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Research Publications (Applied Sciences)

Permanent URI for this collectionhttp://ir-dev.dut.ac.za/handle/10321/213

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Author: search.filters.author.Brassel, Byron P.Subject: search.filters.subject.51 Physical sciences

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    Junction conditions for composite matter in higher dimensions
    (IOP Publishing, 2021-10-07) Maharaj, Sunil D.; Brassel, Byron P.
    We derive the junction conditions for a general higher dimensional spherically symmetric radiating star across a comoving surface with an electromagnetic field. The charged composite interior consists of barotropic matter, a null dust and a null string fluid. The higher dimensional generalised Vaidya geometry describes the exterior radiating atmosphere of the charged composite star. We show at the stellar surface that the pressure is determined by the interior heat flux, anisotropy, null string density, charge distribution and the exterior null string density. The charge distribution affects the stellar pressure in general; the higher dimensional charged Vaidya spacetime is special and does not exhibit this feature. The number of dimensions appears explicitly in the surface pressure showing that the dimensions affect the gravitational dynamics. All previous treatments, for matter which is neutral or charged, emerge as special cases in our treatment.
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    Cosmic censorship and charged radiation in second order Lovelock gravity
    (Elsevier BV, 2022-11) Brassel, Byron P.; Goswami, Rituparno; Maharaj, Sunil D.
    The conditions for naked singularity formation are considered for a radiating metric of Boulware–Deser type within an electromagnetic field in second order Lovelock (or Einstein–Gauss–Bonnet) gravity. The spacetime metric remains real only up to certain maximum charge contribution. This differs from general relativity. Beyond a certain maximal charge, there exists no real and physical spacetime since the metric becomes complex. We establish that, under certain parameters and for specific values of the mass function and charge contribution, this branch singularity is indeed a naked singularity. This is in contrast to the neutral case where the spacetime metric is always real for a positive mass function, and further, a weak, initially naked singularity always occurs before it becomes covered by an event horizon for all future time. We highlight that both neutral and charged collapse under gravity in Einstein–Gauss–Bonnet gravity differ significantly to their general relativistic counterparts.
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    Higher-dimensional inhomogeneous composite fluids : energy conditions
    (Oxford University Press (OUP), 2021-10-07) Brassel, Byron P.; Maharaj, Sunil D.; Goswami, Rituparno
    The energy conditions are studied, in the relativistic astrophysical setting, for higher-dimensional Hawking–Ellis Type I and Type II matter fields. The null, weak, dominant and strong energy conditions are investigated for a higher-dimensional inhomogeneous, composite fluid distribution consisting of anisotropy, shear stresses, non-vanishing viscosity as well as a null dust and null string energy density. These conditions are expressed as a system of six equations in the matter variables where the presence of the higher dimension $N$ is explicit. The form and structure of the energy conditions is influenced by the geometry of the $(N-2)$-sphere. The energy conditions for the higher-dimensional Type II fluid are also generated, and it is shown that under certain restrictions the conditions for a Type I fluid are regained. All previous treatments for four dimensions are contained in our work.
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    A relativistic heat conducting model
    (Springer Science and Business Media LLC, 2018-11-27) Govender, Gabriel; Brassel, Byron P.; Maharaj, Sunil D.
    The interior dynamics of a relativistic fluid in a shear-free spherically symmetric spacetime are investigated. The isotropic matter distribution is an imperfect fluid with a nonvanishing heat flux which is in the radial direction. The pressure isotropy condition is a second-order nonlinear ordinary differential equation with variable coefficients in the gravitational potentials. We impose a particular on these potentials and a new class of solutions are obtained, containing those of Bergmann and Modak. A physical analysis is then performed where the matter variables are graphically plotted and the energy conditions are shown to be satisfied. Causality is also shown not to be violated. An analysis of the temperature profiles indicates that closed form expressions can be generated for both the noncausal and causal cases.
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    Stars and junction conditions in Einstein-Gauss-Bonnet gravity
    (IOP Publishing, 2023-06-15) Brassel, Byron P.; Maharaj, Sunil D.; Goswami, Rituparno
    The junction conditions for a higher dimensional spherically symmetric charged and anisotropic static star are derived in Einstein–Gauss–Bonnet (EGB) gravity with nonvanishing cosmological constant. It is shown that for a timelike boundary hypersurface of zero thickness, the generalised matching conditions across this surface in EGB gravity are satisfied. A sufficient condition is that the Israel-Darmois conditions are valid. Therefore it is possible to generate a complete stellar model in EGB gravity. The interior matches to the exterior higher dimensional charged Boulware–Deser spacetime with cosmological constant. The barotropic radial pressure has to vanish at the boundary of the star which is also the case in general relativity.