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1.
The recently discovered accelerated expansion of the universe is of current interest in theoretical research on the evolution
of the universe. The cause of this behavior is presumably the presence of dark energy, which has been estimated to form up
to 70% of the universe and generates a “repulsive force.” In this paper a cosmological model is constructed which takes the
dark energy into account in a Jordan-Brans-Dicke tensor-scalar model with a dominant, nonminimally coupled scalar field in
the presence of a cosmological scalar. The radiation dominant epoch is discussed.
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Translated from Astrofizika, Vol. 51, No. 1, pp. 151–159 (February 2008). 相似文献
2.
Modern concepts of the universe support the assumed existence of a nongravitational source, known as dark energy, for which
ε + 3 P < 0 (where ε is the energy density and P is the pressure). This ensures accelerated expansion of the universe. This paper examines a tensor-scalar
variant of the theory of gravitation with a conformally coupled scalar field. Various cosmological models are examined and
the possible evolutionary development of the universe with accelerated expansion is discussed.
Translated from Astrofizika, Vol. 51, No. 4, pp. 653–661 (November 2008). 相似文献
3.
Cosmological solutions are examined in the proper representation of the JBD theory with a dominant nonminimally coupled scalar
field. It is shown that only the introduction of a cosmological scalar that transforms to the ordinary cosmological constant
in the Einstein representation enables a phase of evolution with a uniform and then an accelerated expansion of the universe
over cosmological time scales.
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Translated from Astrofizika, Vol. 48, No. 4, pp. 633–640 (November 2005). 相似文献
4.
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
5.
Antonio Alfonso-Faus 《Astrophysics and Space Science》2012,337(1):367-372
Combining the kinematical definitions of the two dimensionless parameters, the deceleration q(x) and the Hubble t
0
H(x), we get a differential equation (where x=t/t
0 is the age of the universe relative to its present value t
0). First integration gives the function H(x). The present values of the Hubble parameter H(1) [approximately t
0
H(1)≈1], and the deceleration parameter [approximately q(1)≈−0.5], determine the function H(x). A second integration gives the cosmological scale factor a(x). Differentiation of a(x) gives the speed of expansion of the universe. The evolution of the universe that results from our approach is: an initial
extremely fast exponential expansion (inflation), followed by an almost linear expansion (first decelerated, and later accelerated).
For the future, at approximately t≈3t
0 there is a final exponential expansion, a second inflation that produces a disaggregation of the universe to infinity. We
find the necessary and sufficient conditions for this disaggregation to occur. The precise value of the final age is given
only with one parameter: the present value of the deceleration parameter [q(1)≈−0.5]. This emerging picture of the history of the universe represents an important challenge, an opportunity for the
immediate research on the Universe. These conclusions have been elaborated without the use of any particular cosmological
model of the universe. 相似文献
6.
7.
Anisotropic universe with cosmic strings and bulk viscosity in a scalar–tensor theory of gravitation
V. U. M. Rao G. Sree Devi Kumari K. V. S. Sireesha 《Astrophysics and Space Science》2011,335(2):635-638
A spatially homogeneous and anisotropic Bianchi type-I cosmological model is obtained in a scalar–tensor theory of gravitation
proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. Some physical
and kinematical properties of the model are discussed. It is observed that the bulk viscosity has a greater role in getting
an accelerated expansion of the universe in this theory. 相似文献
8.
We propose in this paper an interacting holographic dark energy (IHDE) model in chameleon–tachyon cosmology by interaction
between the components of the dark sectors. In the formalism, the interaction term emerges from the scalar field coupling
matter Lagrangian in the model rather than being inserted into the formalism as an external source for the interaction. The
correspondence between the tachyon field and the holographic dark energy (HDE) densities allows to reconstruct the tachyon
scalar field and its potential in a flat FRW universe. The model can show the accelerated expansion of the universe and satisfies
the observational data. 相似文献
9.
This paper investigates the existence of Noether symmetries of isotropic universe model in \(f(R,T)\) gravity admitting minimal coupling of matter and scalar fields. The scalar field incorporates two dark energy models such as quintessence and phantom models. We determine symmetry generators and corresponding conserved quantities for two particular \(f(R,T)\) models. We also evaluate exact solutions and investigate their physical behavior via different cosmological parameters. For the first model, the graphical behavior of these parameters indicate consistency with recent observations representing accelerated expansion of the universe. For the second model, these parameters identify a transition form accelerated to decelerated expansion of the universe. The potential function is found to be constant for the first model while it becomes \(V(\phi )\approx \phi ^{2}\) for the second model. We conclude that the Noether symmetry generators and corresponding conserved quantities appear in all cases. 相似文献
10.
《New Astronomy》2020
A flat FLRW (Friedmann–Lemaitre–Robertson–Walker) cosmological model with perfect fluid comprising of variable Chaplygin gas (VCG) has been studied in the context of f(R, T) gravity with particle creation. The solutions of the modified field equations are obtained through three different considered form of scale factors. The effective pressure is negative throughout the evolution of universe, which leads to accelerated expansion of the universe. In addition to that we have also discussed the importance of particle creation pressure on the cosmological parameters, energy conditions and state-finder diagnostic parameters. It is noticed that the time evolution of source function yields almost constant particle production at late times. 相似文献
11.
