共查询到20条相似文献，搜索用时 46 毫秒
1.
R. K. Tiwari 《Astrophysics and Space Science》2009,321(2):147-150
Einstein field equations with variable gravitational and cosmological constants are considered in the presence of perfect
fluid for Robertson-Walker universe by assuming the cosmological term proportional to the Hubble parameter. This variation
law for vacuum density has recently been proposed by Schützhold on the basis of quantum field estimations in the curved and
expanding background. The cosmological term tends asymptotically to a genuine cosmological constant and the model tends to
a deSitter universe. We obtain that the present universe is accelerating with a large fraction of cosmological density in
the form of cosmological term. 相似文献
2.
R. K. Tiwari 《Astrophysics and Space Science》2008,318(3-4):243-247
Einstein field equations with variable gravitational and cosmological constants are considered in the presence of perfect fluid for Bianchi type-I universe by assuming the cosmological term proportional to the Hubble parameter. This variation law for vacuum density has recently been proposed by Schützhold on the basis of quantum field estimations in the curved and expanding background. The model obtained approaches isotropy. The cosmological term tends asymptotically to a genuine cosmological constant, and the model tends to a deSitter universe. We obtain that the present universe is accelerating with a large fraction of cosmological density in the form of cosmological term. 相似文献
3.
The solutions of Einstein’s equations with cosmological constant (Λ) in the presence of a creation field have been obtained for general class of anisotropic cosmological models. We have obtained the cosmological solutions for two different scenarios of average scale factor. In first case, we have discussed three different types of physically viable cosmological solutions of average scale factor for the general class of Bianchi cosmological models by using a special law for deceleration parameter which is linear in time with a negative slope. In second case, we have discussed another three different forms of cosmological solutions by using the average scale factor in three different scenarios like Intermediate scenario, Logamediate scenario and Emergent scenario. All physical parameters are calculated and discussed in each physical viable cosmological model. We examine the nature of creation field and cosmological constant is dominated the early Universe but they do not survive for long time and finally tends to zero for large cosmic time t. We have also discussed the all energy conditions in each cases. 相似文献
4.
5.
Nonzero cosmological constant favours cosmological models with larger Hubble constant. The evolution of ionization during
decoupling period in a universe with nonzero cosmological constant is computed by using a corrected recombination coefficient.
Also presented in this paper is the redshift distribution of the last scattering surfaces of the cosmic background photons
while the cosmological constant is nonvanishing. Finally, we give a brief estimation about the influence of He on the last scattering surfaces.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
U. Kasper 《Astronomische Nachrichten》1988,309(4):259-261
Scalar fields are an important ingredient of modern cosmological models describing the very early universe. If they are of the Higgs field type, scalar fields offer a possibility to understand why the cosmological constant is such a small quantity. This is because of the fact that different ground states are possible for a Higgs field. The unstable ground state gives an inflationary stage of the cosmic evolution and a large cosmological constant whereas the stable ground state has a vanishing cosmological constant and is decisive for the late time behaviour with an Einstein-De Sitter — like expansion law. 相似文献
7.
Rishi Kumar Tiwari 《中国天文和天体物理学报》2010,10(4)
Einstein's field equations with variable gravitational and cosmological constants are considered in the presence of perfect fluid for the Bianchi type-Ⅰ universe by assuming that the cosmological term is proportional to R-m(R is a scale factor and m is a constant).A variety of solutions are presented.The physical significance of the respective cosmological models are also discussed. 相似文献
8.
A Bianchi type-I cosmological model in the presence of a magnetic flux along a cosmological string is considered. The first
objective of this study is to investigate Einstein equations using a tractable assumption usually accepted in the literature.
Quantum effects of the present cosmological model are examined in the framework of loop quantum cosmology. Finally we draw
a parallel between the classical and quantum approaches. 相似文献
9.
Bianchi type I perfect fluid cosmological model is investigated with a variable cosmological term. Einstein’s field equations
are solved for any arbitrary cosmic scale factor. The main result of the study is the expression for cosmological term as
a power law of scale factor. The age of the universe can also be readily calculated. 相似文献
10.
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
11.
P. Ruiz-Lapuente 《Astrophysics and Space Science》2004,290(1-2):43-59
This review gives an update of the cosmological use of SNe Ia and the progress made in testing their properties from the local universe to high-z. The cosmological road from high-z supernovae down to Galactic SNe Ia is followed in search of the answer to standing questions on their nature and their validity as cosmological indicators. 相似文献
12.
Can Battal Kılınç 《Astrophysics and Space Science》2004,289(1-2):103-109
Einstein's equations with variable gravitational and cosmological constants are considered in the presence of a perfect fluid for the anizotropic Bianchi I universe in a way which conserving the energy-momentum tensor. Two solutions are found, one of which the cosmological term varies inversely with power law of time. The other of which cosmological term is constant. 相似文献
13.
