Rituparno Goswami | University of KwaZulu-Natal (original) (raw)

Uploads

Papers by Rituparno Goswami

International Journal of Modern Physics D, 2016

Generalizing earlier results on dust collapse in higher dimensions, we show here that cosmic cens... more Generalizing earlier results on dust collapse in higher dimensions, we show here that cosmic censorship can be restored in gravitational collapse with tangential pressure present if we take the spacetime dimension to be Nge6N\ge6Nge6. This is under conditions to be motivated physically, such as the smoothness of initial data from which the collapse develops. The models considered here incorporating a non-zero tangential pressure include the Einstein cluster spacetime.

Physical Review D Particles and Fields, 2008

We investigate whether or not an Einstein static universe is a solution to the cosmological equat... more We investigate whether or not an Einstein static universe is a solution to the cosmological equations in f(R) gravity. It is found that only one class of f(R) theories admits an Einstein static model, and that this class is neutrally stable with respect to vector and tensor perturbations for all equations of state on all scales. Scalar perturbations are only stable on all scales if the matter fluid equation of state satisfies cs2>(5-1)/(6)≈0.21. This result is remarkably similar to the general relativity case, where it was found that the Einstein static model is stable for cs2>(1)/(5).

We recast the system of Einstein field equations for Locally Rotationally Symmetric spacetimes in... more We recast the system of Einstein field equations for Locally Rotationally Symmetric spacetimes into an autonomous system of covariantly defined geometrical variables. The analysis of this autonomous system gives all the important global features of the maximal extension of these spacetimes. We conclude that the dynamical system analysis can be a powerful mathematical tool for qualitative understanding of the global structure of spacetimes covariantly, without actually solving the field equations.

This paper examines the various arguments that have been put forward suggesting either that time ... more This paper examines the various arguments that have been put forward suggesting either that time does not exist, or that it exists but its flow is not real. I argue that (i) time both exists and flows; (ii) an Evolving Block Universe (`EBU') model of spacetime adequately captures this feature, emphasizing the key differences between the past, present, and future; (iii) the associated surfaces of constant time are uniquely geometrically and physically determined in any realistic spacetime model based in General Relativity Theory; (iv) such a model is needed in order to capture the essential aspects of what is happening in circumstances where initial data does not uniquely determine the evolution of spacetime structure because quantum uncertainty plays a key role in that development. Assuming that the functioning of the mind is based in the physical brain, evidence from the way that the mind apprehends the flow of time prefers this evolving time model over those where there is no flow of time.

We study the structure of neutron stars in f(R)=R+alphaR2f(R)=R+\alpha R^{2}f(R)=R+alphaR2 theory of gravity (Starobinsky m... more We study the structure of neutron stars in f(R)=R+alphaR2f(R)=R+\alpha R^{2}f(R)=R+alphaR2 theory of gravity (Starobinsky model), in an exact and non-perturbative approach. In this model, apart from the standard General Relativistic junction conditions, two extra conditions, namely the continuity of the curvature scalar and its first derivative needs to be satisfied. For an exterior Schwarzschild solution, the curvature scalar and its derivative has to be zero at the stellar surface. We show that for some equation of state (EoS) of matter, matching all conditions at the surface of the star is impossible. Hence the model brings two major fine-tuning problems: (i) only some particular classes of EoS are consistent with Schwarzschild at the surface and (ii) given that EoS, only a very particular set of boundary conditions at the centre of the star will satisfy the given boundary conditions at the surface. Hence we show that this model (and subsequently many other f(R) models where uniqueness theorem is valid) is highly unnatural, for the existence of compact astrophysical objects. This is because the EoS of a compact star should be completely determined by the physics of nuclear matter at high density and not the theory of gravity.

Eprint Arxiv 0707 1122, Jul 9, 2007

We present a new class of solutions to Einstein equations for the spherical collapse of dustlike ... more We present a new class of solutions to Einstein equations for the spherical collapse of dustlike matter coupled with heat flux. In this family of solutions spacetime shear is necessarily non-zero. Also these solutions have an interesting property that there is always a bounce before the singularity, which is caused entirely due to the dissipative processes. We show there exist open sets of initial data for which the bounce occurs before any trapped surface formation, making the star explode away to infinity. We also discuss the role of heat flow in generating spacetime shear and in modifying the effective inertial mass of the matter cloud.

Int J Mod Phys D, 2011

The phenomena of collapse and dispersal for a massless scalar field have drawn considerable inter... more The phenomena of collapse and dispersal for a massless scalar field have drawn considerable interest in recent years, mainly from a numerical perspective. We give here a sufficient condition for the dispersal to take place for a scalar field that initially begins with a collapse. It is shown that the change of the gradient of the scalar field from a timelike to a spacelike vector must be accompanied by the dispersal of the scalar field. This result holds independently of any symmetries of the spacetime. We demonstrate the result explicitly by means of an example, which is the scalar field solution given by Roberts. The implications of the result are discussed.

