1,235 research outputs found
Reionization, SLOAN, and WMAP: is the Picture Consistent?
I show that advanced simulations of cosmological reionization are able to fit
the observed data on the mean transmitted flux in the hydrogen Lyman-alpha line
at z~6. At the same time, posteriori models can be constructed that also
produce a large value (20%) for the Thompson scattering optical depth,
consistent with the WMAP measurements. Thus, it appears that a consistent
picture emerges in which early reionization (as suggested by WMAP) is complete
by z~6 in accord with the SLOAN data.Comment: accepted for publication in Ap
Probing the universe with the Lyman-alpha forest: II. The column density distribution
I apply the well controlled Hydro-PM approximation of Gnedin & Hui to model
the column density distribution of the Lyman-alpha forest for 25 different flat
cosmological scenarios, including variants of the standard CDM, tilted CDM, CDM
with a cosmological constant, and CHDM models. I show that within the accuracy
of the HPM approximation the slope of the column density distribution reflects
the degree of nonlinearity of the cosmic gas distribution and is a function of
the rms linear density fluctuation at the characteristic filtering scale only.
The amplitude of the column density distribution, expressed as the value for
the ionizing intensity, is derived as a function of the cosmological parameters
(to about 40% accuracy). The observational data are currently consistent with
the value for the ionizing intensity being constant in the redshift interval
z~2-4.Comment: Revised version; submitted to MNRA
Sub-diffusion in External Potential: Anomalous hiding behind Normal
We propose a model of sub-diffusion in which an external force is acting on a
particle at all times not only at the moment of jump. The implication of this
assumption is the dependence of the random trapping time on the force with the
dramatic change of particles behavior compared to the standard continuous time
random walk model. Constant force leads to the transition from non-ergodic
sub-diffusion to seemingly ergodic diffusive behavior. However, we show it
remains anomalous in a sense that the diffusion coefficient depends on the
force and the anomalous exponent. For the quadratic potential we find that the
anomalous exponent defines not only the speed of convergence but also the
stationary distribution which is different from standard Boltzmann equilibrium.Comment: 6 pages, 3 figure
Emergence of L\'{e}vy Walks in Systems of Interacting Individuals
Recent experiments (G. Ariel, et al., Nature Comm. 6, 8396 (2015)) revealed
an intriguing behavior of swarming bacteria: they fundamentally change their
collective motion from simple diffusion into a superdiffusive L\'{e}vy walk
dynamics. We introduce a nonlinear non-Markovian persistent random walk model
that explains the emergence of superdiffusive L\'{e}vy walks. We show that the
alignment interaction between individuals can lead to the superdiffusive growth
of the mean squared displacement and the power law distribution of run length
with infinite variance. The main result is that the superdiffusive behavior
emerges as a nonlinear collective phenomenon, rather than due to the standard
assumption of the power law distribution of run distances from the inception.
At the same time, we find that the repulsion/collision effects lead to the
density dependent exponential tempering of power law distributions. This
qualitatively explains experimentally observed transition from superdiffusion
to the diffusion of mussels as their density increases (M. de Jager et al.,
Proc. R. Soc. B 281, 20132605 (2014))
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