共查询到20条相似文献,搜索用时 31 毫秒
1.
2.
Zhong Chao Wu 《General Relativity and Gravitation》2006,38(2):381-386
In the Kaluza-Klein model with a cosmological constant Λ and a flux, the external spacetime of the created universe from aS
s
× S
n−s
seed instanton can be identified in quantum cosmology. One can also show that in the internal space theeffective cosmological
constant is most probably zero. 相似文献
3.
Paul S. Wesson 《Foundations of Physics Letters》2006,19(3):285-291
NAbstract Following fresh attempts to resolve the problem of the energy density of the vacuum, we reconsider the case where the cosmological
constant is derived from a higher-dimensional version of general relativity, and interpret the gauge-dependence of Λ as a
dynamical effect. This leads to a relation between the change in Λ and the line element (action) which is independent of gauge
choices and fundamental constants: dΛds2 = −6. This implies that the (classical) vacuum is unstable, with implications for particle production. 相似文献
4.
We investigate the possibility of inducing the cosmological constant from extra dimensions by embedding our four-dimensional
Riemannian space-time into a five-dimensional Weyl integrable space. Following the approach of the space-time-matter theory
we show that when we go down from five to four dimensions, the Weyl field may contribute both to the induced energy-tensor
as well as to the cosmological constant Λ, or more generally, it may generate a time-dependent cosmological parameter Λ(t). As an application, we construct a simple cosmological model in which Λ(t) has some interesting properties. 相似文献
5.
Vilson Tonin-Zanchin Erasmo Recami José A. Roversi Luis A. Brasca-Annes 《Foundations of Physics Letters》1994,7(2):167-179
Within a purely classical formulation of “strong gravity,” we associated hadron constituents (and even hadrons themselves)
with suitable stationary, axisymmetric solutions of certain new Einsteintype equations supposed to describe the strong field
inside hadrons. Such equations are nothing but Einstein equations—with cosmological term—suitably scaled down. As a consequence,
the cosmological constant Λ and the massesM result in our theory to be scaled up, and transformed into a “hadronic constant” and into “strong masses,” respectively.
Due to the unusual range of Λ andM values considered, we met a series of solutions of the Kerr-Newman-de Sitter (KNdS) type with rather interesting properties:
aim of the present work is putting forth such results, while “translating” them into the more popular language of ordinary
gravity.
The requirement that those solutions be stable, i.e., that their temperature (or surface gravity) bevanishingly small, implies the coincidence of at least two of their (in general, three) horizons. Imposing the stability condition of a certain
horizon does yield (once chosen the values ofJ, q and Λ) mass and radius of the associated black hole.
In the case of ordinary Einstein equations and for stable blackholes of the KNdS type, we get in particular Regge-like relations
among massM, angular momentumJ, chargeq and cosmological constant Λ; which did not receive enough attention in the previous literature. For instance, with the standard
definitionsQ
2 = Gq2/(4πε
0
c
4), a ≡ J/(Mc), m ≡GM/c
2, in the case Λ=0 in whichm
2=a2+Q2 and ifq is negligible, we findm
2=J. When considering, for simplicity, Λ>0 andJ=0 (andq still negligible), then we obtainm
2 = 1/(9Λ). In the most general case, the condition, for instance, of “triple coincidence” among the three horizons yields
for |Λa
2|<< 1 the couple of independent relationsm
2 = 2/(9Λ) andm
2 = 8(a
2 + Q2.
Another interesting point is that—with few exceptions—all such relations (amongM, J, q, Λ) lead to solutions that can be regarded as (stable) cosmological models.
Work partially supported by INFN, MURST, and CNR and by CNPq, FAPESP, and CAPES. 相似文献
6.
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
7.
F. Rahaman M. Kalam M. Sarker A. Ghosh B. Raychaudhuri 《General Relativity and Gravitation》2007,39(2):145-151
It has been suggested that the cosmological constant is a variable dynamical quantity. A class of solution has been presented
for the spherically symmetric space time describing wormholes by assuming the erstwhile cosmological constant Λ to be a space
variable scalar, viz., Λ = Λ (r) . It is shown that the averaged null energy condition (ANEC) violating exotic matter can be made arbitrarily small. 相似文献
8.
