共查询到20条相似文献，搜索用时 296 毫秒
1.
Murli Manohar Verma 《Astrophysics and Space Science》2010,330(1):101-105
We propose a time-varying cosmological constant with a fixed equation of state, which evolves mainly through its interaction
with the background during most of the long history of the universe. However, such an interaction does not exist in the very
early- and the late-time universe and this produces the acceleration during these eras when it becomes very nearly a constant.
It is found that after the initial inflationary phase, the cosmological constant, which we call the lambda parameter, rolls
down from a large constant value to another but very small constant value and further dominates the present epoch showing
up in the form of dark energy, driving the acceleration. 相似文献
2.
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
3.
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.
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Translated from Astrofizika, Vol. 51, No. 1, pp. 151–159 (February 2008). 相似文献
4.
In this paper, we investigate Bianchi type-VI cosmological model for the universe filled with dark energy and viscous fluid in the presence of cosmological constant. Also, we show accelerating expansion of the universe by drawing volume scale, pressure and energy density versus cosmic time. In order to solve the Einstein’s field equations, we assume the expansion scalar is proportional to a component of the shear tensor. Therefore, we obtain the directional scale factors and show the EOS parameter crosses over phantom divided-line. 相似文献
5.
Sanjay Oli 《Astrophysics and Space Science》2008,314(1-3):95-103
In this paper we present anisotropic, homogeneous two-fluid cosmological models in a Bianchi I space-time. These classes of
cosmological models picture two different scenarios of cosmic history; viz., when the radiation and matter content of the
universe are in interactive phase and another when the two are non-interacting. The universe is highly anisotropic in the
initial stages, however, anisotropy tapers out to insignificance in due course of cosmic evolution. In every model the anisotropy
of the space-time is determined by the density parameter Ω0 at the present epoch. For Ω0=1, the anisotropy is washed out before long. An interesting class of models, having an inflationary epoch in finite future,
is discovered.
相似文献
6.
It is shown that the recently suggested energy-dependent torsion coupling constant can make the spin contributions of matter sources large enough to cancel the cosmological constant term at all stages in the early universe from the Planck epoch. 相似文献
7.
In this paper, we study an anisotropic Bianchi-I space-time model in f(R) theory of gravity in the presence of perfect fluid as a matter contains. The aim of this paper is to find the functional form of f(R) from the field equations and hence the solution of various cosmological parameters. We assume that the deceleration parameter to be a constant, and the shear scalar proportional to the expansion scalar to obtain the power-law form of the scale factors. We find that the model describes the decelerated phases of the universe under the choice of certain constraints on the parameters. The model does not show the acceleration expansion and also transition from past deceleration to present accelerating epoch. We discuss the stability of the functional form of f(R) and find that it is completely stable for describing the decelerating phase of the universe. 相似文献
8.
The neutrinos from the Big Bang or the Cosmic Neutrino Background (CNB) carry precious information from the early epoch when our universe was only 1 s old. Although not yet directly detected, CNB may be revealed indirectly through cosmological observations due to neutrino important cosmological influence.We review the cosmological role of neutrinos and the cosmological constraints on neutrino characteristics. Namely, we discuss the impact of neutrinos in the early universe: the cosmic expansion, neutrino decoupling, the role of neutrinos in the primordial production of light elements, leptogenesis, etc. We briefly discuss the role of neutrino at later stages of the universe.Due to the considerable cosmological influence of neutrinos, cosmological bounds on neutrino properties from observational data exist. We review the cosmological constraints on the effective number of neutrino species, neutrino mass and mixing parameters, lepton number of the universe, presence of sterile neutrino, etc. 相似文献
9.
Future evolution of nearby large-scale structures in a universe dominated by a cosmological constant
《New Astronomy》2003,8(5):439-448
We simulate the future evolution of the observed inhomogeneities in the local universe assuming that the global expansion rate is dominated by a cosmological constant. We find that within two Hubble times (∼30 billion years) from the present epoch, large-scale structures will freeze in co-moving coordinates and the mass distribution of bound objects will stop evolving. The Local Group will get somewhat closer to the Virgo cluster in co-moving coordinates, but will be pulled away from the Virgo in physical coordinates due to the accelerated expansion of the Universe. In the distant future there will only be one massive galaxy within our event horizon, namely the merger product of the Andromeda and the Milky Way galaxies. All galaxies that are not gravitationally bound to the Local Group will recede away from us and eventually exit from our event horizon. More generally, we identify the critical interior overdensity above which a shell of matter around an object will remain bound to it at late times. 相似文献
10.
