1,721,742 research outputs found
Considerações sobre espaços-tempos LRS Bianchi-I
Full text linkIn this text we study the Einstein equations in the spatially flat spacetimes (Bianchi-I)
which are endowed with an extra locally rotational symmetry (LRS). We develop a new
local coordinate representation where we use the components of the energy-momentum
tensor directly in the metric, the energy density being the "time" coordinate. As an
application, some general classes of exact solutions are obtained which are of physical and
mathematical interest. In particular, the general barotropic perfect fluid solution is given,
p = p( ).Agência 1Neste texto estudamos as equações de Einstein em espaços-tempos espacialmente planos
(Bianchi-I) que são dotados de uma simetria extra, a saber, localmente rotacional
(LRS). Desenvolvemos uma nova representação local de coordenadas na qual usamos as
componentes do tensor de energia-momento diretamente na métrica, sendo a densidade
de energia a coordenada tipo "tempo". Como aplicação, são obtidas classes gerais de
soluções exatas, que são de interesses físico e matemático. Em particular, é fornecida a
solução geral com um fluido perfeito barotrópico, p = p( )
0 Bianchi-I kozmičkim strunama
Full text linkEinstein field equations with the cosmological term are solved for an axially symmetric Bianchi-I Letelier string coupled with a magnetic field. It is shown that in the presence of Λ, the initial singularity occurs faster. A formula to obtain numerical value of Φ2 = A2/B2 is given.Rijeđena je Einsteinova jednadžba polja s kozmološkim članom za aksijalno-simetričnu Bianchi-I Letelierovu strunu vezanu za magnetsko polje. Pokazano je da se u prisustvu Λ početni singularitet brze pojavljuje
0 Bianchi-I kozmičkim strunama
Full text linkEinstein field equations with the cosmological term are solved for an axially symmetric Bianchi-I Letelier string coupled with a magnetic field. It is shown that in the presence of Λ, the initial singularity occurs faster. A formula to obtain numerical value of Φ2 = A2/B2 is given.Rijeđena je Einsteinova jednadžba polja s kozmološkim članom za aksijalno-simetričnu Bianchi-I Letelierovu strunu vezanu za magnetsko polje. Pokazano je da se u prisustvu Λ početni singularitet brze pojavljuje
Bianchi-I cosmological model and crossing singularities
No full textWe consider a rather simple method for the description of the big bang-big crunch cosmological singularity crossing. For the flat Friedmann universe this method gives the same results as more complicated methods, usingWeyl symmetry or the transitions between the Jordan and Einstein frames. It is then easily generalized for the case of a Bianchi-I anisotropic universe. We also present early-time and latetime asymptotic solutions for a Bianchi-I universe, filled with a conformally coupled massless scalar field
Conformal motions in Bianchi I spacetime.
Full text linkThesis (M.Sc.)-University of KwaZulu-Natal, 1992.In this thesis we study the physical properties of the manifold in general relativity that admits a conformal motion. The results obtained are general as the metric tensor field is not specified. We obtain the Lie derivative along a conformal Killing vector of the kinematical and dynamical quantities for the general energy-momentum tensor of neutral matter. Equations obtained previously are regained as special cases from our results. We also find the Lie derivative of the energy-momentum tensor for the electromagnetic field. In particular we comprehensively study conformal symmetries in the Bianchi I spacetime. The conformal Killing vector equation is integrated to obtain the general conformal Killing vector and the conformal factor subject to
integrability conditions. These conditions place restrictions on the metric functions. A particular solution is exhibited which demonstrates that these conditions have a nonempty solution set. The solution obtained is a generalisation of the results of Moodley (1991) who considered locally rotationally symmetric spacetimes. The Killing vectors are regained as special cases of the conformal solution. There do not exist any proper special conformal Killing vectors in the Bianchi I spacetime. The homothetic vector is found for a nonvanishing constant conformal factor. We establish that the vacuum Kasner solution is the only Bianchi I spacetime that admits a homothetic vector. Furthermore we isolate a class of vectors from the solution which causes the Bianchi I model to degenerate into a spacetime of higher symmetry
Physical evolution in loop quantum cosmology: The example of the vacuum Bianchi i model
