共查询到20条相似文献,搜索用时 31 毫秒
1.
Vitaly N. Melnikov 《Frontiers of Physics》2009,4(1):75-93
A review of different cosmological models in diverse dimensions leading to a relatively small time variation in the effective gravitational constant G is presented. Among them: the 4-dimensional (4-D) general scalar-tensor model, the multidimensional vacuum model with two curved Einstein spaces, the multidimensional model with the multicomponent anisotropic “perfect fluid”, the S-brane model with scalar fields and two form fields, etc. It is shown that there exist different possible ways of explaining relatively small time variations of the effective gravitational constant G compatible with present cosmological data (e.g. acceleration): 4-dimensional scalar-tensor theories or multidimensional cosmological models with different matter sources. The experimental bounds on ? may be satisfied either in some restricted interval or for all allowed values of the synchronous time variable. 相似文献
2.
Dipanjali Behera Sunil K. Tripathy Tushar R. Routray 《International Journal of Theoretical Physics》2010,49(10):2569-2581
Bulk Viscous anisotropic Bianchi-III cosmological models are investigated with time dependent gravitational and cosmological
constants in the framework of Einstein’s general relativity. In order to get some useful information about the time varying
nature of G and Λ, we have assumed an exponentially decaying rest energy density of the universe. The extracted Newtonian gravitational
constant G varies with time but its time varying nature depends on bulk viscosity and the anisotropic nature of the model. The cosmological
constant Λ is found to decrease with time to a small but positive value for the models. 相似文献
3.
In order to analyze how the gravitational constant, G, and the cosmological constant, Λ, may vary we study through symmetry principles the form of the functions in the generalized scalar-tensor theories under
the self-similar hypothesis. The results obtained are absolutely general and valid for all the Bianchi models and the flat
FRW one. We study the concrete example of the Kantowski–Sachs model finding some new exact self-similar solutions. 相似文献
4.
José A. Belinchón 《Central European Journal of Physics》2012,10(4):789-798
We study how the constants G and Λ may vary in different theoretical models (general relativity (GR) with a perfect fluid, scalar cosmological models (SM) (“quintessence”) with and without interacting scalar and matter fields and three scalar-tensor theories (STT) with a dynamical Λ) in order to explain some observational results. We apply the program outlined in section II to study the Bianchi I models, under the self-similarity hypothesis. We put special emphasis on calculating exact power-law solutions which allow us to compare the different models. In all the studied cases we conclude that the solutions are isotropic and noninflationary. We also arrive at the conclusion that in the GR model with time-varying constants, Λ vanishes while G is constant. In the SM all the solutions are massless i.e. the potential vanishes and all the interacting models are inconsistent from the thermodynamical point of view. The solutions obtained in the STT collapse to the perfect fluid one obtained in the GR model where G is a true constant and Λ vanishes as in the GR and SM frameworks. 相似文献
5.
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
6.
C-field cosmological models based on Hoyle-Narlikar theory with variable gravitational constant G in the frame work of FRW (Friedmann-Robertson-Walker) space–time for positive and negative curvatures are investigated. To
get the deterministic solutions in terms of cosmic time t, we have assumed G=R
n
and discussed for n=−1, −2, R being scalar factor. In both the cases, creation field C increases with time, the gravitational constant G and matter density (ρ) decrease with time in the model (21). In the model (41) G decreases with time and matter density (ρ) is constant. The other physical aspects of the models are also discussed. 相似文献
7.
8.
Samad Khakshournia 《General Relativity and Gravitation》2008,40(8):1791-1796
Within our thick brane approach previously used to obtain the cosmological evolution equations on a thick brane embedded in
a five-dimensional Schwarzschild Anti-de Sitter spacetime it is explicitly shown that the consistency of these equations with
the energy conservation equation requires that, in general, the thickness of the brane evolves in time. This varying brane
thickness entails the possibility that both Newton’s gravitational constant G and the effective cosmological constant Λ4 are time dependent. 相似文献
9.
Marcelo Samuel Berman 《International Journal of Theoretical Physics》2010,49(1):232-237
After reviewing the scalar-tensor inflationary solutions by Berman and Trevisan (Int. J. Theor. Phys. 29, 1411–1414, 2009), we obtain solutions for the amplification of gravitational waves in the models. The solutions consider a perfect gas equation
of state, with cosmic pressure proportional to the energy density, the proportionality constant being smaller than −2/3, and
a cosmological term. 相似文献
10.
Mubasher Jamil Farook Rahaman Mehedi Kalam 《The European Physical Journal C - Particles and Fields》2009,60(1):149-156
The standard model of cosmology is investigated using a time-dependent cosmological constant Λ and Newton gravitational constant G. The total energy content is described by the modified Chaplygin gas equation of state. It is found that the time-dependent
constants coupled with the modified Chaplygin gas interpolate between the earlier matter to the later dark-energy dominated
phase of the universe. We also achieve a convergence of the parameter ω→−1, almost at the present time. Thus our model fairly alleviates the cosmic-coincidence problem, which demands ω=−1 at the present time. 相似文献
11.
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
12.
