1,349 research outputs found
Angpow: a software for the fast computation of accurate tomographic power spectra
The statistical distribution of galaxies is a powerful probe to constrain
cosmological models and gravity. In particular the matter power spectrum
brings information about the cosmological distance evolution and the galaxy
clustering together. However the building of from galaxy catalogues
needs a cosmological model to convert angles on the sky and redshifts into
distances, which leads to difficulties when comparing data with predicted
from other cosmological models, and for photometric surveys like LSST.
The angular power spectrum between two bins located at
redshift and contains the same information than the matter power
spectrum, is free from any cosmological assumption, but the prediction of
from is a costly computation when performed exactly.
The Angpow software aims at computing quickly and accurately the auto
() and cross () angular power spectra between redshift
bins. We describe the developed algorithm, based on developments on the
Chebyshev polynomial basis and on the Clenshaw-Curtis quadrature method. We
validate the results with other codes, and benchmark the performance. Angpow is
flexible and can handle any user defined power spectra, transfer functions, and
redshift selection windows. The code is fast enough to be embedded inside
programs exploring large cosmological parameter spaces through the
comparison with data. We emphasize that the Limber's
approximation, often used to fasten the computation, gives wrong
values for cross-correlations.Comment: Published in Astronomy & Astrophysic
A direct method to compute the galaxy count angular correlation function including redshift-space distortions
In the near future, cosmology will enter the wide and deep galaxy survey area
allowing high-precision studies of the large scale structure of the universe in
three dimensions. To test cosmological models and determine their parameters
accurately, it is natural to confront data with exact theoretical expectations
expressed in the observational parameter space (angles and redshift). The
data-driven galaxy number count fluctuations on redshift shells, can be used to
build correlation functions on and between shells which
can probe the baryonic acoustic oscillations, the distance-redshift distortions
as well as gravitational lensing and other relativistic effects. Transforming
the model to the data space usually requires the computation of the angular
power spectrum but this appears as an artificial and
inefficient step plagued by apodization issues. In this article we show that it
is not necessary and present a compact expression for
that includes directly the leading density and redshift space distortions terms
from the full linear theory. It can be evaluated using a fast integration
method based on Clenshaw-Curtis quadrature and Chebyshev polynomial series.
This new method to compute the correlation functions without any Limber
approximation, allows us to produce and discuss maps of the correlation
function directly in the observable space and is a significant step towards
disentangling the data from the tested models
Photonic Hall Effect in ferrofluids: Theory and Experiments
An experimental and theoretical study on the Photonic Hall Effect (PHE) in
liquid and gelled samples of ferrofluids is presented. The ferrofluids are
aqueous colloidal suspensions of Fe(_{2})CoO(_{4}) particles, which can be
considered as anisotropic and absorbing Rayleigh scatterers.
The PHE is found to be produced by the orientation of the magnetic moments of
the particles, as is also the case for the Faraday effect. The dependence of
the PHE with respect to the concentration of the scatterers, the magnetic field
and the polarization of the incident light is measured in liquid and in gelled
samples and is compared to a simple model based on the use of a scattering
matrix and the single scattering approximation.Comment: 20 pages, 11 figures, submitte
Nonlinear Schrodinger Equations and N=1 Superconformal Algebra
By using AKNS scheme and soliton connection taking values in N=1
superconformal algebra we obtain new coupled super Nonlinear Schrodinger
equations.Comment: 12 pages, no figures, LaTex fil
Topological gravity with exchange algebra
A topological gravity is obtained by twisting the effective
super\-gravity. We show that this topological gravity has an infinite number of
BRST invariant quantities with conformal weight . They are a tower of
OSp multiplets and satisfy the classical exchange algebra of OSp.
We argue that these BRST invariant quantities become physical operators in the
quantum theory and their correlation functions are braided according to the
quantum OSp group. These properties of the topological effective gravity
are not shared by the standard topological gravity.Comment: 15 pages, Plain TEX, KUL-TF-93/4
Exact 1/N and Optimized Perturbative Evaluation of mu_c for Homogeneous Interacting Bose Gases
In the framework of the O(N) three-dimensional effective scalar field model
for homogeneous dilute weakly interacting Bose gases we use the 1/N expansion
to evaluate, within the large N limit, the parameter r_c which is directly
related to the critical chemical potential mu_c. This quantity enters the
order-a^2 n^{2/3} coefficient contributing to the critical temperature shift
Delta T_c where a represents the s-wave scattering length and n represents the
density. Compared to the recent precise numerical lattice simulation results,
our calculation suggests that the large N approximation performs rather well
even for the physical case N=2. We then calculate the same quantity but using
different forms of the optimized perturbative (variational) method, showing
that these produce excellent results both for the finite N and large-N cases.Comment: 12 pages, 2 figures. We have performed a refined and extended
numerical analysis to take into account the very recent results of Ref. [15
Singular measures in circle dynamics
Critical circle homeomorphisms have an invariant measure totally singular
with respect to the Lebesgue measure. We prove that singularities of the
invariant measure are of Holder type. The Hausdorff dimension of the invariant
measure is less than 1 but greater than 0
Challenges in Bridging Social Semantics and Formal Semantics on the Web
This paper describes several results of Wimmics, a research lab which names
stands for: web-instrumented man-machine interactions, communities, and
semantics. The approaches introduced here rely on graph-oriented knowledge
representation, reasoning and operationalization to model and support actors,
actions and interactions in web-based epistemic communities. The re-search
results are applied to support and foster interactions in online communities
and manage their resources
Ophthalmological Manifestations of Oculocutaneous and Ocular Albinism: Current Perspectives
Albinism describes a heterogeneous group of genetically determined disorders characterized by disrupted synthesis of melanin and a range of developmental ocular abnormalities. The main ocular features common to both oculocutaneous albinism (OCA), and ocular albinism (OA) include reduced visual acuity, refractive errors, foveal hypoplasia, congenital nystagmus, iris and fundus hypopigmentation and visual pathway misrouting, but clinical signs vary and there is phenotypic overlap with other pathologies. This study reviews the prevalence, genetics and ocular manifestations of OCA and OA, including abnormal development of the optic chiasm. The role of visual electrophysiology in the detection of chiasmal dysfunction and visual pathway misrouting is emphasized, highlighting how age-associated changes in visual evoked potential (VEP) test results must be considered to enable accurate diagnosis, and illustrated further by the inclusion of novel VEP data in genetically confirmed cases. Differential diagnosis is considered in the context of suspected retinal and other disorders, including rare syndromes that may masquerade as albinism
The Fokker-Planck equation for bistable potential in the optimized expansion
The optimized expansion is used to formulate a systematic approximation
scheme to the