共查询到20条相似文献,搜索用时 15 毫秒
1.
Orest?Hrycyna Jakub?Mielczarek Marek?Szyd?owski 《General Relativity and Gravitation》2009,41(5):1025-1049
In this paper, we investigate dynamics of the modified loop quantum cosmology models using dynamical systems methods. Modifications
considered come from the choice of the different field strength operator F̂ and result in different forms of the effective Hamiltonian. Such an ambiguity of the choice of this expression from some
class of functions is allowed in the framework of loop quantization. Our main goal is to show how such modifications can influence
the bouncing universe scenario in the loop quantum cosmology. In effective models considered we classify all evolutional paths
for all admissible initial conditions. The dynamics is reduced to the form of a dynamical system of the Newtonian type on
a two-dimensional phase plane. These models are equivalent dynamically to the FRW models with the decaying effective cosmological
term parameterized by the canonical variable p (or by the scale factor a). We demonstrate that the evolutional scenario depends on the geometrical constant parameter Λ as well as the model parameter
n. We find that for the positive cosmological constant there is a class of oscillating models without the initial and final
singularities. The new phenomenon is the appearance of curvature singularities for the finite values of the scale factor,
but we find that for the positive cosmological constant these singularities can be avoided. The values of the parameter n and the cosmological constant differentiate asymptotic states of the evolution. For the positive cosmological constant the
evolution begins at the asymptotic state in the past represented by the de Sitter contracting (deS−) spacetime or the static Einstein universe H = 0 or H = − ∞ state and reaches the de Sitter expanding state (deS+), the state H = 0 or H = + ∞ state. In the case of the negative cosmological constant we obtain the past and future asymptotic states as the Einstein
static universes. 相似文献
2.
3.
A new dark energy model called “ghost dark energy” was recently suggested to explain the observed accelerating expansion of
the universe. This model originates from the Veneziano ghost of QCD. The dark energy density is proportional to Hubble parameter,
ρ
D
= α H, where α is a constant of order LQCD3{\Lambda_{\rm QCD}^3} and ΛQCD ~ 100 MeV is QCD mass scale. In this Letter, we extend the ghost dark energy model to the universe with spatial curvature
in the presence of interaction between dark matter and dark energy. We study cosmological implications of this model in detail.
In the absence of interaction the equation of state parameter of ghost dark energy is always w
D
> −1 and mimics a cosmological constant in the late time, while it is possible to have w
D
< −1 provided the interaction is taken into account. When k = 0, all previous results of ghost dark energy in flat universe are recovered. For the observational test, we use Supernova
type Ia Gold sample, shift parameter of cosmic microwave background radiation and the correlation of acoustic oscillation
on the last scattering surface and the baryonic acoustic peak from Sloan Digital Sky Survey are used to confine the value
of free parameter of mentioned model. 相似文献
4.
In this paper, we propose a simple finite temperature λφ4 cosmological model to show that a new type singularity free cosmological model could be established by taing a series of impoftant quantum and statistical effects into consideration such as spontaneoui rymetry breaking, trace anomaly and pirtide creation, symmetry restoration at hish temperature through.phase transition and others. To begin with, the state of the universe would be a cold singu1arity.free and horizon free Beltrami-Anti de Sitter one rather than a hot one. Then associated with the,particle creatipl, the temperature would, become higher and higher and as soon. as the temperature reached a critical value, Tc a second-order phase transition would take place and the universe would transfer to a hot radiation dominated Friedmann state. 相似文献
5.
6.
Marek Demiański Marek Szydłowski Jerzy Szczesny 《General Relativity and Gravitation》1990,22(11):1217-1227
We propose to call a dynamical dimensional reduction effective if the corresponding dynamical system possesses a single attracting critical point representing expanding physical space-time and static internal space. We show that theBV × T
D multidimensional cosmological model with a hydrodynamic energy-momentum tensor provides an example of effective dimensional reduction. We also study the dynamics of the multidimensional cosmological model of typeBI × T
D with an energy-momentum tensor representing low temperature quantum effects, monopole contribution and the cosmological constant. It turns out that anisotropy and the cosmological constant are crucial for the process of dimensional reduction to be effective. We argue that this is the general property of homogeneous multidimensional cosmological models. 相似文献
7.
