Where are the degrees of freedom responsible for black hole entropy?

Considering the entanglement between quantum field degrees of freedom inside and outside the horizon as a plausible source of black hole entropy, we address the question: {\it where are the degrees of freedom that give rise to this entropy located?} When the field is in ground state, the black hole area law is obeyed and the degrees of freedom near the horizon contribute most to the entropy. However, for excited state, or a superposition of ground state and excited state, power-law corrections to the area law are obtained, and more significant contributions from the degrees of freedom far from the horizon are shown.Comment: 6 pages, 4 figures, Invited talk at Theory Canada III, Edmonton, Alberta, Canada, June 16, 200

What can we say about seed fields for galactic dynamos?

We demonstrate that a quasi-uniform cosmological seed field is a much less suitable seed for a galactic dynamo than has often been believed. The age of the Universe is insufficient for a conventional galactic dynamo to generate a contemporary galactic magnetic field starting from such a seed, accepting conventional estimates for physical quantities. We discuss modifications to the scenario for the evolution of galactic magnetic fields implied by this result. We also consider briefly the implications of a dynamo number that is significantly larger than that given by conventional estimates

Reverse undercompressive shock structures in driven thin film flow

We show experimental evidence of a new structure involving an undercompressive and reverse undercompressive shock for draining films driven by a surface tension gradient against gravity. The reverse undercompressive shock is unstable to transverse perturbations while the leading undercompressive shock is stable. Depending on the pinch-off film thickness, as controlled by the meniscus, either a trailing rarefaction wave or a compressive shock separates from the reverse undercompressive shock

A comprehensive set of UV and x-ray radiative transition rates for Fe XVI

Sodium-like Fe XVI is observed in collisionally ionized plasmas such as stellar coronae and coronal line regions of active galactic nuclei including black hole-accretion disc environments. Given its recombination edge from neon-like Fe XVII at ~25 Å, the Fe XVI bound-bound transitions lie in the soft x-ray and EUV (extreme ultraviolet) range. We present a comprehensive set of theoretical transition rates for radiative dipole allowed E1 transitions including fine structure for levels with nℓ(SLJ) ≤ 10, ℓ ≤ 9 using the relativistic Breit-Pauli R-matrix (BPRM) method. In addition, forbidden transitions of electric quadrupole (E2), electric octupole (E3), magnetic dipole (M1) and magnetic quadrupole (M2) type are presented for levels up to 5g(SLJ) from relativistic atomic structure calculations in the Breit-Pauli approximation using code SUPERSTRUCTURE. Some of the computed levels are autoionizing, and oscillator strengths among those are also provided. BPRM results have been benchmarked with the relativistic coupled cluster method and the atomic structure Dirac-Fock code GRASP. Levels computed with the electron collision BPRM codes in bound state mode were identified with a procedure based on the analysis of quantum defects and asymptotic wavefunctions. The total number of Fe XVI levels considered is 96, with 822 E1 transitions. Tabulated values are presented for the oscillator strengths f, line strengths S and Einstein radiative decay rates A. This extensive dataset should enable spectral modelings up to highly excited levels, including recombination-cascade matrices.This work was partially supported by the NASA Astronomy and Physics Research Program and the Astrophysical Theory Program. The computational work was carried out at the Ohio Supercomputer Center in Columbus, Ohio. CS acknowledges discussions with Professor B P Das, Professor D Mukherjee and Professor R K Chaudhuri

Where are the black hole entropy degrees of freedom ?

Understanding the area-proportionality of black hole entropy (the `Area Law') from an underlying fundamental theory has been one of the goals of all models of quantum gravity. A key question that one asks is: where are the degrees of freedom giving rise to black hole entropy located? Taking the point of view that entanglement between field degrees of freedom inside and outside the horizon can be a source of this entropy, we show that when the field is in its ground state, the degrees of freedom near the horizon contribute most to the entropy, and the area law is obeyed. However, when it is in an excited state, degrees of freedom far from the horizon contribute more significantly, and deviations from the area law are observed. In other words, we demonstrate that horizon degrees of freedom are responsible for the area law.Comment: 5 pages, 3 eps figures, uses Revtex4, References added, Minor changes to match published versio

Magnetic helicity fluxes in interface and flux transport dynamos

Dynamos in the Sun and other bodies tend to produce magnetic fields that possess magnetic helicity of opposite sign at large and small scales, respectively. The build-up of magnetic helicity at small scales provides an important saturation mechanism. In order to understand the nature of the solar dynamo we need to understand the details of the saturation mechanism in spherical geometry. In particular, we want to understand the effects of magnetic helicity fluxes from turbulence and meridional circulation. We consider a model with just radial shear confined to a thin layer (tachocline) at the bottom of the convection zone. The kinetic alpha owing to helical turbulence is assumed to be localized in a region above the convection zone. The dynamical quenching formalism is used to describe the build-up of mean magnetic helicity in the model, which results in a magnetic alpha effect that feeds back on the kinetic alpha effect. In some cases we compare with results obtained using a simple algebraic alpha quenching formula. In agreement with earlier findings, the magnetic alpha effect in the dynamical alpha quenching formalism has the opposite sign compared with the kinetic alpha effect and leads to a catastrophic decrease of the saturation field strength with increasing magnetic Reynolds numbers. However, at high latitudes this quenching effect can lead to secondary dynamo waves that propagate poleward due to the opposite sign of alpha. Magnetic helicity fluxes both from turbulent mixing and from meridional circulation alleviate catastrophic quenching.Comment: 9 pages, 14 figures, submitted to A &