Sukratu Barve

Spherical gravitational collapse: Tangential pressure and related equations of state

We derive an equation for the acceleration of a fluid element in the spherical gravitational collapse of a bounded compact object made up of an imperfect fluid. We show that non-singular as well as singular solutions arise in the collapse of a fluid initially at rest and having only a tangential pressure. We obtain an exact solution of the Einstein equations, in the form of an infinite series, for collapse under tangential pressure with a linear equation of state. We show that if a singularity forms in the tangential pressure model, the conditions for the singularity to be naked are exactly the same as in the model of dust collapse.

PARTICLE CREATION IN THE MARGINALLY BOUND, SELF-SIMILAR COLLAPSE OF INHOMOGENEOUS DUST

Abstract We consider the evaporation of the (shell-focusing) naked singularity formed during the self-similar collapse of a marginally bound inhomogeneous dust cloud, in the geometric optics approximation. We show that, neglecting the back reaction of the space-time, the radiation on I +tends to infinity as the Cauchy horizon is approached. Twoconsequences can be expected from this result: (a) the back reaction of space-time will be large and eventually halt the formation of a naked singularity thus preserving the Cosmic Censorship Hypothesis, and (b) matter attempting to collapse into a naked singularity will radiate away energy at an intense rate, thereby possibly providing experimental signatures of quantum effects in curved space-times.

Quantum stress tensor in self-similar spherical dust collapse

We calculate the quantum stress tensor for a massless scalar field in the 2D self-similar spherical dust collapse model which admits a naked singularity. We find that the outgoing radiation flux diverges on the Cauchy horizon. This may have two consequences. The resultant back reaction may prevent the naked singularity from forming, thus preserving cosmic censorship through quantum effects. The divergent flux may lead to an observable signature differentiating naked singularities from black holes in astrophysical observations.

A simple derivation of the naked singularity in spherical dust collapse

We describe a simple method of determining whether the singularity that forms in the spherically symmetric collapse of inhomogeneous dust is naked or covered. This derivation considerably simplifies the analysis given in the earlier literature, while giving the same results as have been obtained previously.

Are Naked Singularities Forbidden by the Second Law of Thermodynamics

By now, many examples of naked singularities in classical general relativity are known. It may, however, be that a physical principle over and above the general theory prevents the occurrence of such singularities in nature. Assuming the validity of the Weyl curvature hypothesis, we propose that naked singularities are forbidden by the second law of thermodynamics.

Noise kernel for the self-similar Tolman-Bondi metric: Fluctuations on the Cauchy horizon

We attempt to calculate the point separated Noise Kernel for self similar Tolman Bondi metric, using a method similar to that developed by Eftekharzadeh et. al for ultra-static spacetimes referring to the work by Page. In case of formation of a naked singularity, the Noise Kernel thus obtained is found to be regular except on the Cauchy horizon, where it diverges. The behavior of the noise in case of the formation of a covered singularity is found to be regular. This result seemingly renders back reaction non-negligible which questions the stability of the results obtained from the semiclassical treatment of the self similar Tolman Bondi metric.

Depletion of energy from naked singular regions during gravitational collapse

A distinguishable and observable physical property of Naked Singular Regions of the spacetime formed during a gravitational collapse has important implications for both experimental and theoretical relativity. We examine here whether energy can escape physically from naked singular regions to reach either a local or a distant observer within the framework of general relativity. We find that in case of imploding null dust collapse scenarios field outgoing singular null geodesics including the cauchy horizon can be immersed between two Vaidya spacetimes as null boundary layers with non vanishing positive energy density. Thus energy can transported from the naked singularity to either a local or a distant observer. And example illustrating that similar considerations can be applied to dust models is given.

Divergence of the quantum stress tensor on the Cauchy horizon in 2D dust collapse

We prove that the quantum stress tensor for a massless scalar field in two dimensional non-self-similar Tolman-Bondi dust collapse and Vaidya radiation collapse models diverges on the Cauchy horizon, if the latter exists. The two dimensional model is obtained by suppressing angular coordinates in the corresponding four dimensional spherical model.

Sabot Discard: Effect of Scoop Shape

Separation of sabot petals is a critical procedure in intermediate ballistics affecting accuracy of the final hit of the penetrator of the fired ammunition round. Design of sabot petals has been an important concern supported by modeling and simulation studies of sabot separation. Computational fluid dynamics studies of the same have hitherto addressed feasibility of coupled fluid dynamics and 6 DOF simulations employing fixed sabot designs. The use of dynamical meshes for supersonic air flow is in its infancy as each simulation run for a given sabot geometry is highly computationally intensive. Good support can still be offered to sabot petal design by using analytical models, like the one by Reinecke and Guillot and its subsequent improvements. We present and study an improved model which better captures the role of the sabot petal scoop. In particular, we study how changes in the shape of the scoop affects the sabot separation kinematics. The sabot assembly assumed to be axisymmetric in shape and other major design parameters are kept the same when the scoop shape is changed.

Assessing the purity of CNT from its composite with polymer - A new approach

This study focuses on calculating the electrical properties of composites of CNTs with polystyrene within the framework of percolation theory. Nano-composites, with multiwall carbon nanotubes (MWCNTs) as a filler; especially synthesized in a dc arc plasma system in presence of focusing electric field, have been shown to exhibit metallic behavior. The purity of the MWCNTs prepared under the optimized focusing voltage of 400 V is then seen to be much higher as compared to the one synthesized at zero focusing voltage.