Naked and Thunderbolt Singularities in Black Hole Evaporation

If an evaporating black hole does not settle down to a non radiating remnant, a description by a semi classical Lorentz metric must contain either a naked singularity or what we call a thunderbolt, a singularity that spreads out to infinity on a spacelike or null path. We investigate this question in the context of various two dimensional models that have been proposed. We find that if the semi classical equations have an extra symmetry that make them solvable in closed form, they seem to predict naked singularities but numerical calculations indicate that more general semi classical equations, such as the original CGHS ones give rise to thunderbolts. We therefore expect that the semi classical approximation in four dimensions will lead to thunderbolts. We interpret the prediction of thunderbolts as indicating that the semi classical approximation breaks down at the end point of black hole evaporation, and we would expect that a full quantum treatment would replace the thunderbolt with a burst of high energy particles. The energy in such a burst would be too small to account for the observed gamma ray bursts.Comment: 21 pages (10 diagrams available on request

The Bousso entropy bound in selfgravitating gas of massless particles

The Bousso entropy bound is investigated in a static spherically symmetric spacetime filled with an ideal gas of massless bosons or fermions. Especially lightsheets generated by spheres are considered. Statistical description of the gas is given. Conditions under which the Bousso bound can be violated are discussed and it is shown that a possible violating region cannot be arbitrarily large and it is contained inside a sphere of unit Planck radius if number of independent polarization states $g_s$ is small enough. It is also shown that central temperature must exceed the Planck temperature to get a violation of the Bousso bound for $g_s$ not too large.Comment: 14 pages, 4 figures, a paragraph added, version published in Gen. Rel. Gra

Fluctuations along supersymmetric flat directions during Inflation

We consider a set of scalar fields, consisting of a single flat direction and one or several non-flat directions. We take our cue from the MSSM, considering separately D-flat and F-flat directions, but our results apply to any supersymmetric scenario containing flat directions. We study the field fluctuations during pure de Sitter Inflation, following the evolution of the infrared modes by numerically solving the appropriate Langevin equations. We demonstrate that for the Standard Model U(1), SU(2) or SU(3) gauge couplings, as well as for large enough Yukawa couplings, the fluctuations along the non-flat directions effectively block the fluctuations along the flat directions. The usual expected behaviour \propto N, with N the number of efolds, may be strongly violated, depending on the coupling strengths. As a consequence, those cosmological considerations, which are derived assuming that during inflation flat directions fluctuate freely, should be revised.Comment: 19 pages, 5 figures, Submitted to JCA

Conformal Properties of an Evaporating Black Hole Model

We use a new, conformally-invariant method of analysis to test incomplete null geodesics approaching the singularity in a model of an evaporating black hole for the possibility of extensions of the conformal metric. In general, a local conformal extension is possible from the future but not from the past

Bell's theorem as a signature of nonlocality: a classical counterexample

For a system composed of two particles Bell's theorem asserts that averages of physical quantities determined from local variables must conform to a family of inequalities. In this work we show that a classical model containing a local probabilistic interaction in the measurement process can lead to a violation of the Bell inequalities. We first introduce two-particle phase-space distributions in classical mechanics constructed to be the analogs of quantum mechanical angular momentum eigenstates. These distributions are then employed in four schemes characterized by different types of detectors measuring the angular momenta. When the model includes an interaction between the detector and the measured particle leading to ensemble dependencies, the relevant Bell inequalities are violated if total angular momentum is required to be conserved. The violation is explained by identifying assumptions made in the derivation of Bell's theorem that are not fulfilled by the model. These assumptions will be argued to be too restrictive to see in the violation of the Bell inequalities a faithful signature of nonlocality.Comment: Extended manuscript. Significant change

Feasibility of human lung ventilation imaging using highly polarized naturally abundant xenon and optimized three-dimensional steady-state free precession

Purpose To demonstrate the potential for high quality MRI of pulmonary ventilation using naturally abundant xenon (NAXe) gas. Methods MRI was performed at 1.5 Tesla (T) and 3 T on one healthy smoker and two healthy never-smokers. 129Xe gas was polarized to ∼25% using an in-house spin-exchange optical pumping polarizer fitted with a laser diode array with integrated volume holographic grating and optical train system. Volunteers inhaled 1 L of NAXe for an 8 to 15 s breathhold while MR images were acquired with full-lung coverage using a three-dimensional steady-state free precession sequence, optimized for maximum signal-to-noise ratio (SNR) at a given spatial resolution. For the purpose of image quality comparison, the MR acquisition was repeated at 1.5 T with 400 mL enriched xenon and 200 mL 3He. Results All NAXe lung images were of high quality, with mean SNRs of 25–40 (voxel 4.2 × 4.2 × 8/10 mm3) and ∼30% improvement at 3 T versus 1.5 T. The high SNR permitted identification of minor ventilation defects in the healthy smoker's lungs. NAXe images were of comparable SNR to those obtained with enriched xenon and 3He. Conclusion Optimization of MR pulse sequences and advances in polarization technology have facilitated high quality pulmonary ventilation imaging with inexpensive NAXe gas. Magn Reson Med 74:346–352, 2015

