Gravitational collapse in generalized Vaidya space-time for Lovelock gravity theory

In this work, we have assumed the generalized Vaidya solution in Lovelock theory of gravity in (n+2)-dimensions. It has been shown that Gauss-Bonnet gravity, dimensionally continued Lovelock gravity and pure Lovelock gravity can be constructed by suitable choice of parameters. We have investigated the occurrence of singularities formed by the gravitational collapse in above three particular forms of Lovelock theory of gravity. The dependence of the nature of singularity on the existence of radial null geodesic for Vaidya space-time has been specially considered. In all the three models, we have shown that the nature of singularities (naked singularity or black hole) completely depend on the parameters. Choices of various parameters are shown in tabular form. In Gauss-Bonnet gravity theory, it can be concluded that the possibility of naked singularity increases with increase in dimensions. In dimensionally continued Lovelock gravity, the naked singularity is possible for odd dimensions for several values of parameters. In pure Lovelock gravity, only black hole forms due to the gravitational collapse for any values of parameters. It has been shown that when accretion is taking place on a collapsing object, it is highly unlikely to get a black hole. Finally on considering the phantom era in the expanding universe it is observed that there is no possibility of formation of a black hole if we are in the Gauss-Bonnet gravity considering the accreting procedure upon a collapsing object.     

Junction conditions and consequences of quasi-spherical space-time with electro-magnetic field and Vaidya metric

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.      

Role of chameleon field in accelerating Universe

In this work, we have considered a model of the flat Friedmann–Robertson–Walker (FRW) Universe filled with cold dark matter and a chameleon field where the scale function is taken as (i) intermediate expansion and (ii) logamediate expansion. In both cases we find the expressions of the chameleon field, chameleon potential, statefinder parameters, and slow-roll parameters. Also, it has been shown that the potential always decreases with the chameleon field in both scenarios. The nature of the slow-roll parameters has been shown diagrammatically.     

Gravitational collapse, black holes and naked singularities

This article gives an elementary review of gravitational collapse and the cosmic censorship hypothesis. Known models of collapse resulting in the formation of black holes and naked singularities are summarized. These models, when taken together, suggest that the censorship hypothesis may not hold in classical general relativity. The nature of the quantum processes that take place near a naked singularity, and their possible implication for observations, is briefly discussed.     

Protostellar evolution during time-dependent anisotropic collapse

The formation and collapse of a protostar involves the simultaneous infall and outflow of material in the presence of magnetic fields, self-gravity and rotation. We use self-similar techniques to self-consistently model the anisotropic collapse and outflow by using a set of angle-separated self-similar equations. The outflow is quite strong in our model, with the velocity increasing in proportion to radius, and material formally escaping to infinity in the finite time is required for the central singularity to develop.
Analytically tractable collapse models have been limited mainly to spherically symmetric collapse, with neither magnetic field nor rotation. Other analyses usually employ extensive numerical simulations, or either perturbative or quasistatic techniques. Our model is unique as an exact solution to the non-stationary equations of self-gravitating magnetohydrodynamics (MHD), which features co-existing regions of infall and outflow.
The velocity and magnetic topology of our model is quadrupolar, although dipolar solutions may also exist. We provide a qualitative model for the origin and subsequent evolution of such a state. However, a central singularity forms at late times, and we expect the late-time behaviour to be dominated by the singularity, rather than depend on the details of its initial state. Our solution may, therefore, have the character of an attractor among a much more general class of self-similarity.     

Elimination of the gravitational singularities within the framework of Einstein's general theory of relativity

In this paper two examples are given, showing that the existence of black holes in the Universe violates, in its consequences, the principle of causality. The solution presented is based on the idea that the primordial black holes have zero-mass-energy and consequently zero-radius of the event horizon. Despite the existence of the surface of last influence, gravitational collapse does not produce black holes during a finite time interval as measured by an external observer. The only singularity, possible to accept (if any), is the initial and final cosmological singularity.     

The effect of external pressure & magnetic field in star formation: The critical mass model

Theoretical work addressing the role of external pressure with magnetic fields in collapsing molecular clouds is important in building a comprehensive theory of star formation(SF). In many SF studies turbulence, magnetic fields, and self-gravity are described as the key dynamical processes involved in SF. However, the importance of external pressure in collapsing strongly magnetized clouds has not yet been particularly explained. Magnetic fields transport excess angular momentum from the central core while external pressure compresses the cloud. Thus the outflow of angular momentum, and on the other hand, the compression made by external pressure are the cause for matter falling onto the central core from the envelope. Therefore, external pressure facilitates the collapse of gas and the inflow of matter. In this work, we show theoretically how the strong magnetic field is dragged inward by the external pressure during the core collapse and formulate the critical mass of the core in the presence of external pressure.     

