共查询到20条相似文献，搜索用时 31 毫秒
1.
Bijan Saha 《Astrophysics and Space Science》2007,312(1-2):3-11
We consider a system of nonlinear spinor and a Bianchi type I gravitational fields in presence of viscous fluid. The nonlinear
term in the spinor field Lagrangian is chosen to be λ
F, with λ being a self-coupling constant and F being a function of the invariants I an J constructed from bilinear spinor forms S and P. Self-consistent solutions to the spinor and BI gravitational field equations are obtained in terms of τ, where τ is the volume scale of BI universe. System of equations for τ and ε, where ε is the energy of the viscous fluid, is deduced. This system is solved numerically for some special cases.
相似文献
2.
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
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.
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
5.
6.
The dominance of dark energy in the universe has necessitated the introduction of a repulsive gravity source to make q0 negative. The models for dark energy range from a simple Λ term to quintessence, Chaplygin gas, etc. We look at the possibility
of how change of behaviour of missing energy density, from DM to DE, may be determined by the change in the equation of state
of a background fluid instead of a form of potential. The question of cosmic acceleration can be discussed within the framework
of theories which do not necessarily include scalar fields. 相似文献
7.
8.
We look for cosmologies with a scalar field (dark energy without cosmological constant), which mimic the standard ΛCDM cosmological
model yielding exactly the same large-scale geometry described by the evolution of the Hubble parameter (i.e. photometric
distance and angular diameter distance as functions on z). Asymptotic behavior of the field solutions is studied in the case of spatially flat Universe with pressureless matter and
separable scalar field Lagrangians; the cases of power-law kinetic term and power-law potential are considered. Exact analytic
solutions are found in some special cases. A number of models have the field solutions with infinite behavior in the past
or even singular behavior at finite redshifts. We point out that introduction of the cosmological scalar field involves some
degeneracy leading to lower precision in determination of Ω
m
. To remove this degeneracy additional information is needed besides the data on large-scale geometry.
The article is published in the original. 相似文献
9.
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
10.
Farook Rahaman 《Astrophysics and Space Science》2003,283(2):155-159
W.A. Hiscock (1990, Class. Quantum Gravitation
7, L235) obtained the vacuum expectation value of the stress-energy tensor of an arbitrary collection of conformal massless
free quantum fields (scalar, spinor and vectors) in the space-time of a static global monopole. With this stress-energy tensor,
the semi-classical Einstein equations are solved retaining terms up to first order in ħ in Lyra geometry.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
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
12.
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
13.
The features of a homogeneous scalar field ϕ with classical Lagrangian L = ϕ;i
ϕ;i
/2 − V(ϕ) and tachyon field Lagrangian L = −V(ϕ)√1 − ϕ;i
ϕ;i
causing the observable accelerated expansion of the Universe are analyzed. The models with constant equation-of-state parameter
w
de = p
de/ρde < −1/3 are studied. For both cases the fields ϕ(a) and potentials V(a) are reconstructed for the parameters of cosmological model of the Universe derived from the observations. The effect of
rolling down of the potential V(ϕ) to minimum is shown.
Published in Ukrainian in Kinematika i Fizika Nebesnykh Tel, 2008, Vol. 24, No. 5, pp. 345–359.
The article was translated by the authors. 相似文献
14.
Antonio Pasqua A. Khodam-Mohammadi Mubasher Jamil R. Myrzakulov 《Astrophysics and Space Science》2012,340(1):199-208
Motivated by the holographic principle, it has been suggested that the dark energy density may be inversely proportional to the area A of the event horizon of the universe. However, such a model would have a causality problem. In this work, we consider the entropy-corrected version of the holographic dark energy model in the non-flat FRW universe and we propose to replace the future event horizon area with the inverse of the Ricci scalar curvature. We obtain the equation of state (EoS) parameter ω Λ, the deceleration parameter q and WD￠\Omega_{D}' in the presence of interaction between Dark Energy (DE) and Dark Matter (DM). Moreover, we reconstruct the potential and the dynamics of the tachyon, K-essence, dilaton and quintessence scalar field models according to the evolutionary behavior of the interacting entropy-corrected holographic dark energy model. 相似文献
15.
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
16.
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
17.
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
18.
Quasinormal modes of a black hole with quintessence-like matter and a deficit solid angle: scalar and gravitational perturbations 总被引：1，自引：0，他引：1
Ping Xi 《Astrophysics and Space Science》2009,321(1):47-51
In the previous paper (Li et al. in Phys. Lett. B 666:125–130, 2008), we show the solutions of Einstein equations with static spherically-symmetric quintessence-like matter surrounding a global
monopole. Furthermore, this monopole become a black hole with quintessence-like matter and a deficit solid angle when it is
swallowed by an ordinary black hole. We study its quasinormal modes by WKB method in this paper. The numerical results show
that both the real part of the quasinormal frequencies and the imaginary part decrease as the state parameter w, for scalar and gravitational perturbations. And we also show variations of quasinormal frequencies of scalar and gravitational
fields via different ε (deficit solid angel parameter) and different ρ
0 (density of static spherically-symmetric quintessence-like matter at r=1), respectively. 相似文献
19.
Koblandy Yerzhanov Kuralay Yesmakhanova Petr Tsyba Nurgissa Myrzakulov Gulgasyl Nugmanova Ratbay Myrzakulov 《Astrophysics and Space Science》2012,341(2):681-688
In the present work we analyze the g-essence model for the particular Lagrangian: . The g-essence models were proposed recently as an alternative and a generalization of the scalar k-essence models. We have presented the three types of the solutions for the g-essence model. We reconstructed the corresponding potentials and the dynamics of the scalar and fermionic fields according the evolution of the scale factor. The results show that the g-essence model predicts that our universe can be in both of the decelerated and accelerated expansion phases. In late time limit, we show that there is a family of exact solutions in which the free parameter may be remains in the range of m>−1. Further we discuss the existence of the de Sitter solutions in such a model. 相似文献
20.
Malcolm Fairbairn 《Astrophysics and Space Science》2003,283(4):613-618
Precision measurements of gauge couplings at particle accelerators strongly suggest that the standard model fields are unified
at about 1016 GeV. This talk will consist of a brief review of this evidence and also how in such a setting variations in α
em
lead to much larger variations in Λ
QCD
. I will then describe how variations in the strong force will impact upon nuclear astrophysics, considering in particular
the stability of di-nucleons and stellar and primordial nucleosynthesis. (Based upon collaborations with T. Dent and with
A. Csoto, H. Oberhummer and H. Schlattl.)
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献