using the dynamical equivalence between f(R) gravity and scalar-tensor theories, we construct a point Lagrangian in the flat FRW spacetime. Applying Noether gauge symmetry approach for this f(R) Lagrangian we find out the form of f(R) and the exact solution for cosmic scale factor. It is shown that the resulting form of f(R) yield a power-law expansion for the scale factor of the universe. 相似文献
15.
In this paper, we solve the field equations in metric f(R) gravity for Bianchi type VI
0 spacetime and discuss evolution of the expanding universe. We find two types of non-vacuum solutions by taking isotropic
and anisotropic fluids as the source of matter and dark energy. The physical behavior of these solutions is analyzed and compared
in the future evolution with the help of some physical and geometrical parameters. It is concluded that in the presence of
isotropic fluid, the model has singularity at [(t)\tilde]=0\tilde{t}=0 and represents continuously expanding shearing universe currently entering into phantom phase. In anisotropic fluid, the
model has no initial singularity and exhibits the uniform accelerating expansion. However, the spacetime does not achieve
isotropy as t→∞ in both of these solutions. 相似文献
16.
Antonio Alfonso-Faus Màrius Josep Fullana i Alfonso 《Astrophysics and Space Science》2013,348(2):527-531
From the equivalence principle, one gets the strength of the gravitational effect of a mass M on the metric at position r from it. It is proportional to the dimensionless parameter β 2=2GM/rc 2, which normally is ?1. Here G is the gravitational constant, M the mass of the gravitating body, r the position of the metric from the gravitating body and c the speed of light. The seeable universe is the sphere, with center at the observer, having a size such that it shall contain all light emitted within it. For this to occur one can impose that the gravitational effect on the velocity of light at r is zero for the radial component, and non zero for the tangential one. Light is then trapped. The condition is given by the equality R g =2GM/c 2, where R g represents the radius of the seeable universe. It is the gravitational radius of the mass M. The result has been presented elsewhere as the condition for the universe to be treated as a black hole. According to present observations, for the case of our universe taken as flat (k=0), and the equation of state as p=?ρc 2, we prove here from the Einstein’s cosmological equations that the universe is expanding in an accelerated way as t 2, a constant acceleration as has been observed. This implies that the gravitational radius of the universe (at the event horizon) expands as t 2. Taking c as constant, observing the galaxies deep in space this means deep in time as ct, linear. Then, far away galaxies from the observer that we see today will disappear in time as they get out of the distance ct that is <R g . The accelerated expanding vacuum will drag them out of sight. This may be a valid test for the present ideas in cosmology. Previous calculations are here halved by our results. 相似文献
17.
The present study deals with locally rotationally symmetric (LRS) Bianchi type II cosmological model representing massive
string. The energy-momentum tensor for such string as formulated by Letelier (Phys. Rev. D 28:2414, 1983) is used to construct massive string cosmological model for which we assume that the expansion (θ) in the model is proportional to the shear (σ). This condition leads to A=B
m
, where A and B are the metric coefficients and m is proportionality constant. For suitable choice of constant m, it is observed
that in early stage of the evolution of the universe string dominates over the particle whereas the universe is dominated
by massive string at the late time. Our model is in accelerating phase which is consistent to the recent observations of type
Is supernovae. Some physical and geometric behavior of the model is also discussed. 相似文献
18.
19.
String cosmological models with bulk viscosity are investigated in Kantowski-Sachs space-time. To obtain a determinate solution,
it is assumed that the coefficient of bulk viscosity is a power function of the scalar of expansion ζ = kθm and the scalar of expansion is proportional to the shear scalar θ ∝ σ, which leads to a relation between metric potentials
R = AS
n
. The physical and geometrical aspects of the model are also discussed. It is shown that the bulk viscosity has significant
influence on the evolution of the universe. There is a ‘big bang’ start in the model when m ≤ 1 but there is no ‘big bang’
start when m > 1. 相似文献
20.
Assuming the time-dependent equation of state p=λ(t)ρ, five dimensional cosmological models with viscous fluid for an open universe (k=−1) and flat universe (k=0) are presented. Exact solutions in the context of the rest mass varying theory of gravity proposed by Wesson (Astron. Astrophys.
119, 145, 1983) are obtained. It is found that the phenomenon of isotropisation takes place in this theory, i.e. the mass scale factor A(t) which characterizes the rest mass of a typical particle is evolving with cosmic time just as the spatial scale factor R(t). It is further found that rest mass is approximately constant in the present universe. 相似文献