共查询到20条相似文献，搜索用时 31 毫秒
1.
Exact solutions of the semi-classical Einstein equations for conformally invariant free quantum fields in an homogeneous and
isotropic space-time, with cosmological constant and containing a classical scalar field, dust matter, an unquantised Dirac
field and electromagnetic radiation are found. The initial behaviour of the semiclassical models is investigated. Some of
the solutions found avoid the singularity and do not have particle horizons.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
Frank J. Tipler Jessica Graber Matthew McGinley Joshua Nichols-Barrer Christopher Staecker 《Monthly notices of the Royal Astronomical Society》2007,379(2):629-640
We show that it is possible for the information paradox in black hole evaporation to be resolved classically. Using standard junction conditions, we attach the general closed spherically symmetric dust metric to a space–time satisfying all standard energy conditions but with a single point future c-boundary. The resulting Omega Point space–time, which has NO event horizons, nevertheless has black hole type trapped surfaces and hence black holes. However, since there are no event horizons, information eventually escapes from the black holes. We show that a scalar quintessence field with an appropriate exponential potential near the final singularity would give rise to an Omega Point final singularity. 相似文献
3.
Davide Farnocchia Davide Bracali Cioci Andrea Milani 《Celestial Mechanics and Dynamical Astronomy》2013,115(1):21-34
The singularity for the big bang state can be represented using the generalized anisotropic Friedmann equation, resulting in a system of differential equations in a central force field. We study the regularizability of this singularity as a function of a parameter, the equation of state, w. We prove that for w > 1 it is regularizable only for w satisfying relative prime number conditions, and for w ≤ 1 it can always be regularized. This is done by using a McGehee transformation, usually applied in the three and four-body problems. This transformation blows up the singularity into an invariant manifold. The relationship of this result to other cosmological models is briefly discussed. 相似文献
4.
This paper investigates the validity of generalized second law of thermodynamics using both the power law and logarithmic entropy corrected formulas in a general scalar-tensor gravity. For this purpose, we take non-flat FRW universe model filled with magnetized perfect fluid matter bounded by four different horizons namely Hubble, apparent, particle and event horizons. We introduce a non-minimal interaction between scalar and matter fields and take Lagrangian density of non-linear electromagnetic effects. Finally, we extend this study to anisotropic case by taking Bianchi I universe model bounded by apparent horizon only and investigate the role of anisotropy parameter on the validity of GSLT. In this case, we also explore the behavior of some cosmological parameters. 相似文献
5.
G. Abbas 《Astrophysics and Space Science》2014,352(2):955-961
This paper is devoted to study the effects of electromagnetic on the collapse and expansion of anisotropic gravitating source. For this purpose, we have evaluated the generating solutions of Einstein–Maxwell field equations with spherically symmetric anisotropic gravitating source. We found that a single function generates the various anisotropic solutions. In this case every generating function involves an arbitrary function of time which can be chosen to fit several astrophysical time profiles. Two physical phenomenon occur, one is gravitational collapse and other is the cosmological expanding solution. In both cases electromagnetic field effects the anisotropy of the model. For collapse the anisotropy is increased while for expansion it deceases from maximum value to finite positive value. In case of collapse there exits two horizons like in case of Reissner–Nordström metric. 相似文献
6.
K. H. Lotze 《Astrophysics and Space Science》1986,120(2):191-204
In this paper we investigate the combined influence of both cosmological and electromagnetic particle creation mechanisms upon massive particles with spin 1/2 on the basis of general covariant Dirac theory.Curved space-time, a radiation-dominated Friedmann universe, is treated as an unquantized gravitational field and the low-frequency part of the 2.7 K background radiation is approximated by homogeneous, constant, and parallel external electric and magnetic fields. We calculate the number density of spin 1/2 particles with massm which are created under the influence of both these external fields.We find that the electric field and the magnetic field both amplify the genuine, purely gravitational particle production. This influence of the magnetic field, which is in contrast to its reducing effect as far as the creation of spin-zero particles is concerned, can clearly be traced back to its coupling to the spin of the particles.Under certain conditions the electromagnetic fields in the early universe can influence the particle creation process even more than the gravitational field. 相似文献
7.
In this work the junction conditions between the exterior Reissner-Nordstrom-Vaidya space-time with the interior quasi-spherical
Szekeres space-time have been studied for analyzing gravitational collapse in the presence of a magneto-hydrodynamic fluid
undergoing dissipation in the form of heat flow. We have discussed about the apparent horizon and have evaluated the time
difference between the formation of apparent horizon and central singularity.
