53,507 research outputs found
Remote sensor imagery in urban research - Some potentialities and problem
Imaging techniques of urban data collection for development and plannin
Resolving the Structure of Cold Dark Matter Halos
We examine the effects of mass resolution and force softening on the density
profiles of cold dark matter halos that form within cosmological N-body
simulations. As we increase the mass and force resolution, we resolve
progenitor halos that collapse at higher redshifts and have very high
densities. At our highest resolution we have nearly 3 million particles within
the virial radius, several orders of magnitude more than previously used and we
can resolve more than one thousand surviving dark matter halos within this
single virialised system. The halo profiles become steeper in the central
regions and we may not have achieved convergence to a unique slope within the
inner 10% of the virialised region. Results from two very high resolution halo
simulations yield steep inner density profiles, . The
abundance and properties of arcs formed within this potential will be different
from calculations based on lower resolution simulations. The kinematics of
disks within such a steep potential may prove problematic for the CDM model
when compared with the observed properties of halos on galactic scales.Comment: Final version, to be published in the ApJLetter
Helium Leak Measurements Using CO2 as a Carrier
Detection delay defects are omitted by using helium mass spectrometer leak detector combined with several known procedures. Technique combines carbon dioxide purging, cryogenic separating helium accumulation, and use of carbon dioxide as carrier gas
An Invariant Theory of Spacelike Surfaces in the Four-dimensional Minkowski Space
We consider spacelike surfaces in the four-dimensional Minkowski space and
introduce geometrically an invariant linear map of Weingarten-type in the
tangent plane at any point of the surface under consideration. This allows us
to introduce principal lines and an invariant moving frame field. Writing
derivative formulas of Frenet-type for this frame field, we obtain eight
invariant functions. We prove a fundamental theorem of Bonnet-type, stating
that these eight invariants under some natural conditions determine the surface
up to a motion. We show that the basic geometric classes of spacelike surfaces
in the four-dimensional Minkowski space, determined by conditions on their
invariants, can be interpreted in terms of the properties of the two geometric
figures: the tangent indicatrix, and the normal curvature ellipse. We apply our
theory to a class of spacelike general rotational surfaces.Comment: 23 pages; to appear in Mediterr. J. Math., Vol. 9 (2012
Entanglement entropy of random quantum critical points in one dimension
For quantum critical spin chains without disorder, it is known that the
entanglement of a segment of N>>1 spins with the remainder is logarithmic in N
with a prefactor fixed by the central charge of the associated conformal field
theory. We show that for a class of strongly random quantum spin chains, the
same logarithmic scaling holds for mean entanglement at criticality and defines
a critical entropy equivalent to central charge in the pure case. This
effective central charge is obtained for Heisenberg, XX, and quantum Ising
chains using an analytic real-space renormalization group approach believed to
be asymptotically exact. For these random chains, the effective universal
central charge is characteristic of a universality class and is consistent with
a c-theorem.Comment: 4 pages, 3 figure
Density profiles and substructure of dark matter halos: converging results at ultra-high numerical resolution
Can N-body simulations reliably determine the structural properties of dark
matter halos? Focussing on a Virgo-sized galaxy cluster, we increase the
resolution of current ``high resolution simulations'' by almost an order of
magnitude to examine the convergence of the important physical quantities. We
have 4 million particles within the cluster and force resolution 0.5 kpc/h
(0.05% of the virial radius). The central density profile has a logarithmic
slope of -1.5, as found in lower resolution studies of the same halo,
indicating that the profile has converged to the ``physical'' limit down to
scales of a few kpc. Also the abundance of substructure is consistent with that
derived from lower resolution runs; on the scales explored, the mass and
circular velocity functions are close to power laws of exponents ~ -1.9 and -4.
Overmerging appears to be globally unimportant for suhalos with circular
velocities > 100 km/s. We can trace most of the cluster progenitors from z=3 to
the present; the central object (the dark matter analog of a cD galaxy)is
assembled between z=3 and 1 from the merging of a dozen halos with v_circ \sim
300 km/s. The mean circular velocity of the subhalos decreases by ~ 20% over 5
billion years, due to tidal mass loss. The velocity dispersions of halos and
dark matter globally agree within 10%, but the halos are spatially anti-biased,
and, in the very central region of the cluster, they show positive velocity
bias; however, this effect appears to depend on numerical resolution.Comment: 19 pages, 13 figures, ApJ, in press. Text significantly clarifie
Theory of superradiant scattering of laser light from Bose-Einstein condensates
In a recent MIT experiment, a new form of superradiant Rayleigh scattering
was observed in Bose-Einstein condensates. We present a detailed theory of this
phenomena in which the directional dependence of the scattering rate and
condensate depletion lead to mode competition which is ultimately responsible
for superradiance. The nonlinear response of the system is highly sensitive to
initial quantum fluctuations which cause large run to run variations in the
observed superradiant pulses.Comment: Updated version with new figures,a numerical simulation with
realistic experimental parameters is now included. Featured in September 1999
Physics Today, in Search and Discovery sectio
Quantifying the Effect of Non-Larmor Motion of Electrons on the Pressure Tensor
In space plasma, various effects of magnetic reconnection and turbulence
cause the electron motion to significantly deviate from their Larmor orbits.
Collectively these orbits affect the electron velocity distribution function
and lead to the appearance of the "non-gyrotropic" elements in the pressure
tensor. Quantification of this effect has important applications in space and
laboratory plasma, one of which is tracing the electron diffusion region (EDR)
of magnetic reconnection in space observations. Three different measures of
agyrotropy of pressure tensor have previously been proposed, namely,
, and . The multitude of contradictory measures has
caused confusion within the community. We revisit the problem by considering
the basic properties an agyrotropy measure should have. We show that
, and are all defined based on the sum of the
principle minors (i.e. the rotation invariant ) of the pressure tensor. We
discuss in detail the problems of -based measures and explain why they may
produce ambiguous and biased results. We introduce a new measure
constructed based on the determinant of the pressure tensor (i.e. the rotation
invariant ) which does not suffer from the problems of -based
measures. We compare with other measures in 2 and 3-dimension
particle-in-cell magnetic reconnection simulations, and show that can
effectively trace the EDR of reconnection in both Harris and force-free current
sheets. On the other hand, does not show prominent peaks in
the EDR and part of the separatrix in the force-free reconnection simulations,
demonstrating that does not measure all the non-gyrotropic
effects in this case, and is not suitable for studying magnetic reconnection in
more general situations other than Harris sheet reconnection.Comment: accepted by Phys. of Plasm
Explicit connection between conformal field theory and 2+1 Chern-Simons theory
We give explicit field theoretical representations for the observables of 2+1
dimensional Chern-Simons theory in terms of gauge invariant composites of 2D
WZW fields. To test our identification we compute some basic Wilson loop
correlators reobtaining known results.Comment: 13 pages, Latex file. To appear in Mod.Phys.Lett.
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