共查询到20条相似文献,搜索用时 31 毫秒
1.
《Physics letters. [Part B]》2006,632(5-6):610-616
Using a formalism recently introduced we study the decaying of the cosmological parameter during the early evolution of an universe, whose evolution is governed by a vacuum equation of state. We use a stochastic approach in a nonperturbative treatment of the inflaton field from a Noncompact Kaluza–Klein (NKK) theory, to study the evolution of energy density fluctuations in the early universe. 相似文献
2.
In this paper, we have considered the curved universe which is filled by tachyonic field. We have found the exact solutions
for the field, pressure, density, and scale factor and some cosmological parameters. In such universe, we have investigated
the role of tachyonic field in different stages of k for the evolution of the universe. Finally we draw the graphs for the scale factor, Hubble’s parameter, energy density, pressure,
acceleration parameter, equation of state and potential for the different values of k. Also we obtained the exact form of field which shows that the tachyonic field has the kink form. 相似文献
3.
4.
C. Wolf 《General Relativity and Gravitation》1995,27(10):1031-1042
The earliest phase of cosmological evolution is studied in a universe where spin-generated torsion effects are important. The evolution of the scale factor is studied in cosmologies with a Robertson-Walker line element and a spin density aligned in the radial direction (±r). Solutions are found for the cases of stiff matter, radiation, pressureless matter and decompressive matter admitting a spin density. 相似文献
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6.
In this paper we propose a higher dimensional Cosmology based on FRW model and brane-world scenario. We consider the warp
factor in the brane-world scenario as a scale factor in 5-dimensional generalized FRW metric, which is called as bulk scale factor, and obtain the evolution of it with space-like and time-like extra dimensions. It is then showed that, additional space-like
dimensions can produce exponentially bulk scale factor under repulsive strong gravitational force in the empty universe at
a very early stage. 相似文献
7.
We consider particle production in Robertson-Walker spacetime as particle-antiparticle rotation. We thereby obtain a scale
factor that guarantees particle production. We then study quantum field effects in spatially flat homogeneous and isotropic
spacetime with energy density of created particles and one loop quantum correction as back reaction. In the numerical solution
initial values are determined from particle production simulated scale factor and obtain the evolution of the universe both
at early and late times having a bounce. 相似文献
8.
9.
C. P. Singh 《International Journal of Theoretical Physics》2014,53(5):1533-1546
In this paper we consider a spatially homogenous and anisotropic Bianchi type-V space-time model to investigate the effects of a magnetic field in string cosmology. We assume that the string’s direction and magnetic field are along x-axis. The field equations are solved by using the equation of state for a cloud of strings and variable magnetic permeability. We derive exact solutions for three types of strings: (i) Nambu strings, (ii) string model where the sum of energy density and string tension density is zero and (iii) Takabayasi strings. We examine the behaviour of scale factors and other physical parameters with and without magnetic field and it is found that the magnetic field effects the dynamics of the universe at early time. During late time the universe becomes isotropic even in the presence of magnetic field. The universe expands with decelerated rate during early stages of the evolution of the universe but it goes to marginal inflation at late times. 相似文献
10.
Among the several proposals to solve the incompatibility between the observed small value of the cosmological constant and the huge value obtained by quantum field theories, we can find the idea of a decaying vacuum energy density, leading from high values at early times of universe evolution to the small value observed nowadays. In this paper we consider a variation law for the vacuum density recently proposed by Schützhold on the basis of quantum field estimations in the curved, expanding background, characterized by a vacuum density proportional to the Hubble parameter. We show that, in the context of an isotropic and homogeneous, spatially flat model, the corresponding solutions retain the well established features of the standard cosmology, and, in addition, are in accordance with the observed cosmological parameters. Our scenario presents an initial phase dominated by radiation, followed by a dust era long enough to permit structure formation, and by an epoch dominated by the cosmological term, which tends asymptotically to a de Sitter universe. Taking the matter density equals to half of the vacuum energy density, as suggested by observation, we obtain a universe age given by Ht = 1.1, and a decelerating parameter equals to −1/2. 相似文献
11.
Motivated by Granda and Oliveros (GO) model, we generalize their work to the non-flat case. We obtain the evolution of the
dark energy density, the deceleration and the equation of state parameters for the holographic dark energy model in a non-flat
universe with GO cut-off. In the limiting case of a flat universe, i.e. k=0, all results given in GO model are obtained. 相似文献
12.
In this paper, we have considered a model for Brans-Dicke scalar field in presence of Chaplygin gas and interaction between
them. We have shown that the BD parameter ω is constant or not, the Chaplygin gas provides early deceleration and late time acceleration of the universe. The graphical
representation of statefinder parameters shows the total evolution of the universe starts from radiation era to phantom model. 相似文献
13.
