Quantum evaporation of a naked singularity

We investigate here quantum effects in gravitational collapse of a scalar field model which classically leads to a naked singularity. We show that nonperturbative semiclassical 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.     

Dynamical instability of charged self-gravitating stars in modified gravity

This paper explores the instability limits for the stellar objects in the background of a particular modified gravity theory. In order to accomplish the instability conditions, a spherically symmetric anisotropic charged fluid influenced by the modified gravity is taken under consideration. The modified field equations and the equations of motion are accomplished in background of the Gauss–Bonnet gravity. These equations are perturbed to constitute the collapse equation. The Newtonian and post-Newtonian limits are imposed and found that the dynamical instability of the fluid is explained by the adiabatic index which consists on analytical value depending on static profile of material variables.     

Quantum effects near the singularity in a general cosmological scenario

Joshi and Joshi obtained a differential equation for quantum uncertainty under some very general conditions and inferred the divergence of the uncertainty on the basis of an approximate solution. The present note exactly solves the equation and confirms the divergence.     

On the pomeranchuk singularity in asymptotically free theories

We calculate the high-energy amplitude in the spontaneously broken Yang-Mills model within the leading logarithmic approximation, and find out that the vector meson is reggeised whereas the Pomeranchon is a fixed branch point located at the right hand from j = 1. The asymptotic freedom alters the situation in this model significally.     

The neutrino bursts from the supernova SN 1987 a do not rule out the existence of wave-packet spreading

Contrary to a recent claim, the spreading of ultra-relativistic neutrino wave-packets results in time spreading much smaller than the duration of the observed bursts.     

Quantum noncommutative gravity in two dimensions

We study quantisation of noncommutative gravity theories in two dimensions (with noncommutativity defined by the Moyal star product). We show that in the case of noncommutative Jackiw–Teitelboim gravity the path integral over gravitational degrees of freedom can be performed exactly even in the presence of a matter field. In the matter sector, we study possible choices of the operators describing quantum fluctuations and define their basic properties (e.g., the Lichnerowicz formula). Then we evaluate two leading terms in the heat kernel expansion, calculate the conformal anomaly and the Polyakov action (as an expansion in the conformal field).     

A singularity free solution of the Maxwell-Einstein equations

It is found that the massless charged particle permanently at rest at the origin of spherical polar coordinates in Lovelock's interpretation [1] of Robinson's solution of the Einstein-Maxwell equations [2] will repel all charged test particles, irrespective of the sign of their charges. By a global embedding of the space-time in a flat 6-space we find an absence of singularities where point-charges or point-masses might be located. With the use of the Newman-Penrose method of spin-coefficients [6] it is shown that all the Robinson solutions [2] represent constant electromagnetic fields.     

Quantum spinors and the singularity theorems of general relativity

I suggest that space-time singularities may be removed through the quantum effects of fermionic spinor matter. I support this viewpoint on the basis of an effective negative contribution, coming from spinor fields with anticommutative (“quantum”) nature, to the energy condition of the singularity theorems of general relativity.     

Quantum theory of a free electron laser

Spontaneous emission by electrons moving in a magnetic field constant in time and periodic in space (with circular polarization) is calculated using exact solutions of the corresponding Dirac equation. In addition to the known transitions we find two harmonics connected with spin flip processes, which are, however, strongly suppressed. The gain of the free electron laser is recalculated from the rate for spontaneous emission. The results obtained by classical methods are corroborated.Supported in part by Deutsche Forschungsgemeinschaft     

Simulation of free dendritic crystal growth in a gravity environment

In this paper we simulate the evolution and free particle motion of an individual nucleus that grows into a dendritic crystal. The melt flow and the convective heat transfer around the crystal are simulated as they settle due to gravity. There is an intricate coupling between the settling and the evolution of the crystal. The relative flow induced by the settling enhances the growth at the downward facing parts, which in its turn affects the subsequent settling motion.Simulations have been done in two dimensions using a semi-sharp phase-field model. The flow was constrained to a rigid body motion by using Lagrange multipliers inside the solidified part. The model was formulated using two different meshes. One is a fixed background mesh, which covers the whole domain. The other is an adaptive mesh, where the node points are also translated and rotated with the movement of the solid particle. In the latter, the dendritic growth is simulated by the semi-sharp phase-field method.