We have considered higher dimensional cosmological models of the FRW model with variable G and Λ. The solutions have been obtained for flat model with particular form of cosmological constant. The cosmological parameters
have also been obtained for dust, radiation and stiff matter. Physical parameters of the models are discussed. 相似文献
13.
Bianchi Type-I cosmological models containing perfect fluid with time varying G and Λ have been presented. The solutions obtained represent an expansion scalar θ bearing a constant ratio to the anisotropy in the direction of space-like unit vector λ
i
. Of the two models obtained, one has negative vacuum energy density, which decays numerically. In this model, we obtain Λ
∼ H
2, Λ ∼ R
44/R and Λ ∼ T
−2 (T is the cosmic time) which is in accordance with the main dynamical laws for the decay of Λ. The second model reduces to a
static solution with repulsive gravity.
相似文献
14.
It is proven that no wormholes can be formed in viable scalar-tensor models of dark energy admitting its phantom-like (w < −1) behavior in cosmology, even in the presence of electric or magnetic fields, if the nonminimal coupling function f(Φ) is positive everywhere and the scalar field Φ itself is not a ghost. Some special static, spherically symmetric wormhole
solutions may exist if f(Φ) is allowed to reach zero or to become negative, so that the effective gravitational constant becomes negative in some
region making the graviton a ghost. If f remains non-negative, such solutions require severe fine-tuning and a very peculiar kind of model. If f < 0 is allowed, it is argued (and confirmed by previous investigations) that such solutions are generically unstable under
nonstatic perturbations, the instability appearing right near transition surfaces to negative f.
The text was submitted by the authors in English. 相似文献
15.
V. Fayaz H. Hossienkhani F. Felegary 《International Journal of Theoretical Physics》2012,51(8):2656-2664
We have investigated general Bianchi type I cosmological models which containing a perfect fluid and dark energy with time varying G and Λ that have been presented. The perfect fluid is taken to be one obeying the equation of state parameter, i.e., p=ωρ; whereas the dark energy density is considered to be either modified polytropic or the Chaplygin gas. Cosmological models admitting both power-law which is explored in the presence of perfect fluid and dark energy too. We reconstruct gravitational parameter G, cosmological term Λ, critical density ρ c , density parameter Ω, cosmological constant density parameter Ω Λ and deceleration parameter q for different equation of state. The present study will examine non-linear EOS with a general nonlinear term in the energy density. 相似文献
16.
We have considered a cosmological model of the FRW universe with variable G and Λ. The solutions have been obtained for flat model with particular form of cosmological constant. The cosmological parameters
have also been obtained for dust, radiation and stiff matter. The statefinder parameters are analyzed and have shown that
these depends only on w and ε. Further the lookback time, proper distance, luminosity distance and angular diameter distance have also been calculated
for our model. 相似文献
17.
Varun Sahni 《Pramana》2000,55(1-2):43-52
I present a short overview of current observational results and theoretical models for a cosmological constant. The main motivation
for invoking a small cosmological constant (or A-term) at the present epoch has to do with observations of high redshift Type
Ia supernovae which suggest an accelerating universe. A flat accelerating universe is strongly favoured by combining supernovae
observations with observations of CMB anisotropies on degree scales which give the ‘best-fit’ values ΘA ⋍ 0.7 and Θ
m
⋍ 0.3. A time dependent cosmological A-term can be generated by scalar field models with exponential and power law potentials.
Some of these models can alleviate the ‘fine tuning’ problem which faces the cosmological constant. 相似文献
18.
S. K. Srivastava 《International Journal of Theoretical Physics》1992,31(12):2103-2113
Einstein's equations for the generalized (4+D)-dimensional Robertson-Walker model are solved taking the conformally invariant action for the matter field. Compactification of this model is discussed and the compactification time/compactification mass scale for different values ofD is calculated. The resulting 4-dimensional action for gravity is obtained. It is found that a time-dependent cosmological constant is induced which is very large when the cosmic time is small and very small when the cosmic time is large. 相似文献
19.
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
20.
K. A. Bronnikov B. E. Meierovich 《Journal of Experimental and Theoretical Physics》2008,106(2):247-264
We consider (d
0 + 2)-dimensional configurations with global strings in two extra dimensions and a flat metric in d
0 dimensions, endowed with a warp factor e
2γ depending on the distance l from the string center. All possible regular solutions of the field equations are classified by the behavior of the warp
factor and the extradimensional circular radius r(l). Solutions with r → ∞ and r → const > 0 as l → ∞ are interpreted in terms of thick brane-world models. Solutions with r → 0 as l → l
c > 0, i.e., those with a second center, are interpreted as either multibrane systems (which are appropriate for large enough
distances l
c between the centers) or as Kaluza-Klein-type configurations with extra dimensions invisible due to their smallness. In the
case of the Mexican-hat symmetry-breaking potential, we build the full map of regular solutions on the (ɛ, Γ) parameter plane,
where ɛ acts as an effective cosmological constant and Γ characterizes the gravitational field strength. The trapping properties
of candidate brane worlds for test scalar fields are discussed. Good trapping properties for massive fields are found for
models with increasing warp factors. Kaluza-Klein-type models are shown to have nontrivial warp factor behaviors, leading
to matter particle mass spectra that seem promising from the standpoint of hierarchy problems.
The text was submitted by the authors in English. 相似文献