619 research outputs found
electron-positron-photon plasma around a collapsing star
We describe electron-positron pairs creation around an electrically charged
star core collapsing to an electromagnetic black hole (EMBH), as well as pairs
annihilation into photons. We use the kinetic Vlasov equation formalism for the
pairs and photons and show that a regime of plasma oscillations is established
around the core. As a byproduct of our analysis we can provide an estimate for
the thermalization time scale.Comment: 8 pages, 4 figures, to appear in the Proceedings of QABP200
On Higher Derivatives as Constraints in Field Theory: a Geometric Perspective
We formalize geometrically the idea that the (de Donder) Hamiltonian
formulation of a higher derivative Lagrangian field theory can be constructed
understanding the latter as a first derivative theory subjected to constraints.Comment: 7 page
Plasma expansion in the geometry of a collapsing star
We describe the evolution of an electron-positron-photon plasma created by
Sauter--Heisenberg--Euler--Schwinger mechanism around a collapsing charged star
core in the Reissner-Nordstr\"{o}m geometry external to the core, in view of
the application in the framework of the EMBH theory for gamma ray bursts.Comment: 12 pages, 4 figures, to appear in the Proceedings of QABP200
Domain modeling and grid generation for multi-block structured grids with application to aerodynamic and hydrodynamic configurations
About five years ago, a joint development was started of a flow simulation system for engine-airframe integration studies on propeller as well as jet aircraft. The initial system was based on the Euler equations and made operational for industrial aerodynamic design work. The system consists of three major components: a domain modeller, for the graphical interactive subdivision of flow domains into an unstructured collection of blocks; a grid generator, for the graphical interactive computation of structured grids in blocks; and a flow solver, for the computation of flows on multi-block grids. The industrial partners of the collaboration and NLR have demonstrated that the domain modeller, grid generator and flow solver can be applied to simulate Euler flows around complete aircraft, including propulsion system simulation. Extension to Navier-Stokes flows is in progress. Delft Hydraulics has shown that both the domain modeller and grid generator can also be applied successfully for hydrodynamic configurations. An overview is given about the main aspects of both domain modelling and grid generation
The cosmological constant and the relaxed universe
We study the role of the cosmological constant (CC) as a component of dark
energy (DE). It is argued that the cosmological term is in general unavoidable
and it should not be ignored even when dynamical DE sources are considered.
From the theoretical point of view quantum zero-point energy and phase
transitions suggest a CC of large magnitude in contrast to its tiny observed
value. Simply relieving this disaccord with a counterterm requires extreme
fine-tuning which is referred to as the old CC problem. To avoid it, we discuss
some recent approaches for neutralising a large CC dynamically without adding a
fine-tuned counterterm. This can be realised by an effective DE component which
relaxes the cosmic expansion by counteracting the effect of the large CC.
Alternatively, a CC filter is constructed by modifying gravity to make it
insensitive to vacuum energy.Comment: 6 pages, no figures, based on a talk presented at PASCOS 201
Cosmography beyond standard candles and rulers
We perform a cosmographic analysis using several cosmological observables
such as the luminosity distance moduli, the volume distance, the angular
diameter distance and the Hubble parameter. These quantities are determined
using different data sets: Supernovae type Ia and Gamma Ray Bursts, the
Baryonic Acoustic Oscillations, the cosmic microwave background power spectrum
and the Hubble parameter as measured from surveys of galaxies. This data set
allows to put constraints on the cosmographic expansion with unprecedented
precision. We also present forecasts for the coefficients of the kinematic
expansion using future but realistic data sets: constraints on the coefficients
of the expansions are likely to improve by a factor ten with the upcoming large
scale structure probes. Finally, we derive the set of the cosmographic
parameters for several cosmological models (including CDM) and compare
them with our best fit set. While distance measurements are unable to
discriminate among these models, we show that the inclusion of the Hubble data
set leads to strong constraints on the lowest order coefficients and in
particular it is incompatible with CDM at 3- confidence level.