No full textWe use the vacuum Bianchi I model as an example to investigate the concept of physical evolution in loop quantum cosmology (LQC) in the absence of the massless scalar field which has been used so far in the literature as an internal time. In order to retrieve the system dynamics when no such a suitable clock field is present, we explore different constructions of families of unitarily related partial observables. These observables are parametrized, respectively, by: (i) one of the components of the densitized triad, and (ii)its conjugate momentum; each of them playing the role of an evolution parameter. Exploiting the properties of the considered example, we investigate in detail the domains of applicability of each construction. In both cases the observables possess a neat physical interpretation only in an approximate sense. However, whereas in case (i) such interpretation is reasonably accurate only for a portion of the evolution of the universe, in case (ii) it remains so during all the evolution (at least in the physically interesting cases). The constructed families of observables are next used to describe the evolution of the Bianchi I universe. The performed analysis confirms the robustness of the bounces, also in absence of matter fields, as well as the preservation of the semiclassicality through them. The concept of evolution studied here and the presented construction of observables are applicable to a wide class of models in LQC, including quantizations of the Bianchi I model obtained with other prescriptions for the improved dynamics. © 2009 The American Physical Society.Supported by the Spanish MICINN Project FIS2008-06078-C03-03 and the ConsoliderIngenio 2010 Program CPAN (CSD2007-00042).Financial aid by the I3P Program of CSIC and the European Social Fund, the grant I3P-BPD2006. Also financial support by the Foundation for Polish Science under the grant MasterPeer Reviewe
Hybrid quantization: From Bianchi i to the Gowdy model
No full textWe complete the quantization of the vacuum Bianchi I model within the framework of loop quantum cosmology adopting a new improved dynamics scheme put forward recently. In addition, we revisit the hybrid quantization of the Gowdy T3 cosmologies with linear polarization using that scheme, proving with rigor some steps that remained unconcluded. The family of Gowdy T3 cosmologies is an inhomogeneous model whose subset of homogeneous solutions is given precisely by the vacuum Bianchi I model. Our hybrid approach combines the new loop quantum cosmology description of this homogeneous sector with a Fock quantization of the inhomogeneities. Both in the Bianchi I model and in the Gowdy model the Hamiltonian constraint provides an evolution equation with respect to the volume of the Bianchi I universe, which is a discrete variable with a strictly positive minimum. We show that, in vacuo, this evolution is well defined inasmuch as the associated initial value problem is well posed: physical solutions are completely determined by the data on the initial section of constant Bianchi I volume. This fact allows us first to carry out to completion the quantization of the vacuum Bianchi I model which had not yet been achieved and then to confirm the feasibility of the hybrid procedure when the homogeneous sector is quantized with the new improved dynamics scheme.© 2010 The American Physical Society.This work was supported by the Spanish MICINN Project FIS2008-06078-C03-03, the Spanish Consolider-Ingenio 2010 Program CPAN (CSD2007-00042), the NSF grant PHY0854743, the George A. and Margaret M. Downsbrough Endowment, the Eberly research funds of Penn State, Le Fonds québécois de la recherche sur la nature et les technologies, and the Edward A. and Rosemary A. Mebus funds. M. M.-B. is supported by CSIC and the European Social Fund under the grant I3P-BPD2006. E. W.-E.Peer Reviewe
Induced gravity and minimally and conformally coupled scalar fields in Bianchi-I cosmological models
No full textWe study the cosmological evolution and singularity crossing in the Bianchi-I universe filled with a conformally coupled scalar field and compare them with those of the Bianchi-I universe filled with a minimally coupled scalar field. We also write down the solution for the Bianchi-I Universe in the induced gravity cosmology
Magnetic relaxation in the Bianchi-I universe
No full textThe extended Einstein-Maxwell model and its application to the problem of the evolution of the magnetized Bianchi-I universe are considered. The evolution of medium magnetization is governed by a relaxation type extended constitutive equation. The series of exact solutions to the extended master equations is obtained and discussed. The anisotropic expansion of the Bianchi-I universe is shown to become non-monotonic (accelerated/decelerated) in both principal directions (along the magnetic field and orthogonal to it). A specific type of expansion, the so-called evolution with hidden magnetic field, is shown to appear when the magnetization effectively screens the magnetic field and the latter disappears from the equations for the gravitational field. © 2007 IOP Publishing Ltd
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