相似文献
8.
This paper deals with the collapse and expansion of relativistic anisotropic self-gravitating source. The field equations for non-radiating and non-static plane symmetric anisotropic source have been evaluated. The non-radiating property of the fluid leads to evaluation of the metric functions. We have classified the dynamical behavior of gravitational source as expansion and collapse. The collapse in this case leads to the final stage without the formation of apparent horizons while such horizons exists in case of spherical anisotropic source. The matching of interior and exterior regions provides the continuity of masses over the boundary surface. 相似文献
9.
This study is emphasized to explore the validity of generalized second law of thermodynamics in the context of non-linear electrodynamics (magnetic effects only) with Brans-Dicke chameleon scalar field as dark energy candidate. For this purpose, we consider FRW universe model with perfect fluid matter contents. We evaluate matter energy density and magnetic field by taking interacting and non-interacting cases of magnetic field and matter as well as the power law ansatz for scalar field. The validity of this law is discussed by using the first law of thermodynamics for four different horizons: Hubble, apparent, particle and event horizons. We conclude that this law may hold for all four horizons with small positive red-shift when chameleon mechanism is taken into account in Brans-Dicke gravity. Finally, we investigate the statefinders in order to check the viability of the model. 相似文献
10.
Hawking’s radiation effect of Klein-Gordon scalar field, Dirac particles and Maxwell’s electromagnetic field in the non-stationary
rotating de Sitter cosmological space-time is investigated by using a method of generalized tortoise co-ordinates transformation.
The locations and the temperatures of the cosmological horizons of the non-stationary rotating de Sitter model are derived.
It is found that the locations and the temperatures of the rotating cosmological model depend not only on the time but also
on the angle. The stress-energy regularization techniques are applied to the two dimensional analog of the de Sitter metrics
and the calculated stress-energy tensor contains the thermal radiation effect. 相似文献
11.
The paper is devoted to an investigation of the relationships between the classical Friedmann cosmology and the Dirac Hamiltonian
approach to quantization of the universe, based on the simple but important example of a homogeneous universe filled with
excitations of a scalar field. The method of gaugeless reduction is used to completely separate the sector of physical variables
from the purely gauge sector, making it possible to find the relationship between cosmological observables in the Friedmann
— Einstein sense and observables of the Dirac Hamiltonian formalism in the Narlikar conformai reference frame. Gaugeless reduction
enabled us to establish that in the process of reduction, one of the variables of the nonphysical sector is converted into
an invariant time parameter and cannot be treated as a dynamical variable in either the functional or the operator approach
to quantization. It is shown that in this conversion of a variable into a time parameter, the Hartle-Hawking functional integral
is the reason why the wave function of the Wheeler—De Witt (WDW) equation cannot be normalized and why an infinite gauge factor
arises. The gaugeless reduction provides a certain recipe for mathematical and physical interpretation of the WDW equation
and wave functions, the use of which makes their relationship to observational cosmology clear and transparent. It is shown,
in particular, how the WDW wave function describes the Friedmann evolution with respect to proper time.
Translated from Astrofizika, Vol. 40, No. 2, pp. 303–321, April–June, 1997. 相似文献
12.
In this paper, we analyze higher-dimensional spherical perfect fluid collapse in \(f(R,T)\) theory for minimally coupled models. We use Darmois junction conditions by taking Lemaître-Tolman-Bondi geometry as an interior region and Schwarzschild metric as an exterior spacetime. The solution of field equations is obtained for constant scalar curvature. We determine mass in two regions of the collapsing object and discuss the formation of apparent horizons. We conclude that modified curvature term tends to slow down the collapse rate. 相似文献
13.
K. I. Kellermann R. C. Vermeulen J. A. Zensus M. H. Cohen A. West 《New Astronomy Reviews》1999,43(8-10)
The VLBA has been used over a period of four years to study the internal motions within a sample of quasars and AGN. In most sources, features appear to propagate away from the central engine along a well collimated radio jet with apparent transverse velocities between zero and 10c, with some evidence for apparent accelerations and decelerations. The distribution of apparent velocity is not consistent with any simple ballistic model and appears to require either a spread in intrinsic velocity or a difference between the bulk velocity and pattern velocity. The dependence of apparent angular velocity with redshift is consistent with standard Friedmann world models. Further observations of a larger source sample, especially at large redshift may lead to meaningful constraints on world models. 相似文献
14.