We further extend the cosmological scenario with energy exchange by Barrow and Clifton and our previous work to the more complex
case with energy exchange between three fluids: radiation, matter and vacuum energy. By prescribing the form of energy exchange
function, we construct an infinitely cyclic cosmological model, in which the universe undergoes an endless sequence of cosmic
epoch and each consisting of expansion and contraction, and the cosmological parameters, such as the Hubble parameter H, deceleration parameter q, transition red-shift Z
T, and densities ρ
r
,ρ
m
, and ρ
Λ are consistent with the present observed values. 相似文献
8.
The present study deals with a spatially homogeneous and anisotropic Bianchi type-I (B-I) cosmological models representing
massive strings in normal gauge for Lyra’s manifold by applying the variation law for generalized Hubble’s parameter that
yields a constant value of deceleration parameter. The variation law for Hubble’s parameter generates two types of solutions
for the average scale factor, one is of power-law type and other is of the exponential-law type. Using these two forms, Einstein’s
modified field equations are solved separately that correspond to expanding singular and non-singular models of the universe
respectively. The energy-momentum tensor for such string as formulated by Letelier, P.S.: Phys. Rev. D 28, 2414 (1983) is used to construct massive string cosmological models for which we assume that the expansion (θ) in the model is proportional to the component s1 1\sigma^{1}_{~1} of the shear tensor sj i\sigma^{j}_{~i}. This condition leads to A=(BC)
m
, where A, B and C are the metric coefficients and m is proportionality constant. Our models are in accelerating phase which is consistent to
the recent observations. It has been found that the displacement vector β behaves like cosmological term Λ in the normal gauge treatment and the solutions are consistent with recent observations
of SNe Ia. It has been found that massive strings dominate in the both decelerating and accelerating universes. The strings
dominate in the early universe and eventually disappear from the universe for sufficiently large times. This is in consistent
with the current observations. Some physical and geometric behaviour of these models are also discussed. 相似文献
9.
10.
In this paper we have considered a cosmological model representing a flat viscous universe with variable G and in the context of higher dimensional spacetime. It has been observed that in this model the particle horizon exists and the cosmological term varies as inverse square of time. The deceleration parameter and temperature are well within the observational limits. The model indicates matter and entropy generation in the early stages of the universe. Further, it is shown that our model generates all models obtained by Arbab and Singh et al. in four-dimensional space-time. 相似文献
11.
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
12.
G. S. Khadekar B. Sarada Cenap Ozel 《International Journal of Theoretical Physics》2008,47(6):1542-1551
A homogeneous and isotropic Friedmann-Robertson-Walker (FRW) model with varying gravitational and cosmological constant is
studied in the context of higher dimensional space time. Exact solution of the field equations are obtained by using the “gamma
law” equation of state p=(γ−1)ρ, where γ is adiabatic parameter varies continuously as the universe expands. The functional form γ which is assumed to be the function of scale factor R as proposed by Carvalho (1996) is used to analyse the behavior of scale factor R, cosmological constant Λ and the gravitational constant G for two different phases: inflation and radiation. The various physical aspects of the early cosmological models has also
been discussed in the framework of higher dimensional space time. 相似文献
13.
I. V. Kirnos A. N. Makarenko S. A. Pavluchenko A. V. Toporensky 《General Relativity and Gravitation》2010,42(11):2633-2641
We investigate dynamics of (4 + 1) and (5 + 1) dimensional flat anisotropic Universe filled with a perfect fluid in the Gauss–Bonnet
gravity. An analytical solutions valid for particular values of the equation of state parameter w = 1/3 have been found. For other values of w structure of cosmological singularity have been studied numerically. We found that for w > 1/3 the singularity is isotropic. Several important differences between (4 + 1) and (5 + 1) dimensional cases are discussed. 相似文献
14.
