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1.
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
2.
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
3.
《New Astronomy》2021
Investigation of dark energy models in the presence of scalar fields are attracting several kinds of research because they play a vital role in the discussion of a new scenario of accelerated expansion of the universe. In this paper, we obtain an exact plane-symmetric dark energy cosmological model in the presence of an attractive massive scalar field by solving Einstein field equations using some physically relevant conditions. We have obtained all the cosmological parameters corresponding to the model. We have also presented a physical discussion of our model using a graphical representation of these parameters. The results exhibit an expanding and accelerating dark energy model of the universe, which are consistent with modern cosmological observations. 相似文献
4.
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
5.
A. A. Saaryan 《Astrophysics》1995,38(3):248-260
We carry out an analysis of the exact solution obtained in the first part of this work for a homogeneous and anisotropic flat cosmological model in the general conformal representation of multidimensional low-energy string theory. We show that the singular solutions with power law of variation of the scale factors of maximally symmetric subspaces and scalar field are asymptotes of the general solution in early and late stages of evolution. We study the conditions under which models of exponential, extended, and violent inflation are realized. As an illustration of the general regularities we consider the specific example of an antisymmetric Kalb-Ramond field as a gravitational source. We demonstrate the possibility of dynamic compactification of the extra dimensions.Translated fromAstrofizika, Vol. 38, No. 3, 1995. 相似文献
6.
Metod Saniga 《Astrophysics and Space Science》1992,193(1):155-159
A U(1)-symmetric Yang-Mills-Higgs (i.e., an Abelian Higgs) sunspot's model is recognized to originate from a massless, complex-valued scalar field coupled minimally to electromagnetic gauge potentials in the background of a (globally)conformally symmetric semi-metric spacetime, whose metric structure is described by the generalized Einstein equations with nonvanishing (positive-valued) cosmological constant. It is shown, in particular, that non-linearity (selfcoupling) of the scalar field appears due to a non-zeroness of the cosmological term, whereas its non-zero vacuum amplitude is induced by the (Ricci scalar) curvature of the Sun's spacetime manifold. 相似文献
7.
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
8.
M. Melek 《Astrophysics and Space Science》2000,272(4):417-421
Limits on cosmic time scale variations of gravitational and cosmological `constants' are studied. The study is based on a
function which can measure the temporal variation of the magnitude of the gradient of any scalar field defined inside a medium
exposed to a gravitational field. The cosmic time dependent scalar fields are taken to be the gravitational and cosmological
`constants'. The medium; in which those scalar fields are defined; is taken to be the spatially perturbed Friedman-Robertson-Walker
(FRW) expanding universe.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
A. A. Saaryan 《Astrophysics》1995,38(1):55-67
The low-energy string gravitation is investigated for the case of reduction with a variable (in string units) inner space. A flat cosmological model of the corresponding four-dimensional theory is constructed. The model equations are analyzed qualitatively for a potential-dominated scalar field as the source. It is demonstrated that an extended inflation stage with one purely scalar dilaton field is possible here, in contrast with the case of a constant inner space. The pattern of cosmological evolution in various conformal representations is discussed.Translated from Astrofizika, Vol. 38, No. 1, pp. 99–119, January–March, 1995. 相似文献
10.
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
11.
This paper is part of a series based on a modified Jordan tensor-scalar theory of gravitation. Given the current importance
of research on vacuum phenomena in cosmic evolution, we examine several standard cosmological models with a scalar field and
a physical vacuum, including models that have a dominant scalar field with the vacuum energy taken into account in various
conformal representations of the Jordan theory, as well as models in which ordinary matter that obeys the conventional equations
of state is present. Some noteworthy results are obtained which are, to a certain extent, consistent with currently available
observational data. 相似文献
12.
Marcelo Samuel Berman 《Astrophysics and Space Science》2007,310(3-4):205-207
We find a solution for exponential inflation in Brans–Dicke cosmology endowed with a cosmological term, which includes time-varying
shear and vorticity. We find that the scalar field and the scale factor increase exponentialy while shear, vorticity, energy
density, cosmic pressure and the cosmological term decay exponentialy for negative beta, where beta is defined in the text. 相似文献
13.
M. N. Varma 《Astrophysics and Space Science》1989,161(1):181-184
A spatially flat Robertson-Walker R-W cosmological model filled with a massive scalar field has been obtained for Einstein's field equations in the presence of a cosmological constant. 相似文献
14.
Raychaudhuri-type equations are derived for cosmological models filled with a perfect fluid and obeying the Brans-Dicke equations with a cosmological term depending on the scalar field. In addition, some general results on spatially homogeneous cosmological models are obtained in the theories due to Bergmann and Wagoner and Uehara and Kim. 相似文献
15.
O. Sergijenko Yu. Kulinich B. Novosyadlyj V. Pelykh 《Kinematics and Physics of Celestial Bodies》2009,25(1):17-27
The evolution of scalar perturbations is studied for 2-component (non-relativistic matter and dark energy) cosmological models
at the linear and non-linear stages. The dark energy is assumed to be the scalar field with either classical or tachyonic
Lagrangian and constant equation-of-state parameter w. The fields and potentials were reconstructed for the set of cosmological parameters derived from observations. The comparison
of the calculated within these models and observational large-scale structure characteristics is made. It is shown that for
w = const such analysis can’t remove the existing degeneracy of the dark energy models.
The article is published in the original. 相似文献
16.
We present constraints on the quintessence scalar field model from observational data of the variation of the fine structure constant obtained from the Keck telescope and VLT. Within the theoretical frame proposed by Bekenstein, the constraints on the parameters of the quintessence scalar field model are obtained. Considering the prior of Ωm0 as WMAP 7 suggests, we obtain various results from different sam- ples. Based on these results, we also calculate the probability density function of the coupling constant ζ. The best-fit values show a consistent relationship between ζ and the different experimental results. In our work, we test two different potential models, namely, the inverse power law potential and the exponential potential. The results show that both the large value of the parameters in the potential and the strong coupling can cause a variation in the fine structure constant. 相似文献
17.
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
18.
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.
__________
Translated from Astrofizika, Vol. 51, No. 1, pp. 151–159 (February 2008). 相似文献
19.
In this work we present a phase-space analysis of the cosmological relaxation of generalized gravity theories where the gravitational
constant G varies towards general relativity. We assess the interplay of the two main mechanisms that yield general relativity
as the cosmological attractor: (i) the vanishing of the coupling function α(ϕ), and (ii) the existence of a minimum of the
scalar field potential. The latter mechanism is shown to supersede the first. We classify the fixed points associated to different
types of potentials and discuss the late time self-similar solutions that arise when the scalar field potential exhibits an
asymptotic exponential behaviour from the viewpoint of the relaxation mechanism.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
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