26,293 research outputs found
Stochastic and Discrete Time Models of Long-Range Turbulent Transport in the Scrape-Off Layer
Two dimensional stochastic time model of scrape-off layer (SOL) turbulent
transport is studied. Instability arisen in the system with respect to the
stochastic perturbations of both either density or vorticity reveals itself in
the strong outward bursts of particle density propagating ballistically across
the SOL. The stability and possible stabilization of the cross- field turbulent
system depend very much upon the reciprocal correlation time between density
and vorticity fluctuations. Pdf of the particle flux for the large magnitudes
of flux events can be modelled with a simple discrete time toy model of random
walks concluding at a boundary. The spectra of wandering times feature the pdf
of particle flux in the model and qualitatively reproduce the experimental
statistics of transport events.Comment: 21 pages,11 figure
Homogeneous and Scalable Gene Expression Regulatory Networks with Random Layouts of Switching Parameters
We consider a model of large regulatory gene expression networks where the
thresholds activating the sigmoidal interactions between genes and the signs of
these interactions are shuffled randomly. Such an approach allows for a
qualitative understanding of network dynamics in a lack of empirical data
concerning the large genomes of living organisms. Local dynamics of network
nodes exhibits the multistationarity and oscillations and depends crucially
upon the global topology of a "maximal" graph (comprising of all possible
interactions between genes in the network). The long time behavior observed in
the network defined on the homogeneous "maximal" graphs is featured by the
fraction of positive interactions () allowed between genes.
There exists a critical value such that if , the
oscillations persist in the system, otherwise, when it tends to
a fixed point (which position in the phase space is determined by the initial
conditions and the certain layout of switching parameters). In networks defined
on the inhomogeneous directed graphs depleted in cycles, no oscillations arise
in the system even if the negative interactions in between genes present
therein in abundance (). For such networks, the bidirectional edges
(if occur) influence on the dynamics essentially. In particular, if a number of
edges in the "maximal" graph is bidirectional, oscillations can arise and
persist in the system at any low rate of negative interactions between genes
(). Local dynamics observed in the inhomogeneous scalable regulatory
networks is less sensitive to the choice of initial conditions. The scale free
networks demonstrate their high error tolerance.Comment: LaTeX, 30 pages, 20 picture
Cosmic voids in modified gravity scenarios
Modified gravity (MG) theories aim to reproduce the observed acceleration of
the Universe by reducing the dark sector while simultaneously recovering
General Relativity (GR) within dense environments. Void studies appear to be a
suitable scenario to search for imprints of alternative gravity models on
cosmological scales. Voids cover an interesting range of density scales where
screening mechanisms fade out, which reaches from a density contrast close to their centers to close to their
boundaries. We present an analysis of the level of distinction between GR and
two modified gravity theories, the Hu-Sawicki and the symmetron theory.
This study relies on the abundance, linear bias, and density profile of voids
detected in n-body cosmological simulations. We define voids as connected
regions made up of the union of spheres with a {\it \textup{mean}} density
given by , but disconnected from any
other voids. We find that the height of void walls is considerably affected by
the gravitational theory, such that it increases for stronger gravity
modifications. Finally, we show that at the level of dark matter n-body
simulations, our constraints allow us to distinguish between GR and MG models
with and . Differences of best-fit values for
MG parameters that are derived independently from multiple void probes may
indicate an incorrect MG model. This serves as an important consistency check.Comment: 15 pages, 12 figure
Disc formation in turbulent cloud cores: Circumventing the magnetic braking catastrophe
We present collapse simulations of strongly magnetised, 100 M_sun, turbulent
cloud cores. Around the protostars formed during the collapse Keplerian discs
with typical sizes of up to 100 AU build up in contrast to previous simulations
neglecting turbulence. Analysing the condensations in which the discs form, we
show that the magnetic flux loss is not sufficient to explain the build-up of
Keplerian discs. The average magnetic field is strongly inclined to the disc
which might reduce the magnetic braking efficiency. However, the main reason
for the reduced magnetic braking efficiency is the highly disordered magnetic
field in the surroundings of the discs. Furthermore, due to the lack of a
coherently rotating structure in the turbulent environment of the disc no
toroidal magnetic field necessary for angular momentum extraction can build up.
Simultaneously the angular momentum inflow remains high due to local shear
flows created by the turbulent motions. We suggest that the "magnetic braking
catastrophe" is an artefact of the idealised non-turbulent initial conditions
and that turbulence provides a natural mechanism to circumvent this problem.Comment: 4 pages, 2 figures. To appear in the proceedings of 'The Labyrinth of
Star Formation' (18-22 June 2012, Chania, Greece), published by Springe
Complete factorization of equations of motion for generalized scalar field theories
We demonstrate that the complete factorization of equations of motion into
first-order differential equations can be obtained for real and complex scalar
field theories with non-canonical dynamics.Comment: 5 pages; version published in EP
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