Thermodynamics of a model of nonadiabatic spherical gravitational collapse

In this work, a thermodynamic treatment of a Friedmann-like model of nonadiabatic spherical gravitational collapse is presented. The calculations have been performed according to Eckart's theory of dissipative relativistic fluids, while the diffusion approximation has been adopted for the radiation transport. The conclusions deduced are in agreement with the predictions of the theory of late stellar evolution.     

Gravitational charged perfect fluid collapse in Friedmann universe models

This paper is devoted to the study of gravitational charged perfect fluid collapse in Friedmann universe models with cosmological constant. For this purpose, we assume that the electromagnetic field is so weak that it does not introduce any distortion into the geometry of the spacetime. The results obtained from the junction conditions between the Friedmann and the Reissner–Nordström de Sitter spacetimes are used to solve the field equations. Further, the singularity structure and mass effects of the collapsing system on the time difference between the formation of apparent horizons and singularity have been studied. This analysis provides the validity of the Cosmic Censorship Hypothesis. It is found that the electromagnetic field affects the area of apparent horizons and their time of formation.     

Viscous fluid cosmological models in the presence of the zero-mass scalar field

The curvature-free (k=0) FRW expanding cosmological model is developed corresponding to interacting viscous fluids and zero-mass scalar fields. In the absence of non-static scalar fields the model exhibits the existence of the initial singularity (Q=0). However, with non-negative coefficient of shear viscosity, in the presence of non-static scalar fields we find thatQ has a minimum value (0). If this epoch is treated as the initial one, it may be said that the presence of scalar fields avoids the initial singularity. Other physical behaviour that the model exhibit has been discussed.     

Some enigmatic aspects of the early universe

Matter collapsing to a singularity in a gravitational field is still an intriguing question. Similar situation arises when discussing the very early universe or a universe recollapsing to a singularity. It was suggested that inclusion of mutual gravitational interactions among the collapsing particles can avert a singularity and give finite value for various physical quantities. We also discussed how inclusion of large dark energy term compensates for the net gravity. The discussion is taken further by including the effects of charge, magnetic fields and rotation. The role of large extra dimensions under the extreme initial conditions is discussed and possible connection with the cyclic brane theory is explored. We constrain various cosmic quantities like the net charge, number density of magnetic monopoles, primordial magnetic fields, size of the extra dimensions, etc. We are also able to arrive at the parameters governing the observed universe.     

A viable mechanism for the production of energy in active galactic nuclei

It is suggested that a collapsing supermassive object, which acts as an ultra-high energy particle accelerator, is the precursor of an active galactic nucleus and that the gravitational energy released during the collapse of the object is locked in the quark-gluon plasma permeated by leptons into which the entire matter in the core of the object is converted as a result of the collapse. It is also pointed out that the collapse of the object to a space-time singularity is inhibited by Pauli's exclusion principle as well as by Heisenberg's uncertainty principle and that the object explodes, before it could collapse to a singularity, thereby releasing the enormous amount of energy locked in the quark-gluon plasma.     

Wide field monitoring of the X-ray sky using Rotation Modulation Collimators

Wide field monitoring is of particular interest in X-ray astronomy due to the strong time-variability of most X-ray sources. Not only does the time-profiles of the persistent sources contain characteristic signatures of the underlying physical systems, but, additionally, some of the most intriguing sources have long periods of quiesense in which they are almost undetectable as X-ray sources, interspersed with relatively brief periods of intense outbursts, where we have unique opportunities of studying dynamical effects, in, for instance, the evolution of accretion discs. Another question for which wide field monitors may provide key information, is the origin and nature of the cosmic gamma ray bursts.Rotation Modulation Collimators (RMC's) were originally introduced in X-ray astronomy to provide accurate source localizations over extended fields. This role has since been taken over by the grazing incidence telescope systems. The potential of the RMC's as wide field monitors have recently been demonstrated by the WATCH instruments on GRANAT and EURECA. It now appears likely, that for use on large, 3-axis stabilized spacecraft, a pinhole camera system may provide better sensitivity than an RMC-system of corresponding physical dimensions. But due to its simplicity, low data rate, and ability to work on spin stabilized (micro)satellites, the RMC wide field monitor may still have a role to play in the X-ray astronomy of the future.     

