Sushant G. Ghosh

Collapsing spherical stars in f(R) gravity

We perform a careful investigation of the problem of physically realistic gravitational collapse of massive stars in f(R)-gravity. We show that the extra matching conditions that arise in the modified gravity imposes strong constraints on the stellar structure and thermodynamic properties. In our opinion these constraints are unphysical. We prove that no homogeneous stars with non-constant Ricci scalar can be matched smoothly with a static exterior for any nonlinear function f(R). Therefore, these extra constraints make classes of physically realistic collapse scenarios in general relativity, non-admissible in these theories. We also find an exact solution for an inhomogeneous collapsing star in the Starobinski model that obeys all the energy and matching conditions. However, we argue that such solutions are fine-tuned and unstable to matter perturbations. Possible consequences on black hole physics and the cosmic censorship conjecture are also discussed.

Gravitational collapse of null fluid on the brane

Abstract We first obtain the analogue of Vaidyas solution on the brane for studying the collapse of null fluid onto a flat Minkowski cavity on the brane. Since the back-reaction of the bulk onto the brane is supposed to strengthen gravity on the brane, it would favor formation of black hole as against naked singularity. That is the parameter window in the initial data set giving rise to naked singularity in the 4D Vaidya case would now get partially covered.

Higher dimensional inhomogeneous dust collapse and cosmic censorship

We investigate the occurrence and nature of a naked singularity in the gravitational collapse of an inhomogeneous dust cloud described by higher dimensional Tolman-Bondi space-times. The naked singularities are found to be gravitationally strong in the sense of Tipler. Higher dimensions seem to favor black holes rather than naked singularities.

On naked singularities in higher dimensional Vaidya space-times

We investigate the end state of the gravitational collapse of a null fluid in higher-dimensional space-times. Both naked singularities and black holes are shown to be developing as the final outcome of the collapse. The naked singularity spectrum in a collapsing Vaidya region (4D) gets covered with the increase in dimensions and hence higher dimensions favor a black hole in comparison to a naked singularity. The cosmic censorship conjecture will be fully respected for a space of infinite dimension.

Radiating Kerr-like regular black hole

We derive a radiating regular rotating black hole solution, radiating Kerr-like regular black hole solution. We achieve this by starting from the Hayward regular black hole solution via a complex transformation suggested by Newman–Janis. The radiating Kerr metric, the Kerr-like regular black hole and the standard Kerr metric are regained in the appropriate limits. The structure of the horizon-like surfaces are also determined.

The Lovelock gravity in the critical spacetime dimension

Abstract It is well known that the vacuum in the Einstein gravity, which is linear in the Riemann curvature, is trivial in the critical ( 2 + 1 = 3 ) dimension because vacuum solution is flat. It turns out that this is true in general for any odd critical d = 2 n + 1 dimension where n is the degree of homogeneous polynomial in Riemann defining its higher order analogue whose trace is the nth order Lovelock polynomial. This is the “curvature” for nth order pure Lovelock gravity as the trace of its Bianchi derivative gives the corresponding analogue of the Einstein tensor as defined by Dadhich (2010) [1] . Thus the vacuum in the pure Lovelock gravity is always trivial in the odd critical ( 2 n + 1 ) dimension which means it is pure Lovelock flat but it is not Riemann flat unless n = 1 and then it describes a field of a global monopole. Further by adding Λ we obtain the Lovelock analogue of the BTZ black hole.

NON-SPHERICAL COLLAPSE OF A RADIATING STAR

We study the junction conditions for non-spherical (plane symmetric) collapsing radiating star consisting of a shearing fluid undergoing radial heat flow with outgoing radiation. Radiation of the system is described by plane symmetric Vaidya solution. Physical quantities relating to the local conservation of momentum and surface red-shift are also obtained.

Shadow of five-dimensional rotating Myers-Perry black hole

A black hole casts a shadow as an optical appearance because of its strong gravitational field. We study the shadow cast by the five-dimensional Myers-Perry black hole with equal rotation parameters. We demonstrate that the null geodesic equations can be integrated that allows us to investigate the shadow cast by a black hole. The shadow of a black hole is found to be a dark zone covered by deformed circle. Interestingly, the shapes of the black hole shadow are more distorted and size decreases for larger black hole spins. Interestingly, it turns out that, for fixed values of rotation parameter, the shadow is slightly smaller and less deformed than for its four-dimensional Kerr black counterpart. Further, the shadow of the five-dimensional Kerr black hole is concentric deformed circles. The effect of rotation parameter on the shape and size of a naked singularity shadow is also analyzed.

A class of black holes in dRGT massive gravity and their thermodynamical properties

We present an exact spherical black hole solution in de Rham, Gabadadze, and Tolley (dRGT) massive gravity for a generic choice of the parameters in the theory, and also discuss the thermodynamical and phase structure of the black hole in both the grand canonical and the canonical ensembles (for the charged case). It turns out that the dRGT black hole solution includes other known solutions to the Einstein field equations, such as the monopole-de Sitter–Schwarzschild solution with the coefficients of the third and fourth terms in the potential and the graviton mass in massive gravity naturally generates the cosmological constant and the global monopole term. Furthermore, we compute the mass, temperature and entropy of the dRGT black hole, and also perform thermodynamical stability analysis. It turns out that the presence of the graviton mass completely changes the black hole thermodynamics, and it can provide the Hawking–Page phase transition which also occurs for the charged black holes. Interestingly, the entropy of a black hole is barely affected and still obeys the standard area law. In particular, our results, in the limit