9.
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
10.
We make the cosmological constant, Λ, into a field and restrict the variations of the action with respect to it by causality.
This creates an additional Einstein constraint equation. It restricts the solutions of the standard Einstein equations and
is the requirement that the cosmological wave function possess a classical limit. When applied to the Friedmann metric it
requires that the cosmological constant measured today, t
U
, be L ~ tU-2 ~ 10-122{\Lambda \sim t_{U}^{-2} \sim 10^{-122}} , as observed. This is the classical value of Λ that dominates the wave function of the universe. Our new field equation
determines Λ in terms of other astronomically measurable quantities. Specifically, it predicts that the spatial curvature
parameter of the universe is Wk0 o -k/a02H2=-0.0055{\Omega _{\mathrm{k0}} \equiv -k/a_{0}^{2}H^{2}=-0.0055} , which will be tested by Planck Satellite data. Our theory also creates a new picture of self-consistent quantum cosmological
history. 相似文献
11.
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
12.
Lallan Yadav Vineet K. Yadav T. Singh 《International Journal of Theoretical Physics》2012,51(10):3113-3126
The present paper envisages a spatially homogeneous and anisotropic Bianchi II massive string cosmological models with time-decaying Λ term in general relativity. By using the variation law of Hubble’s parameter, the Einstein’s field equations have been solved for two general cases. The first case involving a power law solution describes the dynamics of universe from big bang to present epoch while the second case admit an exponential solution seems reasonable to project dynamics of future universe. We observed that massive strings dominate in early universe and eventually disappear at late time, which is consistent with the current astronomical observations. It has been found that the cosmological constant (Λ) is a decreasing function of time and it approaches to small positive value at sufficiently large time. The thermodynamic properties of anisotropic Bianchi II universe are studied and also the absolute temperature and entropy distribution are given explicitly. The relations between thermodynamic parameters and cosmological constant Λ has been established. Physical behavior of the derived model is elaborated in detail. 相似文献
13.
It is well known that solutions of general relativity which allow for traversable wormholes require the existence of exotic
matter (matter that violates weak or null energy conditions (WEC or NEC)). In this article, we provide a class of exact solution
for Einstein-Maxwell field equations describing wormholes assuming the erstwhile cosmological term Λ to be space variable,
viz., Λ=Λ(r). The source considered here not only a matter entirely but a sum of matters i.e. anisotropic matter distribution, electromagnetic
field and cosmological constant whose effective parts obey all energy conditions out side the wormhole throat. Here violation
of energy conditions can be compensated by varying cosmological constant. The important feature of this article is that one
can get wormhole structure, at least theoretically, comprising with physically acceptable matters. 相似文献
14.
A. V. Yurov V. A. Yurov A. V. Astashenok A. A. Shpilevoi 《The European Physical Journal C - Particles and Fields》2011,71(10):1740
An anthropic principle has made it possible to answer the difficult question of why the observable value of cosmological constant
(Λ∼10−47 GeV4) is so disconcertingly tiny compared to the predicted value of vacuum energy density ρ
SUSY∼1012 GeV4. Unfortunately, there is a darker side to this argument; being combined with the cosmic heat death scenario, it consequently
leads to another absurd prediction: the probability of randomly selected observer observing Λ=0 ends up being exactly equal to 1. We shall call this controversy an infrared divergence problem. It is shown that the IRD
prediction can be avoided with the help of a singular runaway measure coupled with the calculation of relative Bayesian probabilities
by the means of the doomsday argument. Moreover, it is shown that while the IRD problem occurs for the prediction stage of value of Λ, it disappears at the explanatory stage when Λ has already been measured by the observer. 相似文献
15.
