共查询到20条相似文献,搜索用时 15 毫秒
1.
We discuss how infinite density singularities may be shown to occur in Friedmann-Robertson-Walker universes and orthogonal spatially homogeneous universes, but how very different behaviours are possible in tilted homogeneous cosmologies. After considering various possibilities that arise in this case, we illustrate them by examining the behaviour of exact solutions of Einstein's equations for a homogeneous cosmology which is a locally rotationally symmetric tilted Bianchi type V universe. These universes - which can be arbitrarily similar to a Robertson-Walker universe at late times - show a variety of singular behaviours quite different from those in the ‘orthogonal’ case. In particular, there exist such universes in which two singularities occur at the early stages of the universe, but in which the density of matter is finite at all times. 相似文献
2.
Dieter Lorenz 《Physics letters. A》1980,80(4):235-237
All exact solutions of the Einstein-Maxwell equations of Bianchi type-I which are of physical importance have been found. The solutions represent non-locally rotationally symmetric universes with source-free electromagnetic fields and the matter content is a perfect fluid, with equation of state p=(γ?1)?(1?γ?2). Non-titled Bianchi type-II models are integrated for perfect fluid matter for all values of γ. 相似文献
3.
《Physics letters. [Part B]》1986,167(2):157-162
We present new higher-dimensional Bianchi cosmologies of class A. Our solutions given are of type Mn = R × M3 × TD where N = M3 are of types I, II, VI0, VII0, VIII and IX. This spectrum of solutions includes the higher-dimensional versions of the Kasner solution and the mixmaster universe with stiff matter content. 相似文献
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《Physics letters. A》1987,123(8):379-381
We present here some general Bianchi type I solutions in (9+1)-dimensional space-time in the following cases: (A) vacuum field with (i) non-vanishing, (ii) vanishing cosmological constant Λ, and (B) dust model. We also discuss briefly some physical features of these solutions. 相似文献
6.
By making use of Letelier’s form of energy—momentum tensor for a cloud of stringdust we present some classes of solutions
of general relativistic field equations which describe cosmological string-dust models in Bianchi type I space-time. Some
of the classes of models obey Takabayashi’s equation of state whereas a class of models exhibits inflation in the initial
stage. Two of the classes presented here have Kasner’s space-time as past asymptote 相似文献
7.
K. D. Krori T. Chaudhury Chandra Rekha Mahanta 《General Relativity and Gravitation》1994,26(3):265-274
Some Bianchi type cosmological models-two in four and one in higher dimensions-are here studied in the context of cosmic strings. The physical implications of the models are briefly discussed. It is interesting to note that cosmic strings do not occur in Bianchi type V cosmology. 相似文献
8.
Within the scope of Bianchi type-III spacetime we study the role of spinor field on the evolution of the Universe as well as the influence of gravity on the spinor field. In doing so we have considered a polynomial type of nonlinearity. In this case the spacetime remains locally rotationally symmetric and anisotropic all the time. It is found that depending on the sign of nonlinearity the models allows both accelerated and oscillatory modes of expansion. The non-diagonal components of energy-momentum tensor though impose some restrictions on metric functions and components of spinor field, unlike Bianchi type I, V and V I 0 cases, they do not lead to vanishing mass and nonlinear terms of the spinor field. 相似文献
9.
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
10.
Bianchi type V solutions of the Einstein equations are studied using the Hamiltonian approach. Explicit expressions depending on a single quadrature are given for the metric components in the general orthogonal perfect fluid case. It is shown that the quadrature can be evaluated in terms of elementary or elliptic integrals when the parameter in the equation of statep=(–1) takes the values 1, 10/9, 4/3, 14/9, 5/3, 2. 相似文献
11.
J. G. Miller 《Communications in Mathematical Physics》1977,52(1):1-10
We study two Bianchi type VIII analogues of Taub space and maximal analytic extensions of them. The first one has SL(2,R) as an isometry group, which acts transitively on spacelike hypersurfaces. The maximal extension has all of the pathological features of Taub-NUT space. The second one has the universal covering group of SL(2,R) as an isometry group. The maximal extension of the latter does not have these pathological properties and is geodesically complete.Work supported in part by NSF Grant MPS 74-16311 AO1 相似文献
12.
Gerald E. Tauber 《International Journal of Theoretical Physics》1979,18(5):371-396
There exist nine types of Bianchi cosmologies classified according to the structure constants of the corresponding Lie groups. Each of these types gives rise to a particular form of the line element, the Friedmann universe corresponding to the simplest type I. It is also known that there exists a simple correspondence (transformation) between the Robertson-Walker line element and the conformal line element but restricting the arbitrary function of that line element. This suggests that a classification of conformai flat line elements according to their parameters should yield a classification similar to that of Bianchi. The conformal group has 15 parameters, corresponding to the pure conformal group, Lorentz group, translation, and dilation. A classification of the line element according to these has been carried out, singly and combining several of them. It has been found that the Friedmann universe is a subclass, as expected, with other cosmologies resulting as wider subclasses. Comparison with the Bianchi classification is also made. 相似文献
13.
T. Harkó 《Acta Physica Hungarica A》1996,3(1-2):115-129
The dynamics of a homogeneous, anisotropic, spatially flat Bianchi type I universe filled with a scalar field is studied. Using the usual synchronous form of the line element, general exact solutions for the Einstein field equations are obtained in the case of the exponential-potential scalar field (V=Λexp(k?)) and in the case of the Barrow-Saich potential ( $V \sim \dot \varphi ^2 $ ). Conditions under which inflation can occur are discussed and the late-time behaviour of the models is also considered. 相似文献
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The dynamics of cosmological models with isotropic matter sources (perfect fluids) is extensively studied in the literature; in comparison, the dynamics of cosmological models with anisotropic matter sources is not. In this paper we consider spatially homogeneous locally rotationally symmetric solutions of the Einstein equations with a large class of anisotropic matter models including collisionless matter (Vlasov), elastic matter, and magnetic fields. The dynamics of models of Bianchi types I, II, and IX are completely described; the two most striking results are the following. (i) There exist matter models, compatible with the standard energy conditions, such that solutions of Bianchi type IX (closed cosmologies) need not necessarily recollapse; there is an open set of forever expanding solutions. (ii) Generic type IX solutions associated with a matter model like Vlasov matter exhibit oscillatory behavior toward the initial singularity. This behavior differs significantly from that of vacuum/perfect fluid cosmologies; hence “matter matters”. Finally, we indicate that our methods can probably be extended to treat a number of open problems—in particular, the dynamics of Bianchi type VIII and Kantowski-Sachs solutions. 相似文献
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18.
Egils Sviestins 《General Relativity and Gravitation》1985,17(6):521-523
It is reported on an investigation of cosmologies which constitute explicit, rotating, and expanding solutions of Einstein's field equations, with spacelike, timelike, or null-like homogeneous hypersurfaces of Bianchi type IX, and the source of which is a non-thermalized perfect fluid. 相似文献
19.
The evolution of a class of exact spatially homogeneous cosmological models of Bianchi type VI
h
is discussed. It is known that solutions of type VI
h
cannot approach isotropy asymptotically at large times. Indeed the present class of solutions become asymptotic to an anisotropic vacuum plane wave solution. Nevertheless, for these solutions the initial anisotropy can decay, leading to a stage of finite duration in which the model is close to isotropy. Depending on the choice of parameters in the solution, this quasi-isotropic stage can commence at the initial singularity, in which case the singularity is of the type known as isotropic or Friedmann-like. The existence of this quasi-isotropic stage implies that these models can be compatible in principle with the observed universe. 相似文献