We discuss the reliability of this determination and suggest further
observations which might be of crucial importance for the viability of
cosmographic tests in the next future.Comment: 15 pages, 2 figures, 2 tables, Accepted for publication in PR
Energy Extraction From Gravitational Collapse to Static Black Holes
The mass--energy formula of black holes implies that up to 50% of the energy
can be extracted from a static black hole. Such a result is reexamined using
the recently established analytic formulas for the collapse of a shell and
expression for the irreducible mass of a static black hole. It is shown that
the efficiency of energy extraction process during the formation of the black
hole is linked in an essential way to the gravitational binding energy, the
formation of the horizon and the reduction of the kinetic energy of implosion.
Here a maximum efficiency of 50% in the extraction of the mass energy is shown
to be generally attainable in the collapse of a spherically symmetric shell:
surprisingly this result holds as well in the two limiting cases of the
Schwarzschild and extreme Reissner-Nordstr\"{o}m space-times. Moreover, the
analytic expression recently found for the implosion of a spherical shell onto
an already formed black hole leads to a new exact analytic expression for the
energy extraction which results in an efficiency strictly less than 100% for
any physical implementable process. There appears to be no incompatibility
between General Relativity and Thermodynamics at this classical level.Comment: 7 pages, 2 figures, to appear on Int. Journ. Mod. Phys.
Iterated Differential Forms II: Riemannian Geometry Revisited
A natural extension of Riemannian geometry to a much wider context is
presented on the basis of the iterated differential form formalism developed in
math.DG/0605113 and an application to general relativity is given.Comment: 12 pages, extended version of the published note Dokl. Math. 73, n. 2
(2006) 18
Do Tax Incentives Increase 401(K) Retirement Saving? Evidence from the Adoption of Catch-Up Contributions
Retirement Research Consortium. The opinions and conclusions expressed are solely those of the authors and do not represent the opinions or policy of SSA, any agency of the federal government, or Boston College. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of the contents of this report. Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply endorsement, recommendation or favoring by the United States Government or any agency thereof. The authors are indebted to the indispensable data analysis of Qi Guan, and would also like to thank Robin Jensen for excellent research assistance. The authors are grateful to Kelly Trageser, Gary Benedetto, and Martha Stinson for helping us access the SIPP synthetic data and re-running our code on the actual data
Structural investigation of nucleophosmin interaction with the tumor suppressor Fbw7γ
Nucleophosmin (NPM1) is a multifunctional nucleolar protein implicated in ribogenesis, centrosome duplication, cell cycle control, regulation of DNA repair and apoptotic response to stress stimuli. The majority of these functions are played through the interactions with a variety of protein partners. NPM1 is frequently overexpressed in solid tumors of different histological origin. Furthermore NPM1 is the most frequently mutated protein in acute myeloid leukemia (AML) patients. Mutations map to the C-terminal domain and lead to the aberrant and stable localization of the protein in the cytoplasm of leukemic blasts. Among NPM1 protein partners, a pivotal role is played by the tumor suppressor Fbw7γ, an E3-ubiquitin ligase that degrades oncoproteins like c-MYC, cyclin E, Notch and c-jun. In AML with NPM1 mutations, Fbw7γ is degraded following its abnormal cytosolic delocalization by mutated NPM1. This mechanism also applies to other tumor suppressors and it has been suggested that it may play a key role in leukemogenesis. Here we analyse the interaction between NPM1 and Fbw7γ, by identifying the protein surfaces implicated in recognition and key aminoacids involved. Based on the results of computational methods, we propose a structural model for the interaction, which is substantiated by experimental findings on several site-directed mutants. We also extend the analysis to two other NPM1 partners (HIV Tat and CENP-W) and conclude that NPM1 uses the same molecular surface as a platform for recognizing different protein partners. We suggest that this region of NPM1 may be targeted for cancer treatment
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