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1.
A. A. Saharian 《Astrophysics》1999,42(2):222-233
Homogeneous and Isotropic cosmological models of effective string theory with a curved space are investigated by the methods
of the qualitative theory of dynamical systems. It is shown that for radiation-dominated models, tthe corresponding dynamical
system can be integrated exactly for the general case of dilaton coupling functions. Models in the tree approximation with
a two-dimensional phase space are considered separately. In the general case of loop corrections, all possible stationary
points are found and their character is determined. The results are illustrated using a specific example. Various cases of
fixing the dilaton within the framework of the Damour-Polyakov mechanism are considered.
Translated from Astrofizika, Vol. 42, No. 2, pp. 295–310, April–June, 1999. 相似文献
2.
A. A. Saharian 《Astrophysics》2000,43(2):230-238
A mechanism for stabilization of the dilaton field within the framework of low-energy string gravitation with loop corrections
to the dilaton coupling function was proposed in the first part of this paper. The mechanism is based on the assumption that
loop corrections generate a singular dilaton kinetic function for a certain value of the dilaton field. For a nortgravitational
source with a constant barotropic index, the system of cosmological equations reduces to an autonomous, third-order dynamical
system. The behavior of the general solution in the vicinity of singularities of the dilaton coupling function is investigated
by methods of the qualitative theory of dynamical systems for different values of the singularity index. The conditions under
which solutions with a constant dilaton are attractors for a general solution with a variable dilaton are determined. The
evolution of models is considered, the corresponding phase diagrams are constructed, and the question of the effectiveness
of dilaton stabilization is investigated.
Translated from Astrofizika, Vol. 43, No. 2, pp. 313–324, April–June, 2000. 相似文献
3.
A. A. Saharian 《Astrophysics》2000,43(1):92-101
A new mechanism is proposed for stabilization of the scalar dilaton field within the framework of lowenergy string gravitation
with loop corrections to the dilaton coupling functions. It is based on the assumption that the loop corrections generate
a kinetic dilaton function, which is singular for some finite value of the dilaton field. For a nongravitational source of
the barotropic type, the system of equations describing the evolution of homogeneous and isotropic cosmological models is
represented in the form of a thirdorder, autonomous, dynamical system. The behavior of the general solution in the vicinity
of singularities of the dilaton coupling function is investigated by methods of the qualitative theory of dynamical systems.
It is shown that there is a class of solutions, different from solutions of the general theory of relativity, with a constant
dilaton. The conditions under which these solutions are an attractor for a general solution with a variable dilaton are determined.
Translated from Astrofizika, Vol. 43, No. 1, pp. 123-136, January–March, 2000. 相似文献
4.
A. A. Saharian 《Astrophysics》2002,45(3):380-391
Cosmological evolution is investigated within the framework of low-energy string gravitation with higher-loop corrections to the dilaton coupling functions in the presence of a dilaton potential and a nongravitational source. It is shown that for homogeneous and isotropic models with a flat space, the cosmological system of equations reduces to an autonomous, third-order, dynamical system. Subclasses of models with a constant dilaton, which provide the basis for various cosmological mechanisms of dilaton stabilization, are considered. A class of solutions is distinguished with asymptotic scaling behavior of the energy density of the dilaton field. 相似文献
5.
A. A. Saharian 《Astrophysics》1997,40(2):154-166
The problems of flat directions and supersymmetry breaking are fundamental problems that are still unsolved in the comparison
of string theory with low-energy physics. A possible nonperturbative dilaton potential may play an important role in their
solution. In the present paper we consider D-dimensional, low-energy, string cosmological models with a di/aton potential.
Exact solutions are written for several simple potentials. The picture of cosmological evolution of a gravi-dilaton model
is investigated by methods of the qualitative theory of dynamical systems for different cases of the behavior of the potential
in the domain of strong coupling. Features of models having potentials that take negative values in certain ranges of values
of the dilaton field are discussed. A specific mechanism of generation of a nonperturbative potential, based on gaugino condensation
in the hidden sector of the gauge group, is considered.
Translated from Astrofizika, Vol. 40, No. 2, pp. 233–251, April–June, 1997. 相似文献
6.
Friedmann-Robertson-Walker gravidilaton cosmological models with a dilaton potential generated by gaugino condensation and
by nonperturbative corrections to the Kahler potential are analyzed within the framework of effective string gravitation.
The question of dilaton stabilization by such potentials is investigated. It is shown that the existence of a range of dilaton
values with a negative definite potential results in the possible existence of mixed expansion-contraction models with aflat
space. The corresponding phase portraits are constructed for qualitatively different cases, illustrating the possibility of
dilaton stabilization.
Translated from Astrofizika, Vol. 42, No. 3, pp. 465–476, July–September, 1999. 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.
A. A. Saharian 《Astrophysics》1997,40(4):335-346
Investigations of string cosmology with a nonperturbative dilaton potential, begun in the first part of this work, are continued.
The picture of cosmological evolution of an isotropic, gravi-dilaton model for different cases of the behavior of the potential
in the region of strong coupling is analyzed by methods of the qualitative theory of dynamical systems. Features of models
with potentials that take negative values in certain ranges of values of the dilaton field are discussed. A specific mechanism
of generation of the nonperturbative potential, based on gaugino condensation in a hidden sector of the gauge group, is considered.
Translated from Astrofizika, Vol. 40, No. 4, pp. 517–534, October-December, 1997. 相似文献
9.
