623 research outputs found
Topological phase transition in a network model with preferential attachment and node removal
Preferential attachment is a popular model of growing networks. We consider a
generalized model with random node removal, and a combination of preferential
and random attachment. Using a high-degree expansion of the master equation, we
identify a topological phase transition depending on the rate of node removal
and the relative strength of preferential vs. random attachment, where the
degree distribution goes from a power law to one with an exponential tail.Comment: The final publication is available at http://www.epj.or
Social Balance on Networks: The Dynamics of Friendship and Enmity
How do social networks evolve when both friendly and unfriendly relations
exist? Here we propose a simple dynamics for social networks in which the sense
of a relationship can change so as to eliminate imbalanced triads--relationship
triangles that contains 1 or 3 unfriendly links. In this dynamics, a friendly
link changes to unfriendly or vice versa in an imbalanced triad to make the
triad balanced. Such networks undergo a dynamic phase transition from a steady
state to "utopia"--all friendly links--as the amount of network friendliness is
changed. Basic features of the long-time dynamics and the phase transition are
discussed.Comment: 16 pages, 11 figures, paper based on an invited talk at Dyonet06,
Dynamics on Complex Networks and Applications, Dresden, Germany, Feburary
200
Effects of finite arm-length of LISA on analysis of gravitational waves from MBH binaries
Response of an interferometer becomes complicated for gravitational wave
shorter than the arm-length of the detector, as nature of wave appears
strongly. We have studied how parameter estimation for merging massive black
hole binaries are affected by this complicated effect in the case of LISA. It
is shown that three dimensional positions of some binaries might be determined
much better than the past estimations that use the long wave approximation. For
equal mass binaries this improvement is most prominent at \sim 10^5\sol.Comment: 10 pages, 3 figures, to appear in Phys.Rev.
Effect of the accelerating growth of communications networks on their structure
Motivated by data on the evolution of the Internet and World Wide Web we
consider scenarios of self-organization of the nonlinearly growing networks
into free-scale structures. We find that the accelerating growth of the
networks establishes their structure. For the growing networks with
preferential linking and increasing density of links, two scenarios are
possible. In one of them, the value of the exponent of the
connectivity distribution is between 3/2 and 2. In the other, and
the distribution is necessarily non-stationary.Comment: 4 pages revtex, 3 figure
Soft-bed experiments beneath Engabreen, Norway: regelation infiltration, basal slip and bed deformation
To avoid some of the limitations of studying soft-bed processes through boreholes, a prism of simulated till (1.8 m × 1.6 m × 0.45 m) with extensive instrumentation was constructed in a trough blasted in the rock bed of Engabreen, a temperate glacier in Norway. Tunnels there provide access to the bed beneath 213 m of ice. Pore-water pressure was regulated in the prism by pumping water to it. During experiments lasting 7–12 days, the glacier regelated downward into the prism to depths of 50– 80 mm, accreting ice-infiltrated till at rates predicted by theory. During periods of sustained high pore water pressure (70–100% of overburden), ice commonly slipped over the prism, due to a water layer at the prism surface. Deformation of the prism was activated when this layer thinned to a sub-millimeter thickness. Shear strain in the till was pervasive and decreased with depth. A model of slip by ploughing of ice-infiltrated till across the prism surface accounts for the slip that occurred when effective pressure was sufficiently low or high. Slip at low effective pressures resulted from water-layer thickening that increased non-linearly with decreasing effective pressure. If sufficiently widespread, such slip over soft glacier beds, which involves no viscous deformation resistance, may instigate abrupt increases in glacier velocity
Do the Unidentified EGRET Sources Trace Annihilating Dark Matter in the Local Group?
In a cold dark matter (CDM) framework of structure formation, the dark matter
haloes around galaxies assemble through successive mergers with smaller haloes.
This merging process is not completely efficient, and hundreds of surviving
halo cores, or {\it subhaloes}, are expected to remain in orbit within the halo
of a galaxy like the Milky Way. While the dozen visible satellites of the Milky
Way may trace some of these subhaloes, the majority are currently undetected. A
large number of high-velocity clouds (HVCs) of neutral hydrogen {\it are}
observed around the Milky Way, and it is plausible that some of the HVCs may
trace subhaloes undetected in the optical. Confirming the existence of
concentrations of dark matter associated with even a few of the HVCs would
represent a dramatic step forward in our attempts to understand the nature of
dark matter. Supersymmetric (SUSY) extensions of the Standard Model of particle
physics currently suggest neutralinos as a natural well-motivated candidate for
the non-baryonic dark matter of the universe. If this is indeed the case, then
it may be possible to detect dark matter indirectly as it annihilates into
neutrinos, photons or positrons. In particular, the centres of subhaloes might
show up as point sources in gamma-ray observations. In this work we consider
the possibility that some of the unidentified EGRET -ray sources trace
annihilating neutralino dark matter in the dark substructure of the Local
Group. We compare the observed positions and fluxes of both the unidentified
EGRET sources and the HVCs with the positions and fluxes predicted by a model
of halo substructure, to determine to what extent any of these three
populations could be associated.Comment: 12 Pages, 4 figures, to appear in a special issue of ApSS. Presented
at "The Multiwavelength Approach to Unidentified Gamma-Ray Sources" (Hong
Kong, June 1 - 4, 2004; Conference organizers: K.S. Cheng and G.E. Romero
Instabilities in the Flux Line Lattice of Anisotropic Superconductors
The stability of the flux line lattice has been investigated within
anisotropic London theory. This is the first full-scale investigation of
instabilities in the `chain' state. It has been found that the lattice is
stable at large fields, but that instabilities occur as the field is reduced.
