IRSPAS 2017
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Item Thin shell model for Majumdar Papapetrou spacetimes.(International Research Symposium on Pure and Applied Sciences, 2017 Faculty of Science, University of Kelaniya, Sri Lanka., 2017) Dilini, N. I.; Wimaladharma, N. A. S. N.Einstein – Maxwell field equations are nonlinear partial differential equations which are difficult to solve. Therefore different assumptions are needed to solve them. Also it is very hard to find properties of the matter distributions with black holes. An exact solution to Einstein-Maxwell filed equations describing gravitational fields of the extremely charged thin spherical shells has been found in a Majumdar Papapetrou spacetime. The boundary conditions are applied considering the facts that the metric must be continuous across the shell and the absence of matter in the thin shell and outside of the shell. The metric for the exterior vacuum region of the thin shell is in the same form of conventional Extreme Reissner Nordstorm (ERN) metric which describes the exterior region of a black hole. Therefore, by replacing the black hole by a thin shell so that the centre of the thin shell is on the point of existence of black hole, the singularities of ERN metric can be removed in a Majumdar Papapetrou spacetime. This process has been generalized for N-ERN black holes with any finite number of black holes in Majumdar-Papapetrou spacetimes. In the case of two ERN black holes, the matter densities of each shell which were located on the points of singularities have been calculated. Two spherical shells with different radii and center locations are considered. Calculating the redshift of a light pulse emitted at a point on the interior flat region of the thin shell as observed by an observer at infinity, it is shown that the solution is physically acceptable.Item Black holes as boson stars.(International Research Symposium on Pure and Applied Sciences, 2017 Faculty of Science, University of Kelaniya, Sri Lanka., 2017) Katugampala, K. D. W. J.; De Silva, L. N. K.Kaup (1968) had shown that spin zero bosons could form stable stars. However, there had been no interest in these stars until the Higgs boson was discovered in 2012. Now there is renewed interest in boson stars. Clark (2017) in an article “holy moley” published in the New Scientist (15th July) considers boson stars and black holes as two different types of objects. As far as black holes are considered it can only be said that the matter collapses in to a singularity. Boson stars on the other hand are made of bosons. The above article considers black holes and boson stars to be two different kinds leading to same observations with respect to certain objects. However, they could be the same with black holes constituting bosons of zero mass. In this paper it is suggested that some stars, may be most stars, constitute ordinary matter, some may constitute fermions and bosons, while some others are boson stars. It is not ruled out that some stars could be made of bosons of zero mass such as gluons and/or gravitons. Some stars could be made of bosons including photons, as stars with photons only have been ruled out. de Silva (1970) had considered time like geodesics in the Schwarzschild metric and found that the energy of a particle of rest mass m0 at a coordinate distance r can be written as m0kc2 where and u being the velocity of the particle in the local frame at rest with respect to the frame at infinity. In recent research we have found that the rest mass of a particle in a rest frame, called the local rest frame at coordinate distance r could be considered as m0 (1-2M/r)1/2 in a Schwarzschild metric, where m0 is the rest mass of the particle at infinity and M is the Schwarzschild mass of the central object. The rest mass of the particle in the local rest frame decreases, and ends up as zero as the particle enters the singularity with velocity. It implies that black holes in a Schwarzschild metric could be considered as objects made of bosons with zero mass and observed at large distances with non zero Schwarzschild mass .