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
11.
The universe with adiabatic matter creation is considered. It is thought that the negative pressure caused by matter creation
can play the role of a dark energy component, and drive the accelerating expansion of the universe. Using the Type Ia supernovae
(SNe Ia) data, the observational Hubble parameter data, the Cosmic Microwave Background (CMB) data and the Baryonic Acoustic
Oscillation (BAO) data, we make constraints on the cosmological parameters, assuming a spatially flat universe. Our results
show that the model with matter creation is consistent with the SNe Ia data, while the joint constraints of all these observational
data disfavor this model. If the cosmological constant is taken into account, a traditional model without matter creation
is favored by the joint observations. 相似文献
12.
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
13.
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
14.
Ilian T. Iliev Paul R. Shapiro 《Monthly notices of the Royal Astronomical Society》2001,325(2):468-482
An analytical model is presented for the post-collapse equilibrium structure of virialized objects that condense out of a low-density cosmological background universe, either matter-dominated or flat with a cosmological constant. This generalizes the model we derived previously for an Einstein–de Sitter (EdS) universe. The model is based upon the assumption that cosmological haloes form from the collapse and virialization of 'top-hat' density perturbations, and are spherical, isotropic and isothermal. This leads to the prediction of a unique, non-singular, truncated isothermal sphere (TIS), a particular solution of the Lane–Emden equation (suitably modified when Λ≠0) . The size and virial temperature are unique functions of the mass and redshift of formation of the object for a given background universe. The central density is roughly proportional to the critical density of the universe at the epoch of collapse. This TIS model is in good agreement with observations of the internal structure of dark-matter-dominated haloes on scales ranging from dwarf galaxies to X-ray clusters. It also reproduces many of the average properties of haloes in simulations of the cold dark matter (CDM) model to good accuracy, suggesting that it is a useful analytical approximation for haloes that form from realistic initial conditions. Our TIS model matches the density profiles of haloes in CDM N -body simulations outside the innermost region, while avoiding the steep central cusp of the latter which is in apparent conflict with observations. The TIS model may also be relevant to non-standard CDM models, such as that for self-interacting dark matter, recently proposed to resolve this conflict. 相似文献
15.
Arbab I.Arbab 《中国天文和天体物理学报》2003,3(2):113-118
Exact solutions for a model with variable G,A and bulk viscosity are obtained,Inflationary solutions with constant(de Sitter-type )and variable energy density are found.An expanding anisotropic universe is found to isotropize during its expansion but a static universe cannot isotropize.The gravitational constant is found to increase with time and the cosmological constant decreases with time as A∝t^-2。 相似文献
16.
J. P. Singh 《Astrophysics and Space Science》2008,318(1-2):103-107
In this paper, we investigate a variation law for Hubble’s parameter in the curved, expanding background of spatially homogeneous, anisotropic Bianchi type I space-time. By choosing a particular form of the generalized Hubble’s parameter, which gives an early deceleration and late time acceleration for the anisotropic Bianchi type I cosmological model, we show that the model approaches isotropy and tends to a de Sitter universe at late times. The cosmological term asymptotically tends to a genuine cosmological constant and the solution is consistent with recent observations. 相似文献
17.
Ahmad Rami El-Nabulsi 《Astrophysics and Space Science》2009,324(1):71-75
The recent observational available data for an accelerated expansion state of the present universe, obtained from distant
SNeIa gave strong support to the search of alternative cosmologies. Recently, there have been a number of different attempts
to modify Einstein’s gravity to yield accelerated expansion at late times. Unfortunately, many of the theoretical models discussed
in the literature are plagued with theoretical problems, in particular the singularity problem at the origin of time. In the
present work we have analyzed a multidimensional spacetime Friedmann–Robertson–Walker (FRW) model with a decaying cosmological
constant and a varying gravitational constant. Many interesting consequences are revealed, in particular the behavior of the
scale factor and the shape of the universe in terms of the number of extra dimensions. 相似文献
18.
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
19.
Bianchi type V viscous fluid cosmological model for barotropic fluid distribution with varying cosmological term Λ is investigated. We have examined a cosmological scenario proposing a variation law for Hubble parameter H in the background of homogeneous, anisotropic Bianchi type V space-time. The model isotropizes asymptotically and the presence of shear viscosity accelerates the isotropization. The model describes a unified expansion history of the universe indicating initial decelerating expansion and late time accelerating phase. Cosmological consequences of the model are also discussed. 相似文献
20.
《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