291,240 research outputs found
FASTLens (FAst STatistics for weak Lensing) : Fast method for Weak Lensing Statistics and map making
With increasingly large data sets, weak lensing measurements are able to
measure cosmological parameters with ever greater precision. However this
increased accuracy also places greater demands on the statistical tools used to
extract the available information. To date, the majority of lensing analyses
use the two point-statistics of the cosmic shear field. These can either be
studied directly using the two-point correlation function, or in Fourier space,
using the power spectrum. But analyzing weak lensing data inevitably involves
the masking out of regions or example to remove bright stars from the field.
Masking out the stars is common practice but the gaps in the data need proper
handling. In this paper, we show how an inpainting technique allows us to
properly fill in these gaps with only operations, leading to a new
image from which we can compute straight forwardly and with a very good
accuracy both the pow er spectrum and the bispectrum. We propose then a new
method to compute the bispectrum with a polar FFT algorithm, which has the main
advantage of avoiding any interpolation in the Fourier domain. Finally we
propose a new method for dark matter mass map reconstruction from shear
observations which integrates this new inpainting concept. A range of examples
based on 3D N-body simulations illustrates the results.Comment: Final version accepted by MNRAS. The FASTLens software is available
from the following link : http://irfu.cea.fr/Ast/fastlens.software.ph
On the size of knots in ring polymers
We give two different, statistically consistent definitions of the length l
of a prime knot tied into a polymer ring. In the good solvent regime the
polymer is modelled by a self avoiding polygon of N steps on cubic lattice and
l is the number of steps over which the knot ``spreads'' in a given
configuration. An analysis of extensive Monte Carlo data in equilibrium shows
that the probability distribution of l as a function of N obeys a scaling of
the form p(l,N) ~ l^(-c) f(l/N^D), with c ~ 1.25 and D ~ 1. Both D and c could
be independent of knot type. As a consequence, the knot is weakly localized,
i.e. ~ N^t, with t=2-c ~ 0.75. For a ring with fixed knot type, weak
localization implies the existence of a peculiar characteristic length l^(nu) ~
N^(t nu). In the scaling ~ N^(nu) (nu ~0.58) of the radius of gyration of the
whole ring, this length determines a leading power law correction which is much
stronger than that found in the case of unrestricted topology. The existence of
such correction is confirmed by an analysis of extensive Monte Carlo data for
the radius of gyration. The collapsed regime is studied by introducing in the
model sufficiently strong attractive interactions for nearest neighbor sites
visited by the self-avoiding polygon. In this regime knot length determinations
can be based on the entropic competition between two knotted loops separated by
a slip link. These measurements enable us to conclude that each knot is
delocalized (t ~ 1).Comment: 29 pages, 14 figure
Decision making process on the Antwerp Oosterweel link: lessons learnt
The Oosterweel link (completion of the Antwerp ring road, including a river Scheldt crossing) was planned to be the largest infrastructure project ever built in Belgium. It started as a noiseless process for more than fifteen years, the decision seemed to be taken in 2008: the reference design was approved and a DBFM consortium selected. Then the project became controversial. Action groups dominated the debate and could enforce a public referendum. The project was rejected by the Antwerp citizens. Can the rejection of the project be explained by opening the black box of the planning process? A research of the Antwerp University College Artesis reveals that the decision process of the Oosterweel link can be described within the three streams model (problems–policy alternatives–politics), developed by W. Kingdon. In each stream actors intervene with their own logic (e.g. experts use traffic models, politicians make political deals, and administrations refer to administrative rules...). The process streams were bundled by a policy maker (the governor of the province), creating for a certain period a 'window of opportunity'. But the research confirms that a project idea has its expiry date. From Kingdon's three project survival criteria the weak point of the Oosterweel project is its small problem definition (traffic congestion on the main road system). Major projects should refer to the mobility issue and not only to a traffic problem. Infrastructure planning should not be limited to the physical object to be built, but be embedded in the urban and regional environment (avoiding e.g. white backgrounds in project evaluations and design). Planning processes that only focus on control (of financial and technical issues) and omit interaction (with stakeholders and the general public) have a great risk to fail. This has huge consequences for project management
Bayesian Analysis of Inflation: Parameter Estimation for Single Field Models
Future astrophysical datasets promise to strengthen constraints on models of
inflation, and extracting these constraints requires methods and tools
commensurate with the quality of the data. In this paper we describe ModeCode,
a new, publicly available code that computes the primordial scalar and tensor
power spectra for single field inflationary models. ModeCode solves the
inflationary mode equations numerically, avoiding the slow roll approximation.