Preliminary orbit-determination method having no co-planar singularity

It has long been recognized and demonstrated in the astrodynamic literature that three observations of angular position are not always sufficient to determine a preliminary orbit. One reason for this is due to the fact that as the plane of the observer's motion approaches the plane of the orbit of the observed object, the determination of the orbit of the object becomes indeterminant. Merely changing the coordinate system will not eliminate the inherent indeterminacy or singularity. When the observed object is moving in the same plane as the observer, their relative motion is described in two dimensions rather than three. The problem reduces to defining two components of position and two of velocity given only three angular measures and no solution is possible. Although this singularity is a rather old, albeit infrequently arising problem in celestial mechanics, it has received renewed interest due to the advent of satellite observatories that observe other spacecraft. In this new circumstance the plane of the observer's motion is rather frequently near the plane of the object (12% to 35% of the time) and the co-planar singularity becomes a subject that deserves additional attention.It is the purpose of this paper to develop a practical and simple method of orbit determination using four observations. This method also allows one to avoid the problem of multiple orbit-determination solution roots, and provides numerical indices that are useful in assessing the degree of indeterminacy in any given observer/object geometry. This paper does not dwell at length on the theory of orbital singularities, since they have been already treated in celestial mechanics literature. Instead, the emphasis is on the details of a new computational technique, which has been found to be computationally more efficient than previous four-observation methods, and which is unique in being formulated in the geocentric system and involves only one scalar quantity in the correction process.The equations for the new method are developed and a numerical example is presented that demonstrates the efficiency of the method.     

Accelerated expansion of the universe in a non-trivial extra-dimensional topology

The recent observational available data for an accelerated expansion state of the present universe, obtained from distant SNeIa gave strong support to the search of alternative cosmologies. Recently, there have been a number of different attempts to modify Einstein’s gravity to yield accelerated expansion at late times. Unfortunately, many of the theoretical models discussed in the literature are plagued with theoretical problems, in particular the singularity problem at the origin of time. In the present work we have analyzed a multidimensional spacetime Friedmann–Robertson–Walker (FRW) model with a decaying cosmological constant and a varying gravitational constant. Many interesting consequences are revealed, in particular the behavior of the scale factor and the shape of the universe in terms of the number of extra dimensions.     

Effects of electromagnetic field on the collapse and expansion of anisotropic gravitating source

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.     

Gravitational field of a stationary circular cosmic string loop

Gravitational field of a stationary circular cosmic string loop has been studied in the context of full nonlinear Einstein's theory of gravity. It has been assumed that the radial and tangential stresses of the loop are equal to the energy density of the string loop. An exact solution for the system has been presented which has a singularity at a finite distance from the axis, but is regular for any other distances from the axis of the loop. This revised version was published online in July 2006 with corrections to the Cover Date.     

A smoothing scheme for reducing field anisotropy inN-body simulation

A method is proposed to reduce the field anisotropy inN-body calculations arising from the discretization of Poisson's equation in a particle-mesh scheme. Specifically, the mass is split (in addition to that employed in the mass assignment algorithm) over neighbouring points to create compensating higher-order multipoles. Improvement by factor 2 has been observed in the field calculations, and spurious geometric effects in the example of a 4000 particle free-fall collapse have been reduced.     

Neutrino fluxes from a core-collapse supernova in a model with three sterile neutrinos

The characteristics of the gravitational collapse of a supernova and the fluxes of active and sterile neutrinos produced during the formation of its protoneutron core have been calculated numerically. The relative yields of active and sterile neutrinos in corematter with different degrees of neutronization have been calculated for various input parameters and various initial conditions. A significant increase in the fraction of sterile neutrinos produced in superdense core matter at the resonant degree of neutronization has been confirmed. The contributions of sterile neutrinos to the collapse dynamics and the total flux of neutrinos produced during collapse have been shown to be relatively small. The total luminosity of sterile neutrinos is considerably lower than the luminosity of electron neutrinos, but their spectrum is considerably harder at high energies.     

The final state of charged collapsing core and its optical appearance

We discuss the optical appearance of a charged collapsing stellar core. In the case Q < M , the collapse leads to the formation of a black hole and the phenomenon will be qualitatively similar to the uncharged case, that is, an exponentially increasing redshift and an exponentially decreasing intensity but the time-constant may be very much greater. In the case Q >M, if the collapse ends in a singularity, then infinite redshift and vanishing of light will be seen optically within a finite time. But the formation of singularity does not seem to be a real possibility. More probably, a steady, vibrating configuration will form, which will show very large redshift if Q is close to M.