E. Schmutzer 《Astronomische Nachrichten》2000,321(4):209-224
In several papers the 5‐dimensional Projective Unified Field Theory (PUFT) of the author (Schmutzer 1998, Schmutzer 1999, Schmutzer 2000a) was applied to a homogeneous, isotropic and closed cosmological model without pressure (cosmology of the attraction era). Here we derive a cosmological equation of state for the pressure. This step enables us to treat cosmology for the complete time scale from the big start (Urstart) to the present time (repulsion era and attraction era). Here following subjects are treated, where our specific terminology of PUFT is taken over from our previous publications quoted above: cosmological equation of state for the pressure, numerical integration of the system of the resulting cosmological differential equations and evaluation of the results. Specific outcomes (temporal course and present values) are given for the following cosmological quantities: radius and age of the world, cosmological frequency shift (Hubble factor), deceleration parameter, effective gravitational “constant”, mass density, temperature, pressure, entropy, behavior of the photon gas and of the mechanical particle gas etc. 相似文献
14.
J -L. Atteia 《Astrophysics and Space Science》1995,231(1-2):11-18
We discuss the use of selected characteristics of the burster population (e.g. the distribution of peak fluxes, or durations) to derive the GRB distance scale within the framework of cosmological models. The effects of the cosmological expansion on GRBs are briefly noted and it is shown that intrinsic GRB properties may strongly complicate the search for (and the interpretation of) purely cosmological effects. In this context, bright GRBs provide a sample of reference; they are almost free of cosmological influences and they have been studied for a long time. We also emphasize the need for a GRB distance indicator which could be used for individual events; several recent studies suggest that such a quantity may exist. 相似文献
15.
Cosmological electrovac field equations are studied in Bertotti-Robinson-type space-time, and a class of cosmological solutions is obtained. The nature of the electromagnetic fields and singularities of the solution is studied. A technique is established to generate these solutions from a known vacuum solution with a non-zero cosmological constant. 相似文献
16.
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
17.
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.
__________
Translated from Astrofizika, Vol. 48, No. 4, pp. 633–640 (November 2005). 相似文献
18.
António Vale José P. S. Lemos 《Monthly notices of the Royal Astronomical Society》2001,325(3):1197-1204
There is now evidence that the cosmological constant Λ has a non-zero positive value. Alternative scenarios to a pure cosmological constant model are provided by quintessence, an effective negative pressure fluid permeating the Universe. Recent results indicate that the energy density ρ and the pressure p of this fluid are constrained by − ρ ≤ p ≲−0.6 ρ . As p =− ρ is equivalent to the pure cosmological constant model, it is appropriate to analyse this particular, but important, case further.
We study the linear theory of perturbations in a Friedmann–Robertson–Walker universe with a cosmological constant. We obtain the equations for the evolution of the perturbations in the fully relativistic case, for which we analyse the single-fluid and two-fluid cases. We obtain solutions to these equations in appropriate limits. We also study the Newtonian approximation. We find that for a positive cosmological constant universe (i) the perturbations will grow more slowly in the relativistic regime for a two-fluid composed of dark matter and radiation, and (ii) in the Newtonian regime the perturbations stop growing. 相似文献
We study the linear theory of perturbations in a Friedmann–Robertson–Walker universe with a cosmological constant. We obtain the equations for the evolution of the perturbations in the fully relativistic case, for which we analyse the single-fluid and two-fluid cases. We obtain solutions to these equations in appropriate limits. We also study the Newtonian approximation. We find that for a positive cosmological constant universe (i) the perturbations will grow more slowly in the relativistic regime for a two-fluid composed of dark matter and radiation, and (ii) in the Newtonian regime the perturbations stop growing. 相似文献
19.
Dynamical and kinematic properties of Bianchi-II cosmological models with rotation and expansion are investigated. Exact solutions of Einstein field equations are obtained which describe the evolution of a rotating Universe. Exact solutions of null, timelike and spacelike geodesics are constructed. Two new cosmological tests for rotating universes are discussed: cosmological lens effect and cosmological mirror effect. 相似文献
20.
Bianchi type V cosmological models are studied that contain a relativistic ideal Boltzmann gas. The effect of a cosmological constant upon the space-time geometry is also considered. In both high and low temperature limit the general solution of the Einstein gravitational field equations can be expressed in an exact closed parametric form. At final stages, depending on the presence or absence of the cosmological constant, cosmologies are driven to an isotropic inflationary open de Sitter type Universe or to an isotropic open Friedmann era. 相似文献