S. K. Srivastava 《International Journal of Theoretical Physics》2008,47(7):1966-1978
Using modified gravity with non-linear terms of curvature, R
2 and R
(2+r) (with r being a positive real number and R being the scalar curvature), cosmological scenario, beginning at the Planck scale, is obtained. Here a unified picture of
cosmology is obtained from f(R)-gravity. In this scenario, universe begins with power-law inflation followed by deceleration and acceleration in the late universe
as well as possible collapse of the universe in future. It is different from f(R)-dark energy models with non-linear curvature terms assumed as dark energy. Here, dark energy terms are induced by linear as well as non-linear
terms of curvature in Friedmann equation being derived from modified gravity. It is also interesting to see that, in this
model, dark radiation and dark matter terms emerge spontaneously from the gravitational sector. It is found that dark energy,
obtained here, behaves as quintessence in the early universe and phantom in the late universe. Moreover, analogous to brane-tension
in brane-gravity inspired Friedmann equation, a tension term λ arises here being called as cosmic tension, It is found that, in the late universe, Friedmann equation (obtained here) contains
a term −ρ
2/2λ (ρ being the phantom energy density) analogous to a similar term in Friedmann equation with loop quantum effects, if λ>0 and brane-gravity correction when λ<0. 相似文献
15.
16.
17.
G. S. Khadekar Gopal L. Kondawar Cenap Ozel 《International Journal of Theoretical Physics》2008,47(7):1844-1852
The field equations with variable cosmological and gravitational constants are consider in the presence of perfect fluid for
Kaluza-Klein type cosmological model. The exact solutions of the field equations are obtained by using the gamma law equation
of state p=(γ−1)ρ in which the parameter γ depends on scale factor R. The functional form of γ(R) is used to analyze a wide range of cosmological solution at early universe for two phases in cosmic history: inflationary phase and radiation dominated phase. The corresponding physical interpretation of cosmological solution are also discussed in the framework of higher dimensional
space time. 相似文献
18.
19.
Teppo Mattsson 《General Relativity and Gravitation》2010,42(3):567-599
Motivated by the observed cosmic matter distribution, we present the following conjecture: due to the formation of voids and
opaque structures, the average matter density on the path of the light from the well-observed objects changes from Ω
M
≃ 1 in the homogeneous early universe to Ω
M
≃ 0 in the clumpy late universe, so that the average expansion rate increases along our line of sight from EdS expansion
Ht ≃ 2/3 at high redshifts to free expansion Ht ≃ 1 at low redshifts. To calculate the modified observable distance–redshift relations, we introduce a generalized Dyer–Roeder
method that allows for two crucial physical properties of the universe: inhomogeneities in the expansion rate and the growth
of the nonlinear structures. By treating the transition redshift to the void-dominated era as a free parameter, we find a
phenomenological fit to the observations from the CMB anisotropy, the position of the baryon oscillation peak, the magnitude–redshift
relations of type Ia supernovae, the local Hubble flow and the nucleosynthesis, resulting in a concordant model of the universe
with 90% dark matter, 10% baryons, no dark energy, 15 Gyr as the age of the universe and a natural value for the transition
redshift z
0 = 0.35. Unlike a large local void, the model respects the cosmological principle, further offering an explanation for the
late onset of the perceived acceleration as a consequence of the forming nonlinear structures. Additional tests, such as quantitative
predictions for angular deviations due to an anisotropic void distribution and a theoretical derivation of the model, can
vindicate or falsify the interpretation that light propagation in voids is responsible for the perceived acceleration. 相似文献
20.
Kanti Jotania Padminin Yadav S. A. Faruqi 《International Journal of Theoretical Physics》2011,50(5):1424-1443
The present study deals with a spatially homogeneous and anisotropic Bianchi-II cosmological models representing massive strings
by applying the variation law for generalized Hubble’s parameter that yields a constant value of deceleration parameter. We
find that the constant value of deceleration parameter is reasonable for the present day universe. The variation law for Hubble’s
parameter generates two types of solutions for the average scale factor, one is of power-law type and other is of the exponential
form. Using these two forms, Einstein’s field equations are solved separately that correspond to expanding singular and non-singular
models of the universe respectively. The energy-momentum tensor for such string as formulated by Letelier (Phys. Rev. D 28:2414,
1983) is used to construct massive string cosmological models for which we assume that the expansion (θ) in the model is proportional to the component s11\sigma^{1}_{1} of the shear tensor sji\sigma^{j}_{i}. This condition leads to A=(BC)
m
, where A, B and C are the metric coefficients and m is proportionality constant. Our models are in accelerating phase which is consistent to the recent observations. The cosmological
constant Λ is found to be a decreasing function of time and it approaches a small positive value at present epoch which is
in good agreement by the results from recent supernovae observations. Some physical and geometric behaviour of the models
are also discussed. 相似文献