19,845 research outputs found
Average resistance of toroidal graphs
The average effective resistance of a graph is a relevant performance index
in many applications, including distributed estimation and control of network
systems. In this paper, we study how the average resistance depends on the
graph topology and specifically on the dimension of the graph. We concentrate
on -dimensional toroidal grids and we exploit the connection between
resistance and Laplacian eigenvalues. Our analysis provides tight estimates of
the average resistance, which are key to study its asymptotic behavior when the
number of nodes grows to infinity. In dimension two, the average resistance
diverges: in this case, we are able to capture its rate of growth when the
sides of the grid grow at different rates. In higher dimensions, the average
resistance is bounded uniformly in the number of nodes: in this case, we
conjecture that its value is of order for large . We prove this fact
for hypercubes and when the side lengths go to infinity.Comment: 24 pages, 6 figures, to appear in SIAM Journal on Control and
Optimization (SICON
Making sense of the local Galactic escape speed estimates in direct dark matter searches
Direct detection (DD) of dark matter (DM) candidates in the 10 GeV
mass range is very sensitive to the tail of their velocity distribution. The
important quantity is the maximum WIMP speed in the observer's rest frame, i.e.
in average the sum of the local Galactic escape speed and of the
circular velocity of the Sun . While the latter has been receiving
continuous attention, the former is more difficult to constrain. The RAVE
Collaboration has just released a new estimate of (Piffl {\em et
al.}, 2014 --- P14) that supersedes the previous one (Smith {\em et al.},
2007), which is of interest in the perspective of reducing the astrophysical
uncertainties in DD. Nevertheless, these new estimates cannot be used blindly
as they rely on assumptions in the dark halo modeling which induce tight
correlations between the escape speed and other local astrophysical parameters.
We make a self-consistent study of the implications of the RAVE results on DD
assuming isotropic DM velocity distributions, both Maxwellian and ergodic.
Taking as references the experimental sensitivities currently achieved by LUX,
CRESST-II, and SuperCDMS, we show that: (i) the exclusion curves associated
with the best-fit points of P14 may be more constraining by up to %
with respect to standard limits, because the underlying astrophysical
correlations induce a larger local DM density; (ii) the corresponding relative
uncertainties inferred in the low WIMP mass region may be moderate, down to
10-15% below 10 GeV. We finally discuss the level of consistency of these
results with other independent astrophysical constraints. This analysis is
complementary to others based on rotation curves.Comment: 18 pages, 7 figures. V2: improved version that matches to the
published on
Weighing the local dark matter with RAVE red clump stars
We determine the Galactic potential in the solar neigbourhood from RAVE
observations. We select red clump stars for which accurate distances, radial
velocities, and metallicities have been measured. Combined with data from the
2MASS and UCAC catalogues, we build a sample of 4600 red clump stars within a
cylinder of 500 pc radius oriented in the direction of the South Galactic Pole,
in the range of 200 pc to 2000 pc distances. We deduce the vertical force and
the total mass density distribution up to 2 kpc away from the Galactic plane by
fitting a distribution function depending explicitly on three isolating
integrals of the motion in a separable potential locally representing the
Galactic one with four free parameters. Because of the deep extension of our
sample, we can determine nearly independently the dark matter mass density and
the baryonic disc surface mass density. We find (i) at 1kpc Kz/(2piG) = 68.5 pm
1.0 Msun/pc2, and (ii) at 2 kpc Kz/(2piG) = 96.9 pm 2.2 Msun/pc2. Assuming the
solar Galactic radius at R0 = 8.5 kpc, we deduce the local dark matter density
rhoDM (z=0) = 0.0143 pm 0.0011Msun pc3 = 0.542 pm 0.042 Gev/cm3 and the
baryonic surface mass density Sigma = 44.4 pm 4.1 Msun/pc2 . Our results are in
agreement with previously published Kz determinations up to 1 kpc, while the
extension to 2 kpc shows some evidence for an unexpectedly large amount of dark
matter. A flattening of the dark halo of order 0.8 can produce such a high
local density in combination with a circular velocity of 240 km/s . Another
explanation, allowing for a lower circular velocity, could be the presence of a
secondary dark component, a very thick disc resulting either from the deposit
of dark matter from the accretion of multiple small dwarf galaxies, or from the
presence of an effective phantom thick disc in the context of effective
galactic-scale modifications of gravity.Comment: 14 pages, 13 figures, accepted to Astronomy and Astrophysic
These are the echoes: Sound Proof 2008-2012
Article in peer-reviewed journal Culture/Kultura for their thematic issue, Art Media and Cultural Memory.
Based on the conference talk given at CCCS Annual Conference on Cultural Memory 4-7 September 2013
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