共查询到20条相似文献，搜索用时 46 毫秒
1.
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
2.
A class of non-vacuum expanding cosmological solutions of Wesson's 5D theory of gravity with variable rest mass is derived. The models are spatially homogeneous and isotropic and the source of gravitation is a pressureless fluid (dust) plus a cosmological constant term. The general and unified solution is found for the equations and some properties of different limiting cases are studied. Particularly, it is shown that for null cosmological constant the predicted age of the universe is smaller than the ones of the 4D FRW models. 相似文献
3.
In this paper, a general FRW cosmological model has been constructed in f(R,T) gravity reconstruction with variable cosmological constant. A number of solutions to the field equations has been generated by utilizing a form for the Hubble parameter that leads to Berman's law of constant deceleration parameter q = m-1. The possible decelerating and accelerating solutions have been investigated. For(q 0) we get a stable flat decelerating radiation-dominated universe at q = 1. For(q 0) we get a stable accelerating solution describing a flat universe with positive energy density and negative cosmological constant. Nonconventional mechanisms that are expected to address the late-time acceleration with negative cosmological constant have been discussed. 相似文献
4.
Sanjay Oli 《Astrophysics and Space Science》2008,314(1-3):89-94
This paper presents anisotropic, homogeneous two-fluid cosmological models in a Bianchi type I space–time with a variable
gravitational constant G and cosmological constant Λ. In the two-fluid model, one fluid represents the matter content of the universe and another fluid is chosen to model the
CMB radiation. We find a variety of solutions in which the cosmological parameter varies inversely with time t. We also discuss in detail the behavior of associated fluid parameters and kinematical parameters. This paper pictures cosmic
history when the radiation and matter content of the universe are in an interactive phase. Here, Ω is closing to 1 throughout
the cosmic evolution.
相似文献
5.
Bulk viscous cosmological models of universe with variable deceleration parameter in Lyra’s Manifold
N. Ibotombi Singh S. Romaleima Devi S. Surendra Singh A. Sumati Devi 《Astrophysics and Space Science》2009,321(3-4):233-239
FRW models of universe in the presence of viscous fluid are investigated in the cosmological theory based on Lyra’s Manifold. By considering the deceleration parameter to be a variable and the viscosity coefficient of bulk viscous fluid to be a constant, exacts solutions have been obtained from which three forms of model of the universe are derived. The physical properties of the models are also investigated. 相似文献
6.
Rishi Kumar Tiwari 《中国天文和天体物理学报》2011,(7)
Einstein's field equations with variable gravitational and cosmological constants are considered in the presence of perfect fluid for a Robertson-Walker universe by assuming the cosmological term to be proportional to R-m(R is a scale factor and m is a constant).A variety of solutions is presented.The physical significance of the cosmological models has also been discussed. 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.
Anirudh Pradhan Rekha Jaiswal Rajeev Kumar Khare 《Astrophysics and Space Science》2013,343(1):489-497
On getting motivation from increasing evidence for the need of a geometry that resembles Bianchi morphology to explain the observed anisotropy in the WMAP data, Einstein’s field equations with variable cosmological “constant” are considered in presence of perfect fluid for a homogeneous and anisotropic Bianchi type-I space-time. Einstein’s field equations are solved by considering a time dependent deceleration parameter which affords a late time acceleration in the universe. The cosmological constant Λ is found to be a decreasing function of time and it approaches a small positive value at the present epoch which is corroborated by consequences from recent supernovae Ia observations. From recently developed Statefinder pair, the behavior of different stages of the evolution of the universe has been studied. The physical significance of the cosmological models have also been discussed. 相似文献
9.
《New Astronomy》2020
A flat FLRW (Friedmann–Lemaitre–Robertson–Walker) cosmological model with perfect fluid comprising of variable Chaplygin gas (VCG) has been studied in the context of f(R, T) gravity with particle creation. The solutions of the modified field equations are obtained through three different considered form of scale factors. The effective pressure is negative throughout the evolution of universe, which leads to accelerated expansion of the universe. In addition to that we have also discussed the importance of particle creation pressure on the cosmological parameters, energy conditions and state-finder diagnostic parameters. It is noticed that the time evolution of source function yields almost constant particle production at late times. 相似文献
10.