In this work, we have considered that the anisotropic universe is filled with normal matter and phantom field (or tachyonic
field). We have chosen the exponential forms of scale factors a and b in such a way that there is no singularity for evolution of the anisotropic universe. Here we have shown that the emergent
scenario is possible for open, closed or flat universe if the universe contains phantom field or tachyonic field or phantom
tachyonic field. From recently developed statefinder parameters, the behaviour of different stages of the evolution of the
emergent universe have been generated. 相似文献
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15.
V. Majerník 《General Relativity and Gravitation》2004,36(9):2139-2149
In this article we describe a model of the universe consisting of a mixture of the ordinary matter and a so-called cosmic quaternionic field. The basic idea here consists in an attempt to interpret as the energy density of the quaternionic field whose source is any form of energy including the proper energy density of this field. We set the energy density of this field to and show that the ratio of ordinary dark matter energy density assigned to is constant during the cosmic evolution. We investigate the interaction of the quaternionic field with the ordinary dark matter and show that this field exerts a force on the moving dark matter which might possible create the dark matter in the early universe. Such determined fulfils the requirements asked from the dark energy. In this model of the universe, the cosmological constant, the fine-tuning and the age problems might be solved. Finally, we sketch the evolution of the universe with the cosmic quaternionic field and show that the energy density of the cosmic quaternionic field might be a possible candidate for the dark energy. 相似文献
16.
We consider the flat Robertson–Walker model in scalar-tensor theory proposed by Lau and Prokhovnik. In this model, the field
equations are solved by using “gamma-law” form of equation of state p=(γ−1)ρ, where the adiabatic parameter ‘gamma’ (γ) varies continuously as the universe expands. Our aim is to study how the adiabatic parameter γ should vary so that in the course of its evolution the universe goes through a transition from an inflationary to a radiation-dominated
phase. A unified one parameter function of γ has been considered to describe the two early phases of evolution of universe. The solutions show the power-law expansion
and cosmological constant is found to be positive and decreasing function of cosmic time. The solutions are compatible with
the Dirac’s large number hypothesis. The deceleration parameter has been presented in a unified manner in terms of scale factor,
which describes the inflation of the model. The nature of singularity and the physical properties have been discussed in details. 相似文献
17.
We discuss the effects of cosmic phase transition on the spectrum of primordial gravitational waves generated during inflation. The energy density of the scalar condensation responsible for the phase transition may become sizable at the epoch of phase transition, which significantly affects the evolution of the universe. As a result, the amplitudes of the gravitational waves at high frequency modes are suppressed. Thus the gravitational wave spectrum can be a probe of phase transition in the early universe. 相似文献
18.
We present the scalar-tensor gravitational theory with an exponential potential in which Pauli metric is regarded as the physical
space-time metric. We show that it is essentially equivalent to coupled quintessence (CQ) model. However for baryotropic fluid
being radiation there are in fact no coupling between dilatonic scalar field and radiation. We present the critical points
for baryotropic fluid and investigate the properties of critical points when the baryotropic matter is specified to ordinary
matter. It is possible for all the critical points to be attractors as long as the parameters λ and β satisfy certain conditions. To demonstrate the attractor behaviors of these critical points, We numerically plot the phase
plane for each critical point. Finally with the bound on β from the observation and the fact that our universe is undergoing an accelerating expansion, we conclude that present accelerating
expansion is not the eventual stage of universe. Moreover, we numerically describe the evolution of the density parameters
Ω and the decelerating factor q, and computer the present values of some cosmological parameters, which are consistent with current observational data. 相似文献
19.
We consider the effect of torsion in the early universe to see if it is possible to explain the small value (if not zero) of the Cosmological constant at the present time. For the gauge-theoretic formulation of the Einstein-Cartan theory, we find a wormhole instanton solution which has a minimum (baby universe) radius of the Planck length. The basic difficulty with the wormhole approach is stressed. Finally, we give an explicit calculation from the expression for the evolution of the scale factor, which shows that the spin-dominated interaction term in the very early universe can cancel the Cosmological constant term at that epoch. 相似文献
20.
We investigate a class of cosmological solutions of Einstein’s field equations in higher dimensions with a cosmological constant and an ideal fluid matter distribution as a source. We discuss the dynamical evolution of the universe subject to two constraints that (i) the total volume scale factor of the universe is constant and (ii) the effective energy density is constant. We obtain various interesting new dynamics for the external space that yield a time varying deceleration parameter including oscillating cases when the flat/curved external and curved/flat internal spaces are considered. We also comment on how the universe would be conceived by an observer in four dimensions who is unaware of the extra dimensions. 相似文献