848 research outputs found
Covariant Linear Perturbations in a Concordance Model
We present the complete solution of the first order metric and density
perturbation equations in a spatially flat (K=0), Friedmann-Robertson-Walker
(FRW) universe filled with pressureless ideal fluid, in the presence of
cosmological constant. We use covariant linear perturbation formalism and the
comoving gauge condition to obtain the field and conservation equations. The
solution contains all modes of the perturbations, i.e. scalar, vector and
tensor modes, and we show that our results are in agreement with the Sachs &
Wolfe metric perturbation formalism.Comment: 8 page
THEORETISCHE UNTERSUCHUNGEN ÜBER KONSTANTE HORIZONTALE UND VERTIKALE BELEUCHTUNGSSTÄRKEN BEI DER BELEUCHTUNG VON EINBAHNSTRASSEN
Near-Miss Symmetric Polyhedral Cages
Following the experimental discovery of several nearly symmetric protein cages, we define the concept of homogeneous symmetric congruent equivalent near-miss polyhedral cages made out of P-gons. We use group theory to parameterize the possible configurations and we minimize the irregularity of the P-gons numerically to construct all such polyhedral cages for =6 to =20 with deformation of up to 10%
Negative radiation pressure exerted on kinks
The interaction of a kink and a monochromatic plane wave in one dimensional
scalar field theories is studied. It is shown that in a large class of models
the radiation pressure exerted on the kink is negative, i.e. the kink is {\sl
pulled} towards the source of the radiation. This effect has been observed by
numerical simulations in the model, and it is explained by a
perturbative calculation assuming that the amplitude of the incoming wave is
small. Quite importantly the effect is shown to be robust against small
perturbations of the model. In the sine-Gordon (sG) model the time
averaged radiation pressure acting on the kink turns out to be zero. The
results of the perturbative computations in the sG model are shown to be in
full agreement with an analytical solution corresponding to the superposition
of a sG kink with a cnoidal wave. It is also demonstrated that the acceleration
of the kink satisfies Newton's law.Comment: 23 pages, 8 figures, LaTeX/RevTe
Impact of non-Poisson activity patterns on spreading processes
Halting a computer or biological virus outbreak requires a detailed
understanding of the timing of the interactions between susceptible and
infected individuals. While current spreading models assume that users interact
uniformly in time, following a Poisson process, a series of recent measurements
indicate that the inter-contact time distribution is heavy tailed,
corresponding to a temporally inhomogeneous bursty contact process. Here we
show that the non-Poisson nature of the contact dynamics results in prevalence
decay times significantly larger than predicted by the standard Poisson process
based models. Our predictions are in agreement with the detailed time resolved
prevalence data of computer viruses, which, according to virus bulletins, show
a decay time close to a year, in contrast with the one day decay predicted by
the standard Poisson process based models.Comment: 4 pages, 3 figure
Instabilities of Twisted Strings
A linear stability analysis of twisted flux-tubes (strings) in an SU(2)
semilocal theory -- an Abelian-Higgs model with two charged scalar fields with
a global SU(2) symmetry -- is carried out. Here the twist refers to a relative
phase between the two complex scalars (with linear dependence on, say, the
coordinate), and importantly it leads to a global current flowing along the the
string. Such twisted strings bifurcate with the Abrikosov-Nielsen-Olesen (ANO)
solution embedded in the semilocal theory. Our numerical investigations of the
small fluctuation spectrum confirm previous results that twisted strings
exhibit instabilities whose amplitudes grow exponentially in time. More
precisely twisted strings with a single magnetic flux quantum admit a
continuous family of unstable eigenmodes with harmonic dependence, indexed
by a wavenumber . Carrying out a perturbative
semi-analytic analysis of the bifurcation, it is found that the purely
numerical results are very well reproduced. This way one obtains not only a
good qualitative description of the twisted solutions themselves as well as of
their instabilities, but also a quantitative description of the numerical
results. Our semi-analytic results indicate that in close analogy to the known
instability of the embedded ANO vortex a twisted string is also likely to
expand in size caused by the spreading out of its magnetic flux.Comment: 27 pages, 18 figures. Typos corrected, references adde
A chloride channel in rat and human axons
Current recordings from single chloride channels were obtained from excised and cell-attached patches of rat and human axons. In rat axons the channels showed an outwardly rectifying current-voltage relationship with a slope conductance of 33 pS at negative membrane potentials and 65 pS at positive potentials (symmetrical 150 mM CsCl). They were measurably for cations (PNa/PCs/PCl=0.1/0.2/1). Channel currents were independent of cytoplasmatic calcium concentration. Inactivation was not observed and gating was weakly voltage dependent. Cl− channels in human axons showed similar gating behavior but had a lower conductance
Simple solutions of fireball hydrodynamics for self-similar elliptic flows
Simple, self-similar, elliptic solutions of non-relativistic fireball
hydrodynamics are presented, generalizing earlier results for spherically
symmetric fireballs with Hubble flows and homogeneous temperature profiles. The
transition from one dimensional to three dimensional expansions is investigated
in an efficient manner.Comment: 12 pages, 4 figures in 8 .eps files, references to recent data added,
accepted in Physics Letters
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