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1.
The present study deals with a spatially homogeneous and anisotropic Bianchi-I cosmological models representing massive strings.
The energy-momentum tensor, as formulated by Letelier (1983), has been used to construct massive string cosmological models for which we assume the expansion scalar in the models is
proportional to one of the components of shear tensor. The Einstein’s field equations have been solved by applying a variation
law for generalized Hubble’s parameter in Bianchi-I space-time. We have analysed a comparative study of accelerating and decelerating
models in the presence of string scenario. The study reveals that massive strings dominate in the decelerating universe whereas
strings dominate in the accelerating universe. The strings eventually disappear from the universe for sufficiently large times,
which is in agreement with current astronomical observations. 相似文献
2.
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
3.
Einstein’s field equations with variable gravitational and cosmological “constant” are considered in presence of perfect fluid
for Bianchi type-I space-time. Consequences of the four cases of the phenomenological decay of Λ have been discussed which
are consistent with observations. The physical significance of the cosmological models have also been discussed.
相似文献
4.
Bianchi Type-V bulk viscous fluid string dust cosmological model in General Relativity is investigated. It has been shown
that if coefficient of bulk viscosity (ζ) is inversely proportional to the expansion (θ) in the model then string cosmological
model for Bianchi Type-V space-time is possible. In absence of bulk viscosity (ζ), i.e. when ζ → 0, then there is no string
cosmological model for Bianchi Type-V space-time. The physical and geometrical aspects of the model are also discussed. 相似文献
5.
K. S. Adhav S. D. Katore R. S. Rane K. S. Wankhade 《Astrophysics and Space Science》2009,323(1):87-90
In this paper, we have investigated spatially homogeneous isotropic Friedmann cosmological model with bulk viscosity and zero-mass
scalar field in Lyra manifold. The cosmological models are obtained with the help of the special law of variation for Hubble’s
parameter proposed by Bermann (Nuovo Cimento 74B:182, 1983) and power law relation. Some physical properties of the models are discussed. 相似文献
6.
We have studied the evolution of homogeneous and anisotropic Bianchi type-I cosmological models filled with perfect fluid
in Barber second self-creation theory by assuming a special law of variation for Hubble’s parameter that yield a constant
value of deceleration parameter. Some physical consequences of the models have been discussed in case of Zel’dovich fluid
and radiation dominated fluid. 相似文献
7.
An LRS Bianchi Type V bulk viscous fluid dust distribution string cosmological model in General Relativity is investigated.
It has been shown that if coefficient of bulk viscosity (ζ) is proportional to the expansion (θ) in the model then string
cosmological model for Bianchi Type V space-time is possible. In absence of bulk viscosity(ζ) i.e. when ζ → 0 then there is
no string cosmological model for Bianchi Type V space-time. The physical and geometrical aspects are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
J. K. Singh 《Astrophysics and Space Science》2008,314(4):361-365
Bianchi type-V anisotropic cosmological models have been studied in the theory based on Lyra’s geometry in normal gauge. The
physical and kinematical behaviors of the models have been discussed in the presence and absence of the magnetic field.
相似文献
9.
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
10.
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
11.
Hawking’s radiation effect of Klein-Gordon scalar field, Dirac particles and Maxwell’s electromagnetic field in the non-stationary
rotating de Sitter cosmological space-time is investigated by using a method of generalized tortoise co-ordinates transformation.
The locations and the temperatures of the cosmological horizons of the non-stationary rotating de Sitter model are derived.
It is found that the locations and the temperatures of the rotating cosmological model depend not only on the time but also
on the angle. The stress-energy regularization techniques are applied to the two dimensional analog of the de Sitter metrics
and the calculated stress-energy tensor contains the thermal radiation effect. 相似文献
12.
We have obtained Bianchi type-III cosmological model with strange quark matter attached to the string cloud in general relativity.
For solving the Einstein’s field equations the relation [C=A
n
] between metric coefficients C and A is used. Also, some physical and kinematic properties of the model are discussed.The results are analogous to results obtained
by Yilmaz (Gen. Rel. Grav. 38:1397–1406, 2006). 相似文献
13.
A. A. Saharian 《Astrophysics》1999,42(1):87-100
Homogeneous and Isotropic cosmological models of low-energy, string gravitation with loop corrections to the dilaton coupling
functions are investigated by methods of the qualitative theory of dynamical systems. An ideal fluid with a barotropic equation
of state is considered as the nongravitational source. In the general case of curved models, the cosmological equations are
represented in the form of a third-order, autonomous, dynamical system. Phase portraits for different coupling functions are
constructed for flat models. The asymptotic behavior of the general solution in limiting regions is investigated. The stabilization
of the dilaton is analyzed using the Damour-Polyakov mechanism.
Translated from Astrofizika, Vol. 42, No. 1, pp. 117–136, January–March, 1999. 相似文献
14.
In this paper, it is shown that five dimensional LRS Bianchi type-I string cosmological models do not survive for Geometric
and Takabayasi string whereas Barotropic string i.e. ρ=ρ(λ) survives and degenerates string with ρ+λ=0 in scalar tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986). Further we studied some physical and geometrical properties of the model. 相似文献
15.
Bianchi type-I string cosmological models are obtained in bimetric theory of gravitation proposed by Rosen (Gen. Relativ. Gravit. 4:435, 1973). Established the existence of string cosmological models, unlike the earlier authors, in this theory and studied some physical and geometrical properties. 相似文献
16.
In this paper, we solve the Einstein’s field equations for the space-time described by a special plane symmetric metric with
dark energy, and the exact solutions which offer an alternative and complementary approach to study cosmological models are
obtained. The dark energy is given by either the quintessence or the modified Chaplygin gas. We show the models are isotropic
and analyze the expansion scalar and the deceleration parameter of the models. 相似文献
17.
It is shown that Kantowski–Sachs cosmological models do not exist in Rosen's (1973) bimetric theory of gravitation when the
source of gravitation is either perfect fluid or cosmic string. Hence, the vacuum model is constructed. 相似文献
18.
In this paper, we have attempted to construct five dimensional string cosmological models in Riemannian and Lyra geometries.
It is found that cosmic string models do not survive in both the theories. Subsequently, the vacuum cosmological models are
constructed and discussed. 相似文献
19.
T. Padmanabhan 《Journal of Astrophysics and Astronomy》1997,18(4):303-311
I review the constraints on standard big bang model arising from considerations related to structure formation. I will focus
on two specific series of models though similar analysis can be performed for a wider class of models. The first one is a
Ω = 1 model with non-zero cosmological constant and the second one is a Ω < 1 model with zero cosmological constant. The observational
constraints which I shall discuss include the measurement of the Hubble’s constant, the ages of globular clusters, the abundance
of rich clusters, the baryon content of galaxy clusters and the abundance of high redshift objects. These constraints limit
the allowed range of the cosmological parameters and allow. for only a small region to survive. In particular, the aesthetically
pleasing model with Ω = 1 and zero cosmological constant is ruled out by the observations. It seems necessary to fine-tune
the theoretical parameters if they have to fall within the available space. This talk is based on the work in Baglaet al. (1996). 相似文献
20.
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