100 research outputs found

### On the sources of static plane symmetric vacuum space-times

The static vacuum plane spacetimes are considered, which have two non-trivial
solutions: The Taub solution and the Rindler solution. Imposed reflection
symmetry, we find that the source for the Taub solution does not satisfy any
energy conditions, which is consistent with previous studies, while the source
for the Rindler solution satisfies the weak and strong energy conditions (but
not the dominant one). It is argued that the counterpart of the Einstein theory
to the gravitational field of a massive Newtonian plane should be described by
the Rindler solution, which represents also a uniform gravitational field

### Spherical collapse of a heat conducting fluid in higher dimensions without horizon

We consider a scenario where the interior spacetime,described by a heat
conducting fluid sphere is matched to a Vaidya metric in higher
dimensions.Interestingly we get a class of solutions, where following heat
radiation the boundary surface collapses without the appearance of an event
horizon at any stage and this happens with reasonable properties of matter
field.The non-occurrence of a horizon is due to the fact that the rate of mass
loss exactly counterbalanced by the fall of boundary radius.Evidently this
poses a counter example to the so-called cosmic censorship hypothesis.Two
explicit examples of this class of solutions are also given and it is observed
that the rate of collapse is delayed with the introduction of extra
dimensions.The work extends to higher dimensions our previous investigation in
4D.Comment: 6 page

### Estructura de capital de empresas brasileñas : normalidad, crisis financiera global y recesión económica

This article aims to identify the determinants of the capital structure of Brazilian companies and compare it with financial theories. In addition, the normality periods (2007, 2009–2014) and financial crisis periods (2008 and 2015) will be considered in the analysis. The sample has 114 Brazilian public companies in the periods from 2007 to 2015. The methodology used for data analysis was multiple regression for panel data. The results showed that there are differences between the determinants of the capital structure in periods of crisis and of normality. Some of the hypotheses tested were accepted. These hypotheses relate financial theory to empirical analysis. Finally, the research contributed by demonstrating the main determinants of the capital structure in the analyzed periods, showing changes between such determinants641Este artículo tiene como objetivo identificar los determinantes de la estructura de capital de las empresas brasileñas y compararlas con las teorías financieras. Además, los periodos de normalidad (2007, 2009-2014) y los períodos de crisis financiera (2008 y 2015) se considerarán en el análisis. La muestra tiene 114 empresas brasileñas en los períodos de 2007 a 2015. La metodología utilizada para el análisis de los datos fue la regresión múltiple para datos en panel. Los resultados mostraron que existen diferencias entre los determinantes de la estructura de capital en períodos de crisis y de normalidad. Algunas de las hipótesis probadas fueron aceptadas. Estas hipótesis relacionan la teoría financiera con el análisis empírico. Por último, la investigación contribuyó presentando los principales determinantes de la estructura de capital en los períodos analizados y evidenciando cambios entre estos determinante

### Non-adiabatic collapse of a quasi-spherical radiating star

A model is proposed of a collapsing quasi-spherical radiating star with
matter content as shear-free isotropic fluid undergoing radial heat-flow with
outgoing radiation. To describe the radiation of the system, we have considered
both plane symmetric and spherical Vaidya solutions. Physical conditions and
thermodynamical relations are studied using local conservation of momentum and
surface red-shift. We have found that for existence of radiation on the
boundary, pressure on the boundary is not necessary.Comment: 8 Latex pages, No figures, Revtex styl

### Hawking temperature of rotating charged black strings from tunneling

Thermal radiations from spherically symmetric black holes have been studied
from the point of view of quantum tunneling. In this paper we extend this
approach to study radiation of fermions from charged and rotating black
strings. Using WKB approximation and Hamilton-Jacobi method we work out the
tunneling probabilities of incoming and outgoing fermions and find the correct
Hawking temperature for these objects. We show that in appropriate limits the
results reduce to those for the uncharged and non-rotating black strings

### Gravitational Collapse in Higher Dimensional Husain Space-Time

We investigate exact solution in higher dimensional Husain model for a null
fluid source with pressure $p$ and density $\rho$ are related by the following
relations (i) $p=k\rho$, (ii) $p=k\rho-\frac{B(v)}{\rho^{\alpha}}$ (variable
modified Chaplygin) and (iii) $p=k\rho^{\alpha}$ (polytropic). We have studied
the nature of singularity in gravitational collapse for the above equations of
state and also for different choices of the of the parameters $k$ and $B$
namely, (i) $k=0$, $B=$ constant (generalized Chaplygin), (ii) $B=$ constant
(modified Chaplygin). It is found that the nature of singularity is independent
of these choices of different equation of state except for variable Chaplygin
model. Choices of various parameters are shown in tabular form. Finally,
matching of Szekeres model with exterior Husain space-time is done.Comment: 12 latex pages, No figure, RevTex styl

### Thermodynamics of charged and rotating black strings

We study thermodynamics of cylindrically symmetric black holes. Uncharged as
well as charged and rotating objects have been discussed. We derive surface
gravity and hence the Hawking temperature and entropy for all these cases. We
correct some results in the literature and present new ones. It is seen that
thermodynamically these black configurations behave differently from
spherically symmetric objects

### Hawking radiation of Dirac particles from black strings

Hawking radiation has been studied as a phenomenon of quantum tunneling in
different black holes. In this paper we extend this semi-classical approach to
cylindrically symmetric black holes. Using the Hamilton-Jacobi method and WKB
approximation we calculate the tunneling probabilities of incoming and outgoing
Dirac particles from the event horizon and find the Hawking temperature of
these black holes. We obtain results both for uncharged as well as charged
particles

### Junction Conditions and Consequences of Quasi-Spherical Space-Time with Electro-Magnetic Field and Vaidya Matric

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.Comment: 8 latex pages, RevTex style, no figure

### Nature of singularity formed by the gravitational collapse in Husain space-time with electromagnetic field and scalar field

In this work, we have investigated the outcome of gravitational collapse in
Husain space-time in the presence of electro-magnetic and a scalar field with
potential. In order to study the nature of the singularity, global behavior of
radial null geodesics have been taken into account. The nature of singularities
formed has been thoroughly studied for all possible variations of the
parameters. These choices of parameters has been presented in tabular form in
various dimensions. It is seen that irrespective of whatever values of the
parameters chosen, the collapse always results in a naked singularity in all
dimensions. There is less possibility of formation of a black hole. Hence this
work is a significant counterexample of the cosmic censorship hypothesis.Comment: 9 pages, 19 figure

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