100 research outputs found

    On the sources of static plane symmetric vacuum space-times

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    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

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    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 brasile├▒as : normalidad, crisis financiera global y recesi├│n econ├│mica

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    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, 2009ÔÇô2014) 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 art├şculo tiene como objetivo identificar los determinantes de la estructura de capital de las empresas brasile├▒as y compararlas con las teor├şas financieras. Adem├ís, los periodos de normalidad (2007, 2009-2014) y los per├şodos de crisis financiera (2008 y 2015) se considerar├ín en el an├ílisis. La muestra tiene 114 empresas brasile├▒as en los per├şodos de 2007 a 2015. La metodolog├şa utilizada para el an├ílisis de los datos fue la regresi├│n m├║ltiple para datos en panel. Los resultados mostraron que existen diferencias entre los determinantes de la estructura de capital en per├şodos de crisis y de normalidad. Algunas de las hip├│tesis probadas fueron aceptadas. Estas hip├│tesis relacionan la teor├şa financiera con el an├ílisis emp├şrico. Por ├║ltimo, la investigaci├│n contribuy├│ presentando los principales determinantes de la estructura de capital en los per├şodos analizados y evidenciando cambios entre estos determinante

    Non-adiabatic collapse of a quasi-spherical radiating star

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    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

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    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

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    We investigate exact solution in higher dimensional Husain model for a null fluid source with pressure pp and density ¤ü\rho are related by the following relations (i) p=k¤üp=k\rho, (ii) p=k¤üÔłĺB(v)¤ü╬▒p=k\rho-\frac{B(v)}{\rho^{\alpha}} (variable modified Chaplygin) and (iii) p=k¤ü╬▒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 kk and BB namely, (i) k=0k=0, B=B= constant (generalized Chaplygin), (ii) B=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

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    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

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    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

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    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

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    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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