Theory and experiment of the ESR of Co2+^{2+} in Zn2_2 % (OH)PO4_4 and Mg2_2(OH)AsO4_4

Experiments of Electron Spin Resonance (ESR) were performed on Co$$ substituting Zn$^{2+}$ or Mg$^{2+}$ in powder samples of Zn$_2$(OH)PO$_4$ and Mg$_2$(OH)AsO$_4$. The observed resonances are described with a theoretical model that considers the departures from the two perfect structures. It is shown that the resonance in the penta-coordinated complex is allowed, and the crystal fields that would describe the resonance of the Co$^{2+}$ in the two environments are calculated. The small intensity of the resonance in the penta-coordinated complex is explained assuming that this site is much less populated than the octahedral one; this assumption was verified by a molecular calculation of the energies of the two environments, with both Co and Zn as central ions in Zn$_2$(OH)PO$_4$.Comment: 43 pages, LaTex file, 6 figures, EPS. submitted to Journal of Physics Condens

High resolution spectroscopy and chemical shift imaging of hyperpolarized 129 Xe dissolved in the human brain in vivo at 1.5 tesla

Purpose Upon inhalation, xenon diffuses into the bloodstream and is transported to the brain, where it dissolves in various compartments of the brain. Although up to five chemically distinct peaks have been previously observed in 129Xe rat head spectra, to date only three peaks have been reported in the human head. This study demonstrates high resolution spectroscopy and chemical shift imaging (CSI) of 129Xe dissolved in the human head at 1.5 Tesla. Methods A 129Xe radiofrequency coil was built in-house and 129Xe gas was polarized using spin-exchange optical pumping. Following the inhalation of 129Xe gas, NMR spectroscopy was performed with spectral resolution of 0.033 ppm. Two-dimensional CSI in all three anatomical planes was performed with spectral resolution of 2.1 ppm and voxel size 20 mm × 20 mm. Results Spectra of hyperpolarized 129Xe dissolved in the human head showed five distinct peaks at 188 ppm, 192 ppm, 196 ppm, 200 ppm, and 217 ppm. Assignment of these peaks was consistent with earlier studies. Conclusion High resolution spectroscopy and CSI of hyperpolarized 129Xe dissolved in the human head has been demonstrated. For the first time, five distinct NMR peaks have been observed in 129Xe spectra from the human head in vivo

Whole lung morphometry with 3D multiple b-value hyperpolarized gas MRI and compressed sensing.

PURPOSE: To demonstrate three-dimensional (3D) multiple b-value diffusion-weighted (DW) MRI of hyperpolarized (3) He gas for whole lung morphometry with compressed sensing (CS). METHODS: A fully-sampled, two b-value, 3D hyperpolarized (3) He DW-MRI dataset was acquired from the lungs of a healthy volunteer and retrospectively undersampled in the ky and kz phase-encoding directions for CS simulations. Optimal k-space undersampling patterns were determined by minimizing the mean absolute error between reconstructed and fully-sampled (3) He apparent diffusion coefficient (ADC) maps. Prospective three-fold, undersampled, 3D multiple b-value (3) He DW-MRI datasets were acquired from five healthy volunteers and one chronic obstructive pulmonary disease (COPD) patient, and the mean values of maps of ADC and mean alveolar dimension (LmD ) were validated against two-dimensional (2D) and 3D fully-sampled (3) He DW-MRI experiments. RESULTS: Reconstructed undersampled datasets showed no visual artifacts and good preservation of the main image features and quantitative information. A good agreement between fully-sampled and prospective undersampled datasets was found, with a mean difference of +3.4% and +5.1% observed in mean global ADC and LmD values, respectively. These differences were within the standard deviation range and consistent with values reported from healthy and COPD lungs. CONCLUSIONS: Accelerated CS acquisition has facilitated 3D multiple b-value (3) He DW-MRI scans in a single breath-hold, enabling whole lung morphometry mapping. Magn Reson Med, 2016. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited