共查询到20条相似文献，搜索用时 31 毫秒
1.
D. R. K. Reddy 《Astrophysics and Space Science》2006,305(2):139-141
Explicit field equations of a scalar tensor theory of gravitation proposed by Saez and Ballester are obtained with the aid of Einstein–Rosen cylindrically symmetric metric in the presence of cosmic string source. The field equations being highly non–linear static and non–static cases have been considered separately. It is observed that in the static case the geometric strings do not exist while in the non–static case cosmological model does not exist in this theory. 相似文献
2.
A. A. Saharian 《Astrophysics》2010,53(3):434-445
We examine the spectrum of perturbations for a scalar field with an arbitrary curvature coupling parameter in the de Sitter
stage of cosmological expansion when a cosmic string is present. These perturbations are caused by vacuum fluctuations in
the field and serve as seed perturbations for the formation of galaxies in the postinflationary stage. A cosmic string disrupts
the homogeneity of a de Sitter space, so that the spectrum of the perturbations depends on the distance from the string. This
dependence is oscillatory in character with a period on the order of the perturbation wavelength. 相似文献
3.
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
4.
A variant of the Jordan-Brans-Dicke (JBD) theory is examined which contains a cosmological scalar that is written so that on going to the Einstein representation it becomes the ordinary cosmological constant of general relativity theory. This paper is divided into two parts. In Part I we examine the cosmological solutions for the Einstein representation of the JBD theory, i.e., in the presence of a minimally coupled scalar field. In Part II we shall study the cosmological solutions in the proper representation of the JBD theory with a self consistent scalar field. The analysis of these solutions is of interest in connection with modern concepts of the evolution of the universe, in particular, with the observed acceleration of cosmological expansion and estimates of the density of dark matter and dark energy.__________Translated from Astrofizika, Vol. 48, No. 3, pp. 455–462 (August 2005). 相似文献
5.
A. A. Saharian 《Astrophysics》1996,39(2):153-168
Multidimensional, anisotropic cosmological models with Ricci-flat subspaces are investigated within the framework of low-energy string theory. The main properties of these models, their behavior at early and late stages of evolution, in particular, are determined on the basis of a qualitative theory of dynamical systems. The conditions for dynamical compactification of extra dimensions are found. A concrete model with a Kalb — Ramond field as the source is considered as an illustration.Translated from Astrofizika, Vol. 39, No. 2, pp. 287–311, April–June, 1996. 相似文献
6.
A. Zhuk 《Astronomische Nachrichten》1995,316(5):269-274
A multi-dimensional cosmological model with space-time consisting of n(n ≥ 2) Einstein spaces Mi is investigated in the presence of a cosmological constant λ and a homogeneous minimally coupled free scalar field. A generalized de Sitter solution was found for λ > 0 and a Ricci-flat external space for the case of static internal spaces with fine tuning of parameters. 相似文献
7.
F. Rahaman S. Chakraborty S. Das R. Mukherjee M. Hossain N. Begam 《Astrophysics and Space Science》2003,288(4):379-387
We have presented in homogeneous cosmological models within the framework of Lyra geometry. We have considered an inhomogeneous
spherically symmetric higher dimensional model in presence of a mass less scalar field whose potential has a flat part. The
scalar field is considered to be inhomogeneous. Also an inhomogeneous cosmological model is derived in a Kaluza-Klein type
of space time. The matter field is taken as an inhomogeneous distribution of fluid. It is observed that there is no singularity
at finite past in our model and the desirable feature of dimensional reduction is also possible for the extra space.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
Mark Israelit 《Astrophysics and Space Science》1996,240(2):331-344
Dark matter is obtained from a scalar field coupled conformally to gravitation, the scalar being a relict of Dirac's gauge function. This conformally coupled dark matter includes a gas of very light (m 2.25 × 10–34
eV) neutral bosons having spin 0, as well as a time-dependent global scalar field, both pervading all of the cosmic space. The time-development of this dark matter in the expanding F-R-W universe is investigated, and an acceptable cosmological behaviour is obtained. 相似文献
9.
Farkhat Zaripov 《Astrophysics and Space Science》2014,352(1):289-305
This research is an extension of the author’s works, in which conformally invariant generalization of string theory was suggested to higher-dimensional objects. Special cases of the proposed theory are Einstein’s theory of gravity and string theory. This work is devoted to the formation of self-consistent equations of the theory of induced gravity in the presence of matter in the form of a perfect fluid that interacts with scalar fields. The study is done to solve these equations for the case of the cosmological model. In this model time-evolving gravitational and cosmological “constants” take place which are determined by the square of scalar fields. The values of which can be matched with the observational data. The equations that describe the theory have solutions that can both match with the solutions of the standard theory of gravity as well as it can differ from it. This is due to the fact that the fundamental “constants” of the theory, such as gravitational and cosmological, can evolve over time and also depend of the coordinates. Thus, in a rather general case the theory describes the two systems (stages): Einstein and “evolving”. This process is similar to the phenomenon of phase transition, where the different phases (Einstein gravity system, but with different constants) transit into each other. 相似文献
10.
The evolution of a homogeneous, isotropic cosmological model driven by a nonminimally coupled scalar field is studied. The
potential for the quintessential inflation model proposed by Peebles and Vilenkin is selected as a scalar potential. Possible
scenarios for the cosmological dynamics are described in the conformal Einstein and Jordan representations. It is shown that,
unlike in models with a minimal scalar field, here a class of solutions exists for which the scalar field is fixed at finite
values during cosmological expansion.
__________
Translated from Astrofizika, Vol. 49, No. 3, pp. 487–498 (August 2006). 相似文献
11.
We consider cosmological dynamics of a canonical bulk scalar field, which is coupled non-minimally to 5-dimensional Ricci scalar in a DGP setup. We show that presence of this non-minimally coupled bulk scalar field affects the jump conditions of the original DGP model significantly. Within a superpotential approach, we perform some numerical analysis of the model parameter space and consider bulk-brane energy exchange in this setup. Also we show that the normal, ghost-free branch of the DGP solutions in this case has the potential to realize a self-consistent phantom-like behavior and therefore explains late time acceleration of the universe in a consistent way. 相似文献
12.
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
13.
C. P. Singh 《Astrophysics and Space Science》2013,343(2):773-781
The objective of the present paper is to study an anisotropic Bianchi-I cosmological model filled with bulk viscous fluid and magnetic field in string cosmology. The magnetic field is due to an electric current produced along the x-axis. The expansion in the model is considered to be proportional to one of the components of the shear tensor. We obtain two different quadrature forms of volume scale factor by considering two different relations between bulk viscosity and expansion scalar. We discuss the behavior of the classical potential with respect to the volume scale factor in the presence or absence of magnetic field and bulk viscosity in each case. We observe the role of bulk viscosity on the classical potential and also on the choices of bulk viscous pressure. By introduction of magnetic field or bulk viscosity or both into the model it results in changes in the potential as well as in volume scale factors. The physical and geometrical aspects of the solutions are discussed in detail. 相似文献
14.
A. A. Saharian 《Astrophysics》1994,37(2):189-195
We solve the cosmological equations for the bimetric scalar—tensor theory of gravitation (BSTT) for a flat model of the Friedmann type with the equation of state p = a. In the initial stage of expansion, the energy density of the scalar field dominates over the energy density of matter. As a result, the behavior of the solution in this limit does not depend on a. For later stages of expansion of the Universe, the solution obtained goes to a special solution having the form of a power law function of time. In this case, the relative change in the gravitational scalar is proportional to the Hubble parameter. In the limit of large values for the parameter of the theory, only a simple solution with zero value of the constant of integration goes to the corresponding Friedmann solution of general relativity theory.Translated from Astrofizika, Vol. 37, No. 2, pp. 351–362, April–June, 1994.I would like to thank L. Sh. Grigoryan for valuable discussions and support. 相似文献
15.
Relativistic cosmological field equations are obtained for a Robertson-Walker space time interacting with viscous fluid and
massive scalar field. The cosmological solutions to the field equations are obtained and the nature of the scalar field as
well as the viscous fluid are studied. It is found that the solutions obtained are realistic only for a closed Universe.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
D. R. K. Reddy R. L. Naidu S. Atchuta Rao K. N. Devi 《Astrophysics and Space Science》2007,310(3-4):177-180
Field equations in the presence of cosmic string source are obtained in a scalar tensor theory of gravitation proposed by
Brans and Dicke (Phys. Rev. 124, 925 (1961)) with the aid of a five-dimensional Kaluza–Klein metric. An exact string cosmological model is presented which represents
a five-dimensional Reddy string (Astrophys. Space Sci. 286, 2003b) in Brans–Dicke theory. Some physical properties of the model are also discussed 相似文献
17.
M. I. Wanas 《Astrophysics and Space Science》1989,154(2):165-177
The field equations of the generalized field theory constructed by Mikhail and Wanas have been applied to a well-established geometrical structure given earlier by H. P. Robertson in connection with the cosmological problem. A unique solution, representing a specified expanding Universe (withq
0=0, 0=0.75,k=–1) has been obtained. The model obtained has been compared with cosmological observations and with FRW-models of relativistic cosmology. It has been shown that the suggested model is free of particle horizons. The existence of singularities has been discussed.The two cases, when the associated Riemannian-space has a definite or indefinite metric are considered. The case of indefinite metric with signature (+ – – –) is found to be characterized byk=–1, while the case of +ve definite metric is characterized byk=+1. Apart from that difference, the two cases give rise to the same cosmological parameters. It has been shown that energy conditions are satisfied by the material contents in both cases. 相似文献
18.
Homogeneous isotropic cosmological solutions are obtained for a de Sitter type of metric in the presence of a self-gravitating scalar field with cubic nonlinearity. Unlike the usual de Sitter case which is indefinitely expanding it is here interestingly found that in the presence of a nonlinear scalar field the model gives a bounce from a maximum of spatial volume. The possibility of bounce from a maximum, however, disappears when a linear scalar field is considered instead. 相似文献
19.
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
20.
We consider the BSBM(Bekenstein, Sandvik, Barrow and Magueijo) cosmological model in the presence of tachyon potential with the aim of studying the stability of the model and test it against observations. The phase space analysis shows that from fourteen critical points that represent the state of the universe, only one is stable.With a small perturbation, the universe transits from a state of unstable deceleration to stable acceleration. The stability analysis combined with the best fitting process imposes constraints on the cosmological parameters that are in agreement with observation. In the BSBM theory, the variation of fundamental constants is driven from variation of a scalar field. The tachyonic scalar field, responsible for both variation of fundamental constants and universal acceleration, is reconstructed. 相似文献