We perform a study to describe motion of charged particles under the influence of electromagnetic and gravitational fields of a slowly rotating wormhole with nonvanishing magnetic moment. We present analytic expression for potentials of electromagnetic field for an axially symmetric slowly rotating magnetized wormholes. While addressing important issues regarding the subject, we compare our results of motion around black holes and wormholes in terms of the ratio of radii of event horizons of a black hole and of the throat of a wormhole. It is shown that both radial and circular motions of test bodies in the vicinity of a magnetized wormhole could give rise to a peculiar observational astrophysical phenomenon. 相似文献
15.
《Chinese Astronomy and Astrophysics》1982,6(3):243-247
We have analysed the finite temperature λφ4 model in the Robertson Walker metric, taking into consideration spontaneous symmetry breaking, particle production and symmetry recovery through phase transition under a high temperature, and found that it is possible to have a cosmological model free of singularities. Such a model begins in the singularity-free, horizon-free, Beltrami-Anti-de Sitter state. Continual production of particles keeps on raising its temperature until a critical temperature TC is reached, when a phase change takes place, and the universe is transformed into a radiation-dominated, thermally-expanding Friedmann state. This phase transition corresponds to a big-bang without, however, an antecedent singularity. 相似文献
16.
We consider a cosmological model in which a scalar field is non-minimally coupled to scalar torsion and a vector field through two coupling functions in the framework of teleparallel gravity. The explicit forms of the coupling functions and the scalar field potential are explored, under the assumption that the Lagrangian admits the Noether symmetry in the Friedmann–Lemaître–Robertson–Walker (FLRW) space–time. The existence of such symmetry allows to solve the equations of motion and achieve exact solutions of the scale factor, scalar and vector fields. It is found that the vector field contributes significantly in the accelerating expansion of the universe in the early times, while the scalar field plays an essential role in the late times. 相似文献
17.
In this paper, we investigate spherically symmetric perfect fluid gravitational collapse in metric f(R) gravity. We take non-static spherically symmetric metric in the interior region and static spherically symmetric metric
in the exterior region of a star. The junction conditions between interior and exterior spacetimes are derived. The field
equations in f(R) theory are solved using the assumption of constant Ricci scalar. Inserting their solution into junction conditions, the
gravitational mass is found. Further, the apparent horizons and their time of formation is discussed. We conclude that the
constant scalar curvature term f(R
0) acts as a source of repulsive force and thus slows down the collapse of matter. The comparison with the corresponding results
available in general relativity indicates that f(R
0) plays the role of the cosmological constant. 相似文献
18.
Exact solutions of the semi-classical Einstein equations with cosmological constant for conformally invariant free quantum fields in a general Robertson-Walker metric are found when a classical perfect fluid is present. There exist a one-parameter family of time-symmetric bouncing solutions that avoid the singularity and a one-parameter family which does not have particles horizons. The de Sitter solution is found to be stable, while the Einstein universe is unstable. 相似文献
19.
Marek Biesiada 《Astrophysics and Space Science》2003,283(4):511-516
Theory of dynamical systems offers a possibility of investigating the space of all possible solutions. In the context of simple
cosmological models such like Varying Speed of Light Friedman-Robertson-Walker (VSL FRW) models there exists a systematic
method of reducing field equations to certain two-dimensional dynamical system. One of the features of this reduction is the
possibility of representing the model as a Hamiltonian system in which the properties of the potential function V(X) can serve as a tool for qualitative classification of possible evolutions of a(t). Some important features like resolution of the flatness problem, existence of event horizons near the singularity can be
visualized as domains on the phase-space. Then one is able to see how large is the class of solutions (labelled by the initial
conditions) leading to the desired property.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
Roger Blandford 《Astrophysics and Space Science》2003,288(1-2):155-160
An electromagnetic model of relativistic jets is outlined. It is suggested that these jets be interpreted as current flows and that they owe their persistence and collimation to the pinching action of the magnetic field. Such structures are unstable and it is suggested that the nonlinear development of these instabilities involves the formation of an electromagnetic turbulence spectrum. This turbulence may be responsible for the acceleration of relativistic electrons and positrons. It may also provide the electromagnetic field in which these electrons and positrons radiate. Some mechanisms through which circular polarisation may be created in this environment are outlined. 相似文献