It is interfaced with CAMB and CosmoMC to compute cosmic microwave background
angular power spectra and perform likelihood analysis and parameter estimation.
ModeCode is easily extendable to additional models of inflation, and future
updates will include Bayesian model comparison. Errors from ModeCode contribute
negligibly to the error budget for analyses of data from Planck or other next
generation experiments. We constrain representative single field models (phi^n
with n=2/3, 1, 2, and 4, natural inflation, and "hilltop" inflation) using
current data, and provide forecasts for Planck. From current data, we obtain
weak but nontrivial limits on the post-inflationary physics, which is a
significant source of uncertainty in the predictions of inflationary models,
while we find that Planck will dramatically improve these constraints. In
particular, Planck will link the inflationary dynamics with the
post-inflationary growth of the horizon, and thus begin to probe the
"primordial dark ages" between TeV and GUT scale energies.Comment: 16 pages, 9 figures. Updated to match published version (revised and
expanded discussions of reheating uncertainties and slow roll mapping;
references added; results unchanged). Code available at
http://zuserver2.star.ucl.ac.uk/~hiranya/ModeCode
Quantum and classical localisation and the Manhattan lattice
We consider a network model, embedded on the Manhattan lattice, of a quantum
localisation problem belonging to symmetry class C. This arises in the context
of quasiparticle dynamics in disordered spin-singlet superconductors which are
invariant under spin rotations but not under time reversal. A mapping exists
between problems belonging to this symmetry class and certain classical random
walks which are self-avoiding and have attractive interactions; we exploit this
equivalence, using a study of the classical random walks to gain information
about the corresponding quantum problem. In a field-theoretic approach, we show
that the interactions may flow to one of two possible strong coupling regimes
separated by a transition: however, using Monte Carlo simulations we show that
the walks are in fact always compact two-dimensional objects with a
well-defined one-dimensional surface, indicating that the corresponding quantum
system is localised.Comment: 11 pages, 8 figure
Multilevel Topological Interference Management
The robust principles of treating interference as noise (TIN) when it is
sufficiently weak, and avoiding it when it is not, form the background for this
work. Combining TIN with the topological interference management (TIM)
framework that identifies optimal interference avoidance schemes, a baseline
TIM-TIN approach is proposed which decomposes a network into TIN and TIM
components, allocates the signal power levels to each user in the TIN
component, allocates signal vector space dimensions to each user in the TIM
component, and guarantees that the product of the two is an achievable number
of signal dimensions available to each user in the original network.Comment: To be presented at 2013 IEEE Information Theory Worksho
Unbiased sampling of globular lattice proteins in three dimensions
We present a Monte Carlo method that allows efficient and unbiased sampling
of Hamiltonian walks on a cubic lattice. Such walks are self-avoiding and visit
each lattice site exactly once. They are often used as simple models of
globular proteins, upon adding suitable local interactions. Our algorithm can
easily be equipped with such interactions, but we study here mainly the
flexible homopolymer case where each conformation is generated with uniform
probability. We argue that the algorithm is ergodic and has dynamical exponent
z=0. We then use it to study polymers of size up to 64^3 = 262144 monomers.
Results are presented for the effective interaction between end points, and the
interaction with the boundaries of the system
A Gauge Theory in Three Dimensions as an Ensemble of Surfaces
A particular gauge theory defined on the three dimensional
dodecahedral lattice is shown to correspond to a model of oriented
self-avoiding surfaces. Using large reduction it is argued that the model
is partially soluble in the planar limit.Comment: 8p, Phys. Lett. B, to appea
Role of cross-links in bundle formation, phase separation and gelation of long filaments
We predict the thermodynamic and structural behavior of solutions of long
cross-linked filaments. We find that at the mean field level, the entropy of
self-assembled junctions induces an effective attraction between the filaments
that can result in a phase separation into a connected network, in equilibrium
with a dilute phase. A connected network can also be formed in a
non-thermodynamic transition upon increase of the chain, or cross link density,
or with decreasing temperature. For rigid rods, at low temperatures, we predict
a transition from an isotropic network, to anisotropic bundles of rods tightly
bound by cross links, that is triggered by the interplay between the
configurational entropy of the cross-link distribution among the rods, and the
rotational and translational entropy of the rods.Comment: typos and graphics corrected; 6 pages 1 figur
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