Alain Bachelot 《Communications in Mathematical Physics》2008,283(1):127-167
We investigate the global solutions of the Dirac equation on the Anti- de-Sitter Universe. Since this space is not globally
hyperbolic, the Cauchy problem is not, a priori, well-posed. Nevertheless we can prove that there exists unitary dynamics, but its uniqueness crucially depends on the ratio
beween the mass M of the field and the cosmological constant Λ > 0: it appears a critical value, Λ/12, which plays a role similar to the Breitenlohner-Freedman
bound for the scalar fields. When M
2 ≥ Λ/12 there exists a unique unitary dynamics. On the contrary, for the light fermions satisfying M
2 < Λ/12, we construct several asymptotic conditions at infinity, such that the problem becomes well-posed. In all the cases,
the spectrum of the hamiltonian is discrete. We also prove a result of equipartition of the energy. 相似文献
16.
R. K. Tiwari Farook Rahaman Saibal Ray 《International Journal of Theoretical Physics》2010,49(10):2348-2357
A five dimensional Kaluza-Klein space-time is considered in the presence of a perfect fluid source with variable G and Λ.
An expanding universe is found by using a relation between the metric potential and an equation of state. The gravitational
constant is found to decrease with time as G∼t
−(1−ω) whereas the variation for the cosmological constant follows as Λ∼t
−2, L ~ ([(R)\dot]/R)2\Lambda \sim (\dot{R}/R)^{2} and L ~ [(R)\ddot]/R\Lambda \sim \ddot{R}/R where ω is the equation of state parameter and R is the scale factor. 相似文献
17.
Zdeněk Stuchlík Jan Hladík Martin Urbanec Gabriel Török 《General Relativity and Gravitation》2012,44(6):1393-1417
Extremely compact stars (ECS) (having radius R < 3GM/c 2) contain captured null geodesics. Certain part of neutrinos produced in their interior will be trapped, influencing thus their neutrino luminosity and thermal evolution. The trapping effect has been previously investigated for the internal Schwarzschild spacetimes with the uniform distribution of energy density. Here, we extend our earlier study considering the influence of the cosmological constant Λ on the trapping phenomena. Our model for the interior of ECS is based on the internal Schwarzschild-(anti-)de Sitter (S(a)dS) spacetimes with uniform distribution of energy density matched to the external vacuum S(a)dS spacetime with the same cosmological constant. Assuming uniform and isotropic distribution of local neutrino emissivity we determine behavior of the trapping coefficients, i.e., “global” one representing influence on the neutrino luminosity and “local” one representing influence on the cooling process. We demonstrate that the repulsive (attractive) cosmological constant has tendency to enhance (damp) the trapping phenomena. 相似文献
18.
Varun Sahni 《Pramana》1999,53(6):937-944
I briefly review the observational evidence for a small cosmological constant at the present epoch. This evidence mainly comes
from high redshift observations of Type 1a supernovae, which, when combined with CMB observations strongly support a flat
Universe with Ω
m
+ ΩA ⋍ 1. Theoretically a cosmological constant can arise from zero point vacuum fluctuations. In addition ultra-light scalar
fields could also give rise to a Universe which is accelerating driven by a time dependent Λ-term induced by the scalar field
potential. Finally a Λ dominated Universe also finds support from observations of galaxy clustering and the age of the Universe. 相似文献
19.
In a recent paper (Vigoureux et al. in Int. J. Theor. Phys. 47:928, 2007) it has been suggested that the velocity of light and the expansion of the universe are two aspects of one single concept
connecting space and time in the expanding universe. It has then be shown that solving Friedmann’s equations with that interpretation
(and keeping c=constant) can explain number of unnatural features of the standard cosmology (for example: the flatness problem, the problem
of the observed uniformity in term of temperature and density of the cosmological background radiation, the small-scale inhomogeneity
problem…) and leads to reconsider the Hubble diagram of distance moduli and redshifts as obtained from recent observations
of type Ia supernovae without having to need an accelerating universe. In the present work we examine the problem of the cosmological
constant. We show that our model can exactly generate Λ (equation of state P
φ
=−ρ
φ
c
2 with Λ∝
R
−2) contrarily to the standard model which cannot generate it exactly. We also show how it can solve the so-called cosmic coincidence
problem. 相似文献