ABSTRACT We extend Birkhoff's theorem for almost LRS-II vacuum spacetimes to show that th... more ABSTRACT We extend Birkhoff's theorem for almost LRS-II vacuum spacetimes to show that the rigidity of spherical vacuum solutions of Einstein's field equations continues even in the perturbed scenario.

Eprint Arxiv Gr Qc 0410144, Oct 28, 2004

We construct here a class of collapsing scalar field models with a non-zero potential, which resu... more We construct here a class of collapsing scalar field models with a non-zero potential, which result in a naked singularity as collapse end state. The weak energy condition is satisfied by the collapsing configuration. It is shown that physically it is the rate of collapse that governs either the black hole or naked singularity formation as the final state for the dynamical evolution. It is seen that the cosmic censorship is violated in dynamical scalar field collapse.

Phys Rev D, 2007

We investigate here spherically symmetric gravitational collapse in a space-time with an arbitrar... more We investigate here spherically symmetric gravitational collapse in a space-time with an arbitrary number of dimensions and with a general type I matter field, which is a broad class that includes most of the physically reasonable matter forms. We show that given the initial data for matter in terms of the initial density and pressure profiles at an initial surface t=ti from which the collapse evolves, there exist the rest of the initial data functions and classes of solutions of Einstein equations which we construct here, such that the space-time evolution goes to a final state which is either a black hole or a naked singularity, depending on the nature of initial data and evolutions chosen, and subject to validity of the weak energy condition. The results are discussed and analyzed in the light of the cosmic censorship hypothesis in black hole physics. The formalism here combines the earlier results on gravitational collapse in four dimensions in a unified treatment. Also the earlier work is generalized to higher-dimensional space-times to allow a study of the effect of the number of dimensions on the possible final outcome of the collapse in terms of either a black hole or naked singularity. No restriction is adopted on the number of dimensions, and other limiting assumptions such as self-similarity of space-time are avoided, in order to keep the treatment general. Our methodology allows us to consider to an extent the genericity and stability aspects related to the occurrence of naked singularities in gravitational collapse.

Physical Review D Particles and Fields, Jan 5, 2007

We discuss geometrical properties of the horizon surface of five-dimensional rotating black holes... more We discuss geometrical properties of the horizon surface of five-dimensional rotating black holes and black rings. Geometrical invariants characterizing these 3D geometries are calculated. We obtain a global embedding of the 5D rotating black horizon surface into a flat space. We also describe the Kaluza-Klein reduction of the black ring solution (along the direction of its rotation) which, though it is nakedly singular, relates this solution to the 4D metric of a static black hole distorted by the presence of external scalar (dilaton) and vector (“electromagnetic”) fields. The properties of the reduced black hole horizon and its embedding in E3 are briefly discussed.

AIP Conference Proceedings, 2002

We report the electromigration (EM) response of single-domain epitaxial Cu(001) lines on layers o... more We report the electromigration (EM) response of single-domain epitaxial Cu(001) lines on layers of Ta, TaN, and TiN. Epitaxial Cu(001) lines on nitride layers exhibit nearly two orders of magnitude higher mean-time-to-failure (MTTF) values than those on Ta, indicating the strong influence of the underlayer. The activation energy of EM for Cu on the nitrides is ~0.8-1.2 eV, and that

A surprising exact result for the Einstein field equations is that if pressure-free matter is mov... more A surprising exact result for the Einstein field equations is that if pressure-free matter is moving in a shear-free way, then it must be either expansion-free or rotation-free. It has been suggested this result is also true for any barotropic perfect fluid, but a proof has remained elusive. We consider the case of barotropic perfect-fluid solutions linearized about a Robertson-Walker geometry, and prove that the result remains true except for the case of a specific highly nonlinear equation of state. We argue that this equation of state is nonphysical, and hence the result is true in the linearized case for all physically realistic barotropic perfect fluids. This result, which is not true in Newtonian cosmology, demonstrates that the linearized solutions, believed to result in standard local Newtonian theory, do not always give the usual behavior of Newtonian solutions.

Physical Review D Particles and Fields, 2010

We investigate here quantum effects in gravitational collapse of a scalar field model which class... more We investigate here quantum effects in gravitational collapse of a scalar field model which classically leads to a naked singularity. We show that non-perturbative semi-classical modifications near the singularity, based on loop quantum gravity, give rise to a strong outward flux of energy. This leads to the dissolution of the collapsing cloud before the singularity can form. Quantum gravitational effects thus censor naked singularities by avoiding their formation. Further, quantum gravity induced mass flux has a distinct feature which may lead to a novel observable signature in astrophysical bursts.

We analyze here the final fate of complete gravitational collapse of a massless scalar field with... more We analyze here the final fate of complete gravitational collapse of a massless scalar field within general relativity. A class of dynamical solutions with initial data close to the Friedmann-Lemaitre-Robertson-Walker (FLRW) collapse model is explicitly given and the Einstein equations are integrated numerically in a neighborhood of the center. We show that the initial data space is evenly divided between the dynamical evolutions that terminate in a black hole final state and those that produce a locally naked singularity. We comment on the genericity aspects of the collapse end-states and the connection to cosmic censorship conjecture is pointed out.