A. A. Saharian 《Astrophysics》1998,41(2):180-193
Homogeneous cosmological models are investigated within the framework of low- energy string gravitation with loop corrections.
Various conformai representations of the effective action are considered. Without specifying the correction functions in the
Lagrangian, cosmological solutions are found with an arbitrary curvature and with dilaton fields, moduli fields, and Kalb-
Ramond fields corresponding to a source with an extremely stiff equation of state. They generalize previously known solutions
of the tree approximation. The behavior of the solutions in different asymptotic domains is investigated.
Translated from Astrofizika, Vol. 41, No. 2, pp. 277–295, April-June, 1998. 相似文献
10.
A. A. Saharian 《Astrophysics》2000,43(4):474-481
Within the framework of the mechanism of dilaton stabilization proposed in Part I of the present work, an analysis of homogeneous and isotropic cosmological models of low-energy string gravitation with loop corrections is continued. The behavior of models with curved space is investigated by methods of the qualitative theory of dynamical systems for different values of the singularity index of the dilaton kinetic function and the barotropic index of nongravitational matter. The conditions under which dilaton stabilization occurs as a result of cosmological expansion are determined, and the corresponding phase diagrams are constructed. 相似文献
11.
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
12.
In Garfinkle-Horowitz-Strominger dilaton spacetime, we study the property of coupling Scalar field with coupling term R∂
μ
ψ∂
μ
ψ, and the results show that as the charge term ξ increases, the absorption probability at weak coupling case decreases, but the absorption probability is enhanced at strong
coupling case. Next, we investigate the dynamical evolution of coupling field in this spacetime, and we find that the effect
of coupling field leads to the damping of the quasinormal modes at a slightly rapider rate. 相似文献
13.
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
14.
A. A. Saaryan 《Astrophysics》1995,38(2):164-175
We consider multi-dimensional cosmological models in the low-energy field theory of strings with a boson gravitational sector containing a metric, dilaton field, and antisymmetric Kalb-Ramon field. We study the conformal properties of the action and show that in the general conformal representation the theory is equivalent to a generalized scalar-tensor theory with a Lagrangian of nongravitating matter dependent on the dilaton. We find exact solutions of the flat homogeneous anisotropic model with structure R×M1×...×Mn and with equation of state pi=ai in the space Mi. We discuss the picture of cosmological evolution in different conformal representations.Translated fromAstrofizika, Vol. 38, No. 2, 1995. 相似文献
15.
We use the generalized Brans-Dicke theory, in which the Pauli metric is identified to be the physical space-time metric, to study the Universe in different epochs. Exact analytical expressions for dilaton field , cosmological radiusR and density parameter are obtained fork=+1,0,–1 Universe in the radiation-dominated epoch. For matter dominated Epoch, exact analytical expressions for Hubble parameterH, cosmological radius, dilaton field, deceleration factorq, density parameter and the gravitational coupling of the ordinary matter are obtained for the flat Universe. Other important results are: (1) the density parameter is always less than unity for the flat Universe because the dilaton field plays a role as an effective dark matter, and (2) the new Brans-Dicke parameter must be larger than 31.75 in order to consistent with the observed data. 相似文献
16.
In this paper, the generalized second law (GSL) of thermodynamics and entropy is revisited in the context of cosmological
models with bouncing behavior such as chameleon cosmology where the boundary of the universe is assumed to be enclosed by
the dynamical apparent horizon. From a thermodynamic point of view, to link between thermodynamic and geometric parameters
in cosmological models, we introduce “entropy rate of change multiplied by the temperature” as a model independent thermodynamic
state parameter together with the well known {r,s} statefinder to differentiate the dark energy models. 相似文献
17.
S. K. Tripathy S. K. Nayak S. K. Sahu T. R. Routray 《Astrophysics and Space Science》2009,323(1):91-96
Bianchi type-I string cosmological models are studied in Saez-Ballester theory of gravitation when the source for the energy
momentum tensor is a viscous string cloud coupled to gravitational field. The bulk viscosity is assumed to vary with time
and is related to the scalar expansion. The relationship between the proper energy density ρ and string tension density λ are investigated from two different cosmological models. 相似文献
18.
A global excitation variable is introduced, as a conformal factor of the metric that depends only on time, into the Einstein
theory of gravitation formulated in terms of conformally invariant variables. The dynamics of this global excitation is isolated
from the Einstein equations by direct averaging of their dynamical part over large spatial volumes. The conditions are found
under which this dynamics duplicates the dynamics of the Friedmann cosmological model.
Translated from Astrofizika, Vol. 42, No. 3, pp. 457–464, July–September, 1999. 相似文献
19.
C. P. Singh 《Astrophysics and Space Science》2011,331(1):337-342
The evolution and dynamics of a locally-rotationally-symmetric (LRS) Bianchi type-V space-time cosmological models are discussed
with variable gravitational and cosmological “constants” in context of the particle creation. We present the exact solutions
of Einstein field equations by using a power-law form of the average scale factor of the metric in the case of the particle
creation and in the absence of particle creation. The solution describes the particle and entropy generation in the anisotropic
cosmological models. The particle creation rate is uniquely determined by the variation of gravitational and cosmological
“constants”. We observe that the variable gravitational constant does not necessarily imply particle creation. In a generic
situation, models can be interpolated between different phases of the universe. The dynamical behaviors of the solutions and
kinematical parameters of the model are discussed in detail. 相似文献
20.
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