565,977 research outputs found
Bianchi type I universes with dilaton and magnetic fields
We consider the dynamics of a Bianchi type I spacetime in the presence of
dilaton and magnetic fields. The general solution of the Einstein-Maxwell
dilaton field equations can be obtained in an exact parametric form. Depending
on the numerical values of the parameters of the model there are three distinct
classes of solutions. The time evolution of the mean anisotropy, shear and
deceleration parameter is considered in detail and it is shown that a
magnetic-dilaton anisotropic Bianchi type I geometry does not isotropize, the
initial anisotropy being present in the universe for all times.Comment: 10 pages, 4 figure
Experimental and Theoretical Research on Plasma Sheaths and Boundary Layers Around Stagnation Point Electrodes Semiannual Progress Report No. 3, 1 Jul. - 31 Dec. 1965
Plasma sheaths and boundary layers around stagnation point electrode
Characterization of the 4-canonical birationality of algebraic threefolds
In this article we present a 3-dimensional analogue of a well-known theorem
of E. Bombieri (in 1973) which characterizes the bi-canonical birationality of
surfaces of general type. Let be a projective minimal 3-fold of general
type with -factorial terminal singularities and the geometric genus
. We show that the 4-canonical map is {\it not}
birational onto its image if and only if is birationally fibred by a family
of irreducible curves of geometric genus 2 with
where is a general irreducible member in .Comment: 25 pages, to appear in Mathematische Zeitschrif
Nonsingular density profiles of dark matter halos and Strong gravitational lensing
We use the statistics of strong gravitational lenses to investigate whether
mass profiles with a flat density core are supported. The probability for
lensing by halos modeled by a nonsingular truncated isothermal sphere (NTIS)
with image separations greater than a certain value (ranging from zero to ten
arcseconds) is calculated. NTIS is an analytical model for the postcollapse
equilibrium structure of virialized objects derived by Shapiro, Iliev & Raga.
This profile has a soft core and matches quite well with the mass profiles of
dark matter-dominated dwarf galaxies deduced from their observed rotation
curves. It also agrees well with the NFW (Navarro-Frenk-White) profile at all
radii outside of a few NTIS core radii. Unfortunately, comparing the results
with those for singular lensing halos (NFW and SIS+NFW) and strong lensing
observations, the probabilities for lensing by NTIS halos are far too low. As
this result is valid for any other nonsingular density profiles (with a large
core radius), we conclude that nonsingular density profiles (with a large core
radius) for CDM halos are ruled out by statistics of strong gravitational
lenses.Comment: 17 pages, 4 figures, ApJ accepted. Final version matches the proofs.
A curve in figure 2 is corrected, conclusions unchange
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