Exact solutions of the semi-classical Einstein equations with cosmological constant for conformally invariant free quantum fields in a general Robertson-Walker metric are found when a classical perfect fluid is present. There exist a one-parameter family of time-symmetric bouncing solutions that avoid the singularity and a one-parameter family which does not have particles horizons. The de Sitter solution is found to be stable, while the Einstein universe is unstable. 相似文献
11.
In addition to our previous paper (Dehnen and Obregón, 1971) the exact cosmological solutions of Brans and Dicke's scalar-tensor theory allowing a power law between the gravitational constant and the radius of curvatureR of the universe are sought in case that — in contrast to our previous paper — the initial conditionR(t=0)=0 is avoided. There exist two different types of solutions especially for the closed space of positive curvature and for positive values of the freely available parameter of the scalar-tensor theory. The radius of curvature and also the gravitational constant increase at first with respect to time and decrease after reaching a maximum value, in contradiction to Dirac's hypothesis whereafter the gravitational constant should decrease with time in an expanding universe. The age of the universe following from these solutions is in accordance with the observations. 相似文献
12.
Mariusz P. Dąbrowski 《Astrophysics and Space Science》1996,240(1):123-131
It is shown that growing-entropy stiff-fluid Kantowski-Sachs universes become time-symmetric (if they start with time-asymmetric phase) and isotropize. Isotropization happens without any inflationary era during the evolution since there is no cosmological term here. It seems that this approach is an alternative to inflation since the universe gets bigger and bigger, approaching flatness. 相似文献
13.
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
14.
《New Astronomy》2022
In this study, we build up a general formalism for tilted N-component fluid form to investigate the isotropization features of the Bianchi-type models excluding Bianchi-IX. We applied this formalism to Bianchi type I and V models analytically and numerically using the metric approach of Einstein field equations. It is found that only the stiff fluid for Bianchi I model does not isotropize, in the absence of cosmological constant. Other Bianchi type I and V models become isotropic regardless of the type of the fluid or how much component it has. The result does not change with the existence of a cosmological constant. 相似文献
15.
Suresh Kumar 《Astrophysics and Space Science》2011,332(2):449-454
The paper deals with a spatially homogeneous and isotropic FRW space-time filled with perfect fluid and dark energy components.
The two sources are assumed to interact minimally, and therefore their energy momentum tensors are conserved separately. A
special law of variation for the Hubble parameter proposed by Berman (Nuovo Cimento B 74:182, 1983) has been utilized to solve the field equations. The Berman’s law yields two explicit forms of the scale factor governing
the FRW space-time and constant values of deceleration parameter. The role of dark energy with variable equation of state
parameter has been studied in detail in the evolution of FRW universe. It has been found that dark energy dominates the universe
at the present epoch, which is consistent with the observations. The physical behavior of the universe has been discussed
in detail. 相似文献
16.
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
17.
Assuming the time-dependent equation of state p=λ(t)ρ, five dimensional cosmological models with viscous fluid for an open universe (k=−1) and flat universe (k=0) are presented. Exact solutions in the context of the rest mass varying theory of gravity proposed by Wesson (Astron. Astrophys.
119, 145, 1983) are obtained. It is found that the phenomenon of isotropisation takes place in this theory, i.e. the mass scale factor A(t) which characterizes the rest mass of a typical particle is evolving with cosmic time just as the spatial scale factor R(t). It is further found that rest mass is approximately constant in the present universe. 相似文献
18.
19.
The study of Einstein field equations describing the Robertson-Walker universe with the Brans-Dicke field and perfect fluid
has been made. It is found that the exact solutions to the B-D field interacting with perfect fluid reduce to a false vaccum
with cosmological constant and without cosmological constant the solution reduce to the exact solution of the matter dust
distribution in the Brans-Dicke cosmology.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
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