The field at which these instabilities first arise, ,
depends on the anisotropy and the angle at which the
lattice is tilted away from the -axis. These instabilities initially occur
at wavevector , and the component of along the
average direction of the flux lines, , is always finite. As the
instability occurs at finite the dependence of the cutoff on is
important, and we have used a cutoff suggested by Sudb\ospace and Brandt. The
instabilities only occur for values of the anisotropy appropriate to
a material like BSCCO, and not for anisotropies more appropriate to YBCO. The
lower critical field is calculated as a function of the angle
at which the applied field is tilted away from the crystal axis. The
presence of kinks in is seen to be related to instabilities in
the equilibrium flux line structure.Comment: Extensively revised paper, with modified analysis of elastic
instabilities. Calculation of the lower critical field is included, and the
presence of kinks in is seen to be related to the elastic
instabilities. 29 pages including 16 figures, LaTeX with epsf styl
Description and evaluation of tropospheric chemistry and aerosols in the Community Earth System Model (CESM1.2)
The Community Atmosphere Model (CAM), version 5, is now coupled to extensive tropospheric and stratospheric chemistry, called CAM5-chem, and is available in addition to CAM4-chem in the Community Earth System Model (CESM) version 1.2. The main focus of this paper is to compare the performance of configurations with internally derived "free running" (FR) meteorology and "specified dynamics" (SD) against observations from surface, aircraft, and satellite, as well as understand the origin of the identified differences. We focus on the representation of aerosols and chemistry. All model configurations reproduce tropospheric ozone for most regions based on in situ and satellite observations. However, shortcomings exist in the representation of ozone precursors and aerosols. Tropospheric ozone in all model configurations agrees for the most part with ozonesondes and satellite observations in the tropics and the Northern Hemisphere within the variability of the observations. Southern hemispheric tropospheric ozone is consistently underestimated by up to 25%. Differences in convection and stratosphere to troposphere exchange processes are mostly responsible for differences in ozone in the different model configurations. Carbon monoxide (CO) and other volatile organic compounds are largely underestimated in Northern Hemisphere mid-latitudes based on satellite and aircraft observations. Nitrogen oxides (NOx) are biased low in the free tropical troposphere, whereas peroxyacetyl nitrate (PAN) is overestimated in particular in high northern latitudes. The present-day methane lifetime estimates are compared among the different model configurations. These range between 7.8 years in the SD configuration of CAM5-chem and 8.8 years in the FR configuration of CAM4-chem and are therefore underestimated compared to observational estimations. We find that differences in tropospheric aerosol surface area between CAM4 and CAM5 play an important role in controlling the burden of the tropical tropospheric hydroxyl radical (OH), which causes differences in tropical methane lifetime of about half a year between CAM4-chem and CAM5-chem. In addition, different distributions of NOx from lightning explain about half of the difference between SD and FR model versions in both CAM4-chem and CAM5-chem. Remaining differences in the tropical OH burden are due to enhanced tropical ozone burden in SD configurations compared to the FR versions, which are not only caused by differences in chemical production or loss but also by transport and mixing. For future studies, we recommend the use of CAM5-chem configurations, due to improved aerosol description and inclusion of aerosol–cloud interactions. However, smaller tropospheric surface area density in the current version of CAM5-chem compared to CAM4-chem results in larger oxidizing capacity in the troposphere and therefore a shorter methane lifetime
Substructures in lens galaxies: PG1115+080 and B1555+375, two fold configurations
We study the anomalous flux ratio which is observed in some four-image lens
systems, where the source lies close to a fold caustic. In this case two of the
images are close to the critical curve and their flux ratio should be equal to
unity, instead in several cases the observed value differs significantly. The
most plausible solution is to invoke the presence of substructures, as for
instance predicted by the Cold Dark Matter scenario, located near the two
images. In particular, we analyze the two fold lens systems PG1115+080 and
B1555+375, for which there are not yet satisfactory models which explain the
observed anomalous flux ratios. We add to a smooth lens model, which reproduces
well the positions of the images but not the anomalous fluxes, one or two
substructures described as singular isothermal spheres. For PG1115+080 we
consider a smooth model with the influence of the group of galaxies described
by a SIS and a substructure with mass as well as a
smooth model with an external shear and one substructure with mass . For B1555+375 either a strong external shear or two substructures
with mass reproduce the data quite well.Comment: 26 pages, updated bibliography, Accepted for publication in
Astrophysics & Space Scienc
Driving the resonant quantum kicked rotor via extended initial conditions
We study the resonances of the quantum kicked rotor subjected to an extended
initial distribution. For the primary resonances we obtain the dispersion
relation for the map of this system. We find an analytical dependence of the
statistical moments on the shape of the initial distribution. For the secondary
resonances we obtain numerically a similar dependence. This allows us to devise
an extended initial condition which produces an average angular momentum
pointing in a preset direction which increases with time with a preset ratio.Comment: 6 pages, 5 figures, send to EPJ
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