4,150 research outputs found
Spherically Symmetric Gravitational Collapse
In this paper, we discuss gravitational collapse of spherically symmetric
spacetimes. We derive a general formalism by taking two arbitrary spherically
symmetric spacetimes with . Israel's junction conditions are used to
develop this formalism. The formulae for extrinsic curvature tensor are
obtained. The general form of the surface energy-momentum tensor depending on
extrinsic curvature tensor components is derived. This leads us to the surface
energy density and the tangential pressure. The formalism is applied to two
known spherically symmetric spacetimes. The results obtained show the regions
for the collapse and expansion of the shell.Comment: 12 pages, 4 figures, accepted for publication in Mod. Phys. Lett.
Effects of Electromagnetic Field on Gravitational Collapse
In this paper, the effect of electromagnetic field has been investigated on
the spherically symmetric collapse with the perfect fluid in the presence of
positive cosmological constant. Junction conditions between the static exterior
and non-static interior spherically symmetric spacetimes are discussed. We
study the apparent horizons and their physical significance. It is found that
electromagnetic field reduces the bound of cosmological constant by reducing
the pressure and hence collapsing process is faster as compared to the perfect
fluid case. This work gives the generalization of the perfect fluid case to the
charged perfect fluid. Results for the perfect fluid case are recovered.Comment: 17 pages, accepted for publication in Mod. Phys. Lett
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Evaluating the transition of e-Government: A review of local authorities in England
The goal of e-Government is seen as a panacea for governmental authorities. The emerging needs of citizens, their inclusion and engagement in policy development, political and participatory processes have meant new perspectives on e-Government are required. This paper seeks to identify and evaluate the preparedness of 10 UK-based local authorities to transition from basic e-Government to a more sophisticated and integrated e-Government. A categorical assessment of e-Government characteristics is made and these authorities are ranked accordingly. Our findings reveal the majority of local authorities sampled had reached a high percentage of informational and transactional e-Government but few had reached the interactional level and none had achieved assimilation. This suggests that local authorities seem to have focused on basic e-Government services. There is a need now to forge ahead to integration and assimilation of e-Government in order to address the critical objectives of citizen inclusion and engagement, and alignment of institutional processes to provide an infrastructure for the transition to e-governance and e-knowledge
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A novel architecture for a reconfigurable micro machining cell
There is a growing demand for machine tools that are specifically designed for the manufacture of micro-scale components. Such machine tools are integrated into flexible micro-manufacturing systems. Design objectives for such tools include energy efficiency, small footprint and importantly flexibility, with the ability to easily reconfigure the manufacturing system in response to process requirements and product demands. Such systems find application in medical, photonics, automotive and electronic industries.
In this paper, a new architecture for a reconfigurable micro manufacturing system is presented. The proposed architecture comprises a micro manufacturing cell with the key design feature being a hexagonal-base on which three tool heads can be attached to three of its sides. Each such machine-tool head, or processing module, is able to perform a different manufacturing process. These tool heads are interchangeable, enabling the cell to be configured to process a wide range of components with different materials, dimensions, tolerances and specification. Additional components of the cell include manipulation robots and an automated buffer unit. Such cells can be integrated into a manufacturing system via a modular conveyor belt to transfer parts from one cell to another and into assembly. A key consideration of the architecture is a control system that is also modular and reconfigurable; such that when new processing modules are introduced the control system is aware of the change and adjusts accordingly. Further to this coordination, issues between modules and machining cells are also considered. Other design considerations include work-piece holding and manipulation.
This paper provides an overview of the architecture, the key design and implementation challenges as well as a high level operational performance assessment by means of a discrete event simulation model of the micro factory cell
Plasma Wave Properties of the Schwarzschild Magnetosphere in a Veselago Medium
We re-formulate the 3+1 GRMHD equations for the Schwarzschild black hole in a
Veselago medium. Linear perturbation in rotating (non-magnetized and
magnetized) plasma is introduced and their Fourier analysis is considered. We
discuss wave properties with the help of wave vector, refractive index and
change in refractive index in the form of graphs. It is concluded that some
waves move away from the event horizon in this unusual medium. We conclude that
for the rotating non-magnetized plasma, our results confirm the presence of
Veselago medium while the rotating magnetized plasma does not provide any
evidence for this medium.Comment: 20 pages, 15 figures, accepted for publication in Astrophys. Space
Sc
Gravitational Charged Perfect Fluid Collapse in Friedmann Universe Models
This paper is devoted to study the gravitational charged perfect fluid
collapse in the 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-Nordstrm de-Sitter spacetimes are used to solve the field
equations. Further, the singularity structure and mass effects of the
collapsing system on time difference between the formation of apparent horizons
and singularity have been studied. This analysis provides the validity of
Cosmic Censorship Hypothesis. It is found that the electric field affects the
area of apparent horizons and their time of formation.Comment: 17 pages, accepted for publication in Astrophys. Space Sc
Dynamics of Non-adiabatic Charged Cylindrical Gravitational Collapse
This paper is devoted to study the dynamics of gravitational collapse in the
Misner and Sharp formalism. We take non-viscous heat conducting charged
anisotropic fluid as a collapsing matter with cylindrical symmetry. The
dynamical equations are derived and coupled with the transport equation for
heat flux obtained from the Mller-Israel-Stewart causal thermodynamic
theory. We discuss the role of anisotropy, electric charge and radial heat flux
over the dynamics of the collapse with the help of coupled equation.Comment: 15 pages, accepted for publication in Astrophys. Space Sc
Gravitational Collapse: Expanding and Collapsing Regions
We investigate the expanding and collapsing regions by taking two well-known
spherically symmetric spacetimes. For this purpose, the general formalism is
developed by using Israel junction conditions for arbitrary spacetimes. This
has been used to obtain the surface energy density and the tangential pressure.
The minimal pressure provides the gateway to explore the expanding and
collapsing regions. We take Minkowski and Kantowski-Sachs spacetimes and use
the general formulation to investigate the expanding and collapsing regions of
the shell.Comment: 12 pages, 4 figures, accepted for publication in Gen. Relativ. Gra
Phantom Energy Accretion by Stringy Charged Black Hole
We investigate the dynamical behavior of phantom energy near stringy
magnetically charged black hole. For this purpose, we derive equations of
motion for steady-state spherically symmetric flow of phantom energy onto the
stringy magnetically charged black hole. It is found that phantom energy
accreting onto black hole decreases its mass. Further, the location of critical
points of accretion is explored, which yields mass to charge ratio. This ratio
implies that accretion process cannot transform a black hole into an extremal
black hole or a naked singularity, hence cosmic censorship hypothesis remains
valid here.Comment: 7 pages, no figur
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