5,435 research outputs found
Varying speed of light cosmology from a stringy short distance cutoff
It is shown that varying speed of light cosmology follows from a
string-inspired minimal length uncertainty relation. Due to the reduction of
the available phase space volume per quantum mode at short wavelengths, the
equation of state of ultrarelativistic particles stiffens at very high
densities. This causes a stronger than usual deceleration of the scale factor
which competes with a higher than usual propagation speed of the particles.
Various measures for the effective propagation speed are analyzed: the group
and phase velocity in the high energy tail, the thermal average of the group
and phase velocity, and the speed of sound. Of these three groups, only the
first provides a possible solution to the cosmological horizon problem.Comment: 5 pages, 2 figure
Thermonuclear supernova simulations with stochastic ignition
We apply an ad hoc model for dynamical ignition in three-dimensional
numerical simulations of thermonuclear supernovae assuming pure deflagrations.
The model makes use of the statistical description of temperature fluctuations
in the pre-supernova core proposed by Wunsch & Woosley (2004). Randomness in
time is implemented by means of a Poisson process. We are able to vary the
explosion energy and nucleosynthesis depending on the free parameter of the
model which controls the rapidity of the ignition process. However, beyond a
certain threshold, the strength of the explosion saturates and the outcome
appears to be robust with respect to number of ignitions. In the most energetic
explosions, we find about 0.75 solar masses of iron group elements. Other than
in simulations with simultaneous multi-spot ignition, the amount of unburned
carbon and oxygen at radial velocities of a few 1000 km/s tends to be reduced
for an ever increasing number of ignition events and, accordingly, more
pronounced layering results.Comment: 7 pages, 6 figures, accepted for publication in Astron. Astrophys.;
PDF version with full resolution figures available from
http://www.astro.uni-wuerzburg.de/~schmidt/Paper/StochIgnt_AA.pd
Characterization of All-Chromium Tunnel Junctions and Single Electron Tunneling Devices Fabricated by Direct-Writing Multilayer Technique
We report about the fabrication and analysis of the properties of Cr/CrO_x/Cr
tunnel junctions and SET transistors, prepared by different variants of
direct-writing multilayer technique. In all cases, the CrO_x tunnel barriers
were formed in air under ambient conditions. From the experiments on single
junctions, values for the effective barrier height and thickness were derived.
For the Cr/CrO_x/Cr SET transistors we achieved minimal junction areas of 17 x
60 nm^2 using a scanning transmission electron microscope for the e-beam
exposure on Si_3N_4 membrane substrate. We discuss the electrical performance
of the transistor samples as well as their noise behavior.Comment: 19 pages, 9 figure
Type Ia Supernova Explosion Models: Homogeneity versus Diversity
Type Ia supernovae (SN Ia) are generally believed to be the result of the
thermonuclear disruption of Chandrasekhar-mass carbon-oxygen white dwarfs,
mainly because such thermonuclear explosions can account for the right amount
of Ni-56, which is needed to explain the light curves and the late-time
spectra, and the abundances of intermediate-mass nuclei which dominate the
spectra near maximum light. Because of their enormous brightness and apparent
homogeneity SN Ia have become an important tool to measure cosmological
parameters. In this article the present understanding of the physics of
thermonuclear explosions is reviewed. In particular, we focus our attention on
subsonic (``deflagration'') fronts, i.e. we investigate fronts propagating by
heat diffusion and convection rather than by compression. Models based upon
this mode of nuclear burning have been applied very successfully to the SN Ia
problem, and are able to reproduce many of their observed features remarkably
well. However, the models also indicate that SN Ia may differ considerably from
each other, which is of importance if they are to be used as standard candles.Comment: 11 pages, 4 figures. To appear in Proc. 10th Ann. Astrophys. Conf.
"Cosmic Explosions", Univ. of Maryland 1999, eds. S.S. Holt and W.W. Zhan
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