655 research outputs found
Dynamical Properties of Euclidean Solutions in a Multidimensional Cosmological Model
In the framework of the Hartle-Hawking no-boundary proposal, we investigated
quantum creation of the multidimensional universe with a cosmological constant
() but without matter fields. We have found that the classical
solutions of the Euclidean Einstein equations in this model have
``quasi-attractors'', i.e., most trajectories on the a-b plane, where a and b
are the scale factors of external and internal spaces, go around a point. It is
presumed that the wave function of the universe has a hump near this
quasi-attractor point. In the case that both the curvatures of external and
internal spaces are positive, and , there exist Lorentzian solutions
which start near the quasi-attractor, the internal space remains microscopic,
and the external space evolves into our macroscopic universe.Comment: 13 pages and 5 figure
Estimation Prospects of the Source Number Density of Ultra-high-energy Cosmic Rays
We discuss the possibility of accurately estimating the source number density
of ultra-high-energy cosmic rays (UHECRs) using small-scale anisotropy in their
arrival distribution. The arrival distribution has information on their source
and source distribution. We calculate the propagation of UHE protons in a
structured extragalactic magnetic field (EGMF) and simulate their arrival
distribution at the Earth using our previously developed method. The source
number density that can best reproduce observational results by Akeno Giant Air
Shower Array is estimated at about in a simple source
model. Despite having large uncertainties of about one order of magnitude, due
to small number of observed events in current status, we find that more
detection of UHECRs in the Auger era can sufficiently decrease this so that the
source number density can be more robustly estimated. 200 event observation
above in a hemisphere can discriminate between
and . Number of events to discriminate
between and is dependent on EGMF strength.
We also discuss the same in another source model in this paper.Comment: 19 pages, 8 figures, accepted for publication in Astroparticle
Physic
A cell membrane model that reproduces cortical flow-driven cell migration and collective movement
Many fundamental biological processes are dependent on cellular migration.
Although the mechanical mechanisms of single-cell migration are relatively well
understood, those underlying migration of multiple cells adhered to each other
in a cluster, referred to as cluster migration, are poorly understood. A key
reason for this knowledge gap is that many forces-including contraction forces
from actomyosin networks, hydrostatic pressure from the cytosol, frictional
forces from the substrate, and forces from adjacent cells-contribute to cell
cluster movement, making it challenging to model, and ultimately elucidate, the
final result of these forces. This paper describes a two-dimensional cell
membrane model that represents cells on a substrate with polygons and expresses
various mechanical forces on the cell surface, keeping these forces balanced at
all times by neglecting cell inertia. The model is discrete but equivalent to a
continuous model if appropriate replacement rules for cell surface segments are
chosen. When cells are given a polarity, expressed by a direction-dependent
surface tension reflecting the location dependence of contraction and adhesion
on a cell boundary, the cell surface begins to flow from front to rear as a
result of force balance. This flow produces unidirectional cell movement, not
only for a single cell but also for multiple cells in a cluster, with migration
speeds that coincide with analytical results from a continuous model. Further,
if the direction of cell polarity is tilted with respect to the cluster center,
surface flow induces cell cluster rotation. The reason why this model moves
while keeping force balance on cell surface (i.e., under no net forces from
outside) is because of the implicit inflow and outflow of cell surface
components through the inside of the cell.Comment: 5 figure
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