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
12.
《New Astronomy》2020
Taking up four model universes we study the behaviour and contribution of dark energy to the accelerated expansion of the universe, in the modified scale covariant theory of gravitation. Here, it is seen that though this modified theory may be a cause of the accelerated expansion it cannot totally outcast the contribution of dark energy in causing the accelerated expansion. In one case the dark energy is found to be the sole cause of the accelerated expansion. The dark energy contained in these models come out to be of the ΛCDM type and quintessence type comparable to the modern observations. Some of the models originated with a big bang, the dark energy being prevalent inside the universe before the evolution of this era. One of the models predicts big rip singularity, though at a very distant future. It is interestingly found that the interaction between the dark energy and the other part of the universe containing different matters is enticed and enhanced by the gauge function ϕ(t) here. 相似文献
13.
Using the teleparallel gravity versions of the Einstein and Landau–Lifshitz’s energy and/or momentum complexes, I obtain the
energy and momentum of the universe in viscous Kasner-type cosmological models. The energy and momentum components (due to
matter plus field) are found to be zero and this agree with a previous work of Rosen and Johri et al., who investigated the
problem of the energy in Friedmann–Robertson–Walker (FRW) universe. The result that the total energy and momentum components
of the universe in these models is zero same as Bergmann–Thomson’s energy–momentum and props the viewpoint of Tryon. Rosen
found that the energy of the FRW space–time is zero, which agrees with the studies of Tryon.
PACs Numbers: 04.20.-q; 04.50.+h
An erratum to this article is available at . 相似文献
14.
《天文和天体物理学研究(英文版)》2016,(4)
On studying some new models of Robertson-Walker universes with a Brans-Dicke scalar field, it is found that most of these universes contain a dark energy like fluid which confirms the present scenario of the expansion of the universe. In one of the cases, the exact solution of the field equations gives a universe with a false vacuum, while in another it reduces to that of dust distribution in the Brans-Dicke cosmology when the cosmological constant is not in the picture. In one particular model it is found that the universe may undergo a Big Rip in the future, and thus it will be very interesting to investigate such models further. 相似文献
15.
Self-consistent system of spinor, scalar and BI gravitational fields in presence of magneto-fluid and Λ term is considered.
Assuming that the expansion of the BI universe is proportional to the σ1 component of the shear tensor, exact solutions for the metric functions, as well as for scalar and spinor fields are obtained.
For a non-positive Λ, the initially anisotropic space–time becomes isotropic one in the process of expansion; whereas for
Λ > 0, an oscillatory mode of expansion of the BI model occurs.
PACS numbers: 03.65.Pm and 04.20.Ha 相似文献
16.
Alain Blanchard 《Astronomy and Astrophysics Review》2010,18(4):595-645
Evidence for an accelerated expansion of the universe as it has been revealed 10 years ago by the Hubble diagram of distant
type Ia supernovae represents one of the major modern revolutions for fundamental physics and cosmology. It is yet unclear
whether the explanation of the fact that gravity becomes repulsive on large scales should be found within general relativity
or within a new theory of gravitation. However, existing evidences for this acceleration all come from astrophysical observations.
Before accepting a drastic revision of fundamental physics, it is interesting to critically examine the present situation
of the astrophysical observations and the possible limitation in their interpretation. In this review, the main various observational
probes are presented as well as the framework to interpret them with special attention to the complex astrophysics and theoretical
hypotheses that may limit actual evidences for the acceleration of the expansion. Even when scrutinized with skeptical eyes,
the evidence for an accelerating universe is robust. Investigation of its very origin appears as the most fascinating challenge
of modern physics. 相似文献
17.
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
18.
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
19.
In this paper, we discuss energy conditions in modified Gauss-Bonnet gravity for locally rotationally symmetric Bianchi type I universe model with perfect fluid. The matter contents are constructed to discuss the energy conditions bounds. We take two specific f(G) models along with present day values of Hubble, deceleration, jerk and snap parameters. It is found that weak and null energy conditions are satisfied while strong energy conditions are violated for both models which represents the accelerated expansion of the universe. 相似文献
20.
The possibility that the cosmological constant is decaying as the observable universe grows is explored, and we define a cosmological parameter, depending of the vacuum energy and the universe radius, which should be presently ca. 122 orders of magnitude smaller than at the Planck epoch. From it, a new version of the Friedmann equation for a flat universe is obtained, which allows the estimation of the Hubble parameter at any epoch and the reconstruction of the expansion history. The main result is a quasi-linear expansion dynamics in concurrence with a number of previous works. This behavior is compatible with the main features of observational cosmology and avoids the horizon, flatness, cosmological constant, coincidence and age problems without the need of neither inflation nor initial fine-tuning. 相似文献