Faculty of Applied Sciences
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Item 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.Item Charged radiation collapse in Einstein-Gauss-Bonnet gravity(Springer Science and Business Media LLC, 2022-04-25) Brassel, Byron P.; Maharaj, Sunil D.; Goswami, RituparnoWe generalise the continual gravitational col lapse of a spherically symmetric radiation shell of matter in f ive dimensional Einstein–Gauss–Bonnet gravity to include theelectromagneticfield.Thepresenceofchargehasasignif icant effectinthecollapsedynamics.Wenotethatthereexists a maximal charge contribution for which the metric func tions in Einstein–Gauss–Bonnet gravity remain real, which is not the case in general relativity. Beyond this maximal charge the spacetime metric is complex. The final fate of col lapse for the uncharged matter field, with positive mass, is an extended, weak and initially naked central conical singular ity. With the presence of an electromagnetic field, collapse terminates with the emergence of a branch singularity sepa rating the physical spacetime from the complex region. We show that this marked difference in singularity formation is only prevalent in five dimensions. We extend our analysis to higher dimensions and show that for all dimensions N ≥ 5, charged collapse ceases with the above mentioned branch singularity. This is significantly different than the uncharged scenario where a strong curvature singularity forms post col lapse for all N ≥ 6 and a weak conical singularity forms when N =5.Acomparison with charged radiation collapse in general relativity is also given.Item Higher-dimensional inhomogeneous composite fluids : energy conditions(Oxford University Press (OUP), 2021-10-07) Brassel, Byron P.; Maharaj, Sunil D.; Goswami, RituparnoThe 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.Item Higher-dimensional radiating black holes in Einstein-Gauss-Bonnet gravity(American Physical Society (APS), 2019-07-01) Brassel, Byron P.; Maharaj, Sunil D.; Goswami, RituparnoThe higher order curvature corrections in Einstein-Gauss-Bonnet gravity play a significant role in the dynamics of gravitational collapse. We extend the gravitational collapse of radiating shells of matter in Einstein-Gauss-Bonnet gravity to higher dimensions, in the context of the cosmic censorship conjecture. In five dimensions the final collapse terminates with the formation of an extended and weak conical, naked singularity in the central region. For dimensions N>5, we determine that collapse terminates with a strong curvature singularity which may or may not be naked. Cosmic censorship is affected by higher-order curvature corrections. A comparison with the higher-dimensional general relativity counterpart is also given, where the dynamics are affected by the higher dimensions.Item Extended naked conical singularity in radiation collapse in Einstein-Gauss-Bonnet gravity(American Physical Society (APS), 2018-09-11) Brassel, Byron P.; Maharaj, Sunil D.; Goswami, RituparnoIn this paper, we investigate the continual gravitational collapse of a spherically symmetric radiation shell in five-dimensional Einstein-Gauss-Bonnet gravity. We show that the final fate of such a collapse is an extended and weak curvature naked conical singularity at the centre, which then subsequently becomes covered by an apparent horizon. This process is entirely different from the five dimensional general relativity counterpart, where a strong curvature singularity develops at the centre. Since the singularity in the case of Einstein-Gauss-Bonnet gravity is sufficiently weak, we argue that the spacetime can be extended through it, which gives us an elegant way of constructing regular black holes in higher dimensions without violating any energy conditions. We also extend our study to spacetimes with null and string fluids, which are the counterpart of generalized Vaidya spacetimes in general relativity. We show that similar end states are also possible in those cases. Higher-dimensional spacetimes are then considered.Item Collapsing radiating stars with various equations of state(American Physical Society (APS), 2017-06-28) Brassel, Byron P.; Goswami, Rituparno; Maharaj, Sunil D.We study the gravitational collapse of radiating stars in the context of the cosmic censorship conjecture. We consider a generalized Vaidya spacetime with three concentric regions. The local internal atmosphere is a two-component system consisting of standard pressure-free, null radiation and an additional string fluid with energy density and nonzero pressure obeying all physically realistic energy conditions. The middle region is purely radiative which matches to a third region which is the Schwarzschild exterior. We outline the general mathematical framework to study the conditions on the mass function so that future-directed nonspacelike geodesics can terminate at the singularity in the past. Mass functions for several equations of state are analyzed using this framework and it is shown that the collapse in each case terminates at a locally naked central singularity. We calculate the strength of these singularities to show that they are strong curvature singularities which implies that no extension of spacetime through them is possible.Item Diffusive and dynamical radiating stars with realistic equations of state(Springer Science and Business Media LLC, 2017-03) Brassel, Byron P.; Maharaj, Sunil D.; Goswami, RituparnoWe model the dynamics of a spherically symmetric radiating dynamical star with three spacetime regions. The local internal atmosphere is a two-component system consisting of standard pressure-free, null radiation and an additional string fluid with energy density and nonzero pressure obeying all physically realistic energy conditions. The middle region is purely radiative which matches to a third region which is the Schwarzschild exterior. A large family of solutions to the field equations are presented for various realistic equations of state. We demonstrate that it is possible to obtain solutions via a direct integration of the second order equations resulting from the assumption of an equation of state. A comparison of our solutions with earlier well known results is undertaken and we show that all these solutions, including those of Husain, are contained in our family. We then generalise our class of solutions to higher dimensions. Finally we consider the effects of diffusive transport and transparently derive the specific equations of state for which this diffusive behaviour is possible.Item Stars and junction conditions in Einstein-Gauss-Bonnet gravity(IOP Publishing, 2023-06-15) Brassel, Byron P.; Maharaj, Sunil D.; Goswami, RituparnoThe 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.