共查询到20条相似文献,搜索用时 31 毫秒
1.
In this paper we study the evolution of a LRS Bianchi I Universe, filled with a bulk viscous cosmological fluid in the presence
of time varying constants “but” taking into account the effects of a c-variable into the curvature tensor. We find that the only physical models are those which “constants” G and c are growing functions on time t, while the cosmological constant Λ is a negative decreasing function. In such solutions the energy density obeys the ultrastiff
matter equation of state i.e. ω = 1. 相似文献
2.
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
3.
Anisotropic Bianchi-III cosmological model is investigated with variable gravitational and cosmological constants in the framework of Einstein’s general relativity. The shear scalar is considered to be proportional to the expansion scalar. The dynamics of the anisotropic universe with variable G and Λ are discussed. Without assuming any specific forms for Λ and the metric potentials, we have tried to extract the time variation of G and Λ from the anisotropic model. The extracted G and Λ are in conformity with the present day observations. Basing upon the observational limits, the behavior and range of the effective equation of state parameter are discussed. 相似文献
4.
Antonio Alfonso-Faus 《Astrophysics and Space Science》2009,321(1):69-72
It is shown that the usual choice of units obtained by taking G=c=ℏ=1, giving the Planck’s units of mass, length and time, introduces an artificial contradiction between cosmology and particle
physics: the lambda problem that we associate with ℏ. We note that the choice of ℏ=1 does not correspond to the scale of quantum physics. For this scale we prove that the correct value is ℏ≈1/10122, while the choice of ℏ=1 corresponds to the cosmological scale. This is due to the scale factor of 1061 that converts the Planck scale to the cosmological scale. By choosing the ratio G/c
3=constant=1, which includes the choice G=c=1, and the momentum conservation mc=constant, we preserve the derivation of the Einstein field equations from the action principle. Then the product Gm/c
2=r
g
, the gravitational radius of m, is constant. For a quantum black hole we prove that ℏ≈r
g
2≈(mc)2. We also prove that the product Λℏ is a general constant of order one, for any scale. The cosmological scale implies Λ≈ℏ≈1, while the Planck scale gives Λ≈1/ℏ≈10122. This explains the Λ problem. We get two scales: the cosmological quantum black hole (QBH), size ∼1028 cm, and the quantum black hole (qbh) that includes the fundamental particles scale, size ∼10−13 cm, as well as the Planck’ scale, size ∼10−33 cm.
相似文献
5.
In this paper we have considered axially symmetric Bianchi-I, Kantowski Sachs and Bianchi-III space-time models with bulk
viscosity, where the gravitational constant G and the cosmological term Λ vary with time. In Einstein equations this variation in G and Λ are taken in such a way as to preserve the energy momentum tensor. Solutions are obtained with the cosmological term
varying inversely with square of time. 相似文献
6.
Anil Kumar Yadav Anirudh Pradhan Ajay Kumar Singh 《Astrophysics and Space Science》2012,337(1):379-385
The present study deals with spatially homogeneous and totally anisotropic locally rotationally symmetric (LRS) Bianchi type
I cosmological model with variable G and Λ in presence of imperfect fluid. To get the deterministic model of Universe, we assume that the expansion (θ) in the model is proportional to shear (σ). This condition leads to A=ℓB
n
, where A, B are metric potential. The cosmological constant Λ is found to be decreasing function of time and it approaches a small positive
value at late time which is supported by recent Supernovae Ia (SN Ia) observations. Also it is evident that the distance modulus
curve of derived model matches with observations perfectly. 相似文献
7.
F. Darabi 《Astrophysics and Space Science》2012,338(1):171-177
We study a gravitational model in which scale transformations play the key role in obtaining dynamical G and Λ. We take a non-scale invariant gravitational action with a cosmological constant and a gravitational coupling constant.
Then, by a scale transformation, through a dilaton field, we obtain a new action containing cosmological and gravitational
coupling terms which are dynamically dependent on the dilaton field with Higgs type potential. The vacuum expectation value
of this dilaton field, through spontaneous symmetry breaking on the basis of anthropic principle, determines the time variations of G and Λ. The relevance of these time variations to the current acceleration of the universe, coincidence problem, Mach’s cosmological
coincidence and those problems of standard cosmology addressed by inflationary models, are discussed. The current acceleration
of the universe is shown to be a result of phase transition from radiation toward matter dominated eras. No real coincidence
problem between matter and vacuum energy densities exists in this model and this apparent coincidence together with Mach’s
cosmological coincidence are shown to be simple consequences of a new kind of scale factor dependence of the energy momentum
density as ρ∼a
−4. This model also provides the possibility for a super fast expansion of the scale factor at very early universe by introducing
exotic type matter like cosmic strings. 相似文献
8.
