Gravitational collapse of a charged viscous-fluid universe

In the course of obtaining three collapsing models of the charged viscous-fluid distribution, the dynamics of gravitational collapse are investigated with respect to them, and their physical and geometrical properties are studied critically, In all the cases, with a proper choice of the mass, the interior solution can be matched to the exterior Schwarzschild vacuum solution, thereby the solution being continuous at the boundary of the star. In all these models the matter inside is found to be radiating thus giving us the opportunity of identifying the gravitational collapse of a realistic astrophysical object. At the boundary of these model stars the matter distribution comes out to be that of dust and thereby comoving with respect to the coordinate system used. All the models are seen to be physically acceptable; they come out to be realistic models of collapsing astrophysical objects which will be of much interest in studying the phenomenon of black holes in this Universe.     

Gravitational perfect fluid collapse in f(R) gravity

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.     

Bianchi type-III universe with perfect fluid

We study Bianchi type-III cosmological model filled with perfect fluid in the presence of cosmological constant Λ(t). The Hubble law utilised yields a constant value of deceleration parameter. Physical and Kinematical properties of the model have also studied.      

Higher-dimensional perfect fluid collapse in $$ gravity

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.     

Inhomogeneous perfect fluid universe with electromagnetic field

Cylindrically symmetric inhomogeneous cosmological model for perfect fluid distribution with electromagnetic field is obtained. The source of the magnetic field is due to an electric current produced along the z-axis. F ₁₂ is the non-vanishing component of electromagnetic field tensor. To get the deterministic solution, it has been assumed that the expansion θ in the model is proportional to the shear σ. Physical and geometric aspects of the models are also discussed in presence and absence of magnetic field.      

Spherically-symmetric charged perfect fluid distribution in Brans-Dicke theory

Solutions of Brans-Dicke field equations in Dicke's conformally-transformed units are obtained when the source of the gravitational field is that of charged perfect fluid where the line element taken is that of Roberton-Walker.     

Gravitational collapse of a self-gravitating unidirectional fluid flow

This paper explores the collapsing process of a unidirectional isotropic matter configuration. The junction conditions for a static exterior geometry and the non-static interior geometry are expressed in terms of the cosmological constant. The time-lapse for the appearance of the black hole and the cosmological horizon is calculated. It is observed that in the de-Sitter space, the unidirectional perfect fluid flow does not make the collapse disappear. The vacuum energy of the cosmological constant makes the collapsing process quite slow and affects the time-lapse of horizon formation. Moreover, the presence of string tension increases the time-lapse of horizon formation.     

Equilibrium of a static charged perfect fluid sphere

A study is developed for treating Einstein-Maxwell's equations, applied to a static perfect fluid sphere with charge, in a such a manner to generalize the Oppenheimer and Volkoff hydrostatic equilibrium equation.We suggest reasons which could make the generalization of this equation inapplicable. It is shown that Einstein-Maxwell's equations predict such special solutions.     

Gravitational radiation from an isolated perfect fluid in higher multipole moments

The purpose of this paper is to give the unknown angular momentum loss of an isolated perfect fluid in any higher multipole moments in the linear approximation of general relativity theory. Also, we discuss the energy and linear momentum fluxes of the given source in higher multipole moments.     

The Friedmann universe and the world potential

In Section 1 of the paper the energy equation of the Friedmann universe, when matter dominates over radiation, is discussed. It is known that the value of the world potential is constant everywhere in the Universe, despite the pulsation motion of the Universe or a possible transformation of pulsation energy into matter or vice versa. The condition for the Universe being closed is deduced. Furthermore, the possibility to define the mass-energy of the Universe is discussed; and the conclusion is arrived at that the mass-energy of the Universe relative to an observer in the non-metric space outside the Universe is equal to zero; i.e. the Universe originated as a vacuum fluctuation. Finally, the view-point of an external observer is described. Such an observer can claim that our closed Universe is a black hole in a non-metric empty space. Besides, the differences between such a black hole and the astrophysical black holes are indicated.In Section 2 the origin of the gravitational force retarding the expansion is discussed, using the properties of the relativistic gravitational potential. In contradiction to Section 1, the view-point of an inner observer (inside the Universe) is used here. It is concluded that the boundary of the closed Universe is an unlocalizable potential barrier.In Section 3 of the paper the apparent discrepancy between Mach's principle and the general theory of relativity is resolved. The solution is based on the fact that, for the Euclidean open universe, the concept of mass is related to the potential of the background equal to –1, but the concept of the mass-energy is related to the zero-potential of the non-metric background. Because the universe is open and a potential barrier (a boundary of the universe) can be localized-i.e. is geometrically existing — by solution of the field equation, we have to refer to the background with zero-potential. The principal idea of the solution is then that the zero-density means the density of mass-energy, when simultaneously the mass density is equal to the critical value for which the Robertson-Walker metric becomes the Euclidean metric of the Minkowski (i.e., flat) space-time. Further a generalization of Newton's law of inertia is formulated, and the properties of nullgeodesics are touched upon. As a conclusion it is stated that this paper and the two previous ones (see Voráek, 1979a, b)de facto express Mach's principle.     

Electromagnetic mass model of a static charged perfect fluid sphere

Using the results, which they have been already published in two earlier papers (Dionysiou, 1982; Dionysiou and Kostakis, 1983; these papers will hereafter be referred to as Papers I, II, respectively), one can easily see that the total gravitational mass, the density and pressure of a static charged perfect fluid sphere are of electromagnetic origin.     

Robertson-Walker cosmological models with perfect fluid in general relativity

Einstein's field equations with variable gravitational and cosmological constants are considered in the presence of perfect fluid for a Robertson-Walker universe by assuming the cosmological term to be proportional to R-m(R is a scale factor and m is a constant).A variety of solutions is presented.The physical significance of the cosmological models has also been discussed.     

The neutrino in the Friedmann universe: Non-zero rest-mass effects

We have considered the problem of the propagation of nonzero rest-mass neutrinos in the Friedmann dust universes of three types: open, flat, and closed, as well as in the radiation-dominated epoch of thehot universe. The total Lagrangian path of the particle has been calculated, and this is shown to be finite for all the three universes-contrary to the total path of the photon, which is infinite in the open and flat universes.We have found the particle horizon as a function of the relativistic parameter at the emission moment and at the moment of observation. The extreme relativistic and nonrelativistic particle motion and its difference from the photon motion have been investigated.     

Friedmann universe as a limit of dust shell model

We generalize the equations of motion of a shell for a sequence of thin shells and derive the general form of the expansion law of the dust shell model which represents the FRW universe when we take an appropriate limit. The three Friedmann model for the three possible values of k:0,±1 are satisfied.     

Vacuum Friedmann models in self-creation cosmology

The vacuum Friedmann models are investigated in self-creation theory of gravitation proposed by Barber. New solutions are obtained forK=±1, and some properties of the solutions are discussed. Only theK=0 solution in self-creation cosmology was previously shown.