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
9.
Sanjay Oli 《Astrophysics and Space Science》2008,314(1-3):89-94
This paper presents anisotropic, homogeneous two-fluid cosmological models in a Bianchi type I space–time with a variable
gravitational constant G and cosmological constant Λ. In the two-fluid model, one fluid represents the matter content of the universe and another fluid is chosen to model the
CMB radiation. We find a variety of solutions in which the cosmological parameter varies inversely with time t. We also discuss in detail the behavior of associated fluid parameters and kinematical parameters. This paper pictures cosmic
history when the radiation and matter content of the universe are in an interactive phase. Here, Ω is closing to 1 throughout
the cosmic evolution.
相似文献
10.
José Antonio Belinchón 《Astrophysics and Space Science》2012,338(2):381-400
We study how the constants G and Λ may vary in different theoretical models (general relativity with a perfect fluid, scalar cosmological models (“quintessence”)
with and without interacting scalar and matter fields and a scalar-tensor model with a dynamical Λ) in order to explain some
observational results. We apply the program outlined in section II to study three different geometries which generalize the
FRW ones, which are Bianchi V, VII0 and IX, under the self-similarity hypothesis. We put special emphasis on calculating exact power-law solutions which allow
us to compare the different models. In all the studied cases we arrive at the conclusion that the solutions are isotropic
and noninflationary while the cosmological constant behaves as a positive decreasing time function (in agreement with the
current observations) and the gravitational constant behaves as a growing time function. 相似文献
11.
Antonio Alfonso-Faus 《Astrophysics and Space Science》2010,325(1):113-117
We present a necessary and sufficient condition for an object of any mass m to be a quantum black hole (q.b.h.): “The product of the cosmological constant Λ and the Planck’s constant
ℏ, Λ and ℏ
corresponding to the scale defined by this q.b.h., must be of order one in a certain universal system of units”. In this system the numerical values known for Λ are of order one in cosmology and about 10122 for Planck’s scale. Proving that in this system the value of the cosmological ℏ
c
is of order one, while the value of ℏ for the Planck’s scale is about 10−122, both scales satisfy the condition to be a q.b.h., i.e. Λℏ≈1. In this sense the Universe is a q.b.h. We suggest that these objects, being q.b.h.’s, give us the linkage between thermodynamics,
quantum mechanics, electromagnetism and general relativity, at least for the scale of a closed Universe and for the Planck’s
scale. A mathematical transformation may refer these scales as corresponding to infinity (our universe) and zero (Planck’s
universe), in a “scale relativity” sense. 相似文献
12.
Hypersurface–homogeneous cosmological models containing a bulk viscous fluid with time varying G and Λ have been presented. We have shown that the field equations are solvable for any arbitrary cosmic scale function. The
viscosity coefficient of bulk viscous fluid is assumed to be a power function of the energy density. Exact solutions of Einstein’s
field equations are obtained which represent an expanding, shearing and accelerating/decelerating models of the universe.
The physical and kinematical behaviours of the models are also discussed. 相似文献
13.
Recent cosmological observations of large-scale structures (red shift of type Ia supernovae) confirm that the universe is
currently expanding at an accelerating rate and its dominant component is dark energy. This has stimulated the development
of the theory of gravity and led to many alternative variants, including tensor-scalar ones. This paper deals with the role
of conformal transformations in the Jordan-Brans-Dicke theory. Variants of intrinsic, conformally coupled, and Einstein representations
are examined. In the Einstein representation an exact analytic solution for the standard cosmological model is obtained. It
is expressed in terms of the relative energy contributions of ordinary matter Ω
m
, the scalar field Ω
CK
, and a term ΩΛ related to the cosmological constant Λ . Information on the evolution of the universe for the case with a minimally coupled
scalar field is given in the form of graphs. 相似文献
14.
G. S. Khadekar Vaishali Kamdi Anirudh Pradhan Saeed Otarod 《Astrophysics and Space Science》2007,310(1-2):141-147
We assume the four dimensional induced matter of the 5D Ricci flat bouncing cosmological solution contains a perfect fluid.
The big bounce singularity of simple 5D cosmological model is studied with the cosmological term Λ=α
ρ and Λ=β
H
2 where α and β are constants and ρ and H are respectively energy density and Hubble parameter. This big bounce singularity is found to be an event horizon at which
the scale factor and mass density of the universe are finite, while the pressure is infinite.
相似文献
15.
M. Malekjani A. Khodam-Mohammadi N. Nazari-pooya 《Astrophysics and Space Science》2011,334(1):193-201
The generalized Chaplygin gas (GCG) model in spatially flat universe is investigated. The cosmological consequences led by
GCG model including the evolution of EoS parameter, deceleration parameter and dimensionless Hubble parameter are calculated.
We show that the GCG model behaves as a general quintessence model. The GCG model can also represent the pressureless CDM
model at the early time and cosmological constant model at the late time. The dependency of transition from decelerated expansion
to accelerated expansion on the parameters of model is investigated. The statefinder parameters r and s in this model are derived and the evolutionary trajectories in s–r plane are plotted. Finally, based on current observational data, we plot the evolutionary trajectories in s–r and q–r planes for best fit values of the parameters of GCG model. It has been shown that although, there are similarities between
GCG model and other forms of Chaplygin gas in statefinder plane, but the distance of this model from the ΛCDM fixed point
in s–r diagram is shorter compare with standard Chaplygin gas model. 相似文献
16.
A new class of plane-symmetric inhomogeneous cosmological models of perfect fluid distribution with electro-magnetic field
based on Lyra’s geometry is obtained by considering a time dependent displacement field. The source of the magnetic field
is due to an electric current produced along the z-axis. Only F
12 is a non-vanishing component of electromagnetic field tensor. To get the deterministic solutions, the free gravitational
field is assumed to be of Petrov type-II non-degenerate. It has been found that the displacement vector β(t) behaves like cosmological term Λ which is consistent with the recent observations of type Ia supernovae. It is also observed
that β(t) affects entropy. Some geometric and physical behaviour of the models are also discussed in presence of magnetic field.
相似文献
17.
In this paper we compare outcomes of some extended phantom-like cosmologies with each other and also with ΛCDM and ΛDGP. We
focus on the variation of the luminosity distances, the age of the universe and the deceleration parameter versus the redshift
in these scenarios. In a dynamical system approach, we show that the accelerating phase of the universe in the f(R)-DGP scenario is stable if one consider the curvature fluid as a phantom scalar field in the equivalent scalar-tensor theory, otherwise it is a transient and unstable phenomenon. Up
to the parameters values adopted in this paper, the extended F(R,ϕ)-DGP scenario is closer to the ΛCDM scenario than other proposed models. All of these scenarios explain the late-time cosmic
speed-up in their normal DGP branches, but the redshift at which transition to the accelerating phase occurs are different:
while the ΛDGP model transits to the accelerating phase much earlier, the F(R,ϕ)-DGP model transits to this phase much later than other scenarios. Also, within the parameter spaces adopted in this paper,
the age of the universe in the f(R)-DGP model is larger than ΛCDM, but this age in F(G,ϕ)-DGP is smaller than ΛCDM. 相似文献
18.
The cosmological reconstruction of modified F(R) and F(G)F(\mathcal{G}) gravities with agegraphic dark energy (ADE) model in a spatially flat universe without matter field is investigated by using
e-folding “N” as a forward way. After calculating a consistent F(R) in ADE’s framework, we obtain conditions for effective equation of state parameter w
eff, and see that reconstruction is possible for both phantom and non-phantom era. These calculations also are done for F(G)F(\mathcal{G}) gravity and the condition for a consistent reconstruction is obtained. 相似文献
19.
Antonio Alfonso-Faus 《Astrophysics and Space Science》2012,337(1):363-365
Using the relation proposed by Weinberg in 1972, combining quantum and cosmological parameters, we prove that the self gravitational
potential energy of any fundamental particle is a quantum, with physical properties independent of the mass of the particle.
It is a universal quantum of gravitational energy, and its physical properties depend only on the cosmological scale factor
R and the physical constants ℏ and c. We propose a modification of the Weinberg’s relation, keeping the same numerical value, but substituting the cosmological
parameter H/c by 1/R. 相似文献
20.
Mohamed E. Hassani 《Astrophysics and Space Science》2011,331(1):221-236
In this work, which is a supplemental to previous one, we undertake to establish some cosmological thermodynamic equations
in the context of the cyclical universe as the scenario in which the universe itself is considered like an adiabatic thermodynamical
system enclosed in physical volume characterized by periodic reversible transitions. Our model is based on the combination
of local and global cosmological time-dependent temperatures {T
0(τ
0),T(τ)} and volumes {V
0(τ
0),V(τ)} instead of the critical temperature T
c and volume V
c; and the infinitesimal relative variations {dT/T,dV/V}, which are mainly due to the cosmological chaotic fluctuations that are generally ignored in certain oscillating models.
By taking into account all these factors, certain equations in the form of d
ℓ/ℓ=±η
d
τ/τ
H have been established and from them we derive some others to provide a mechanism that is responsible for the thermodynamic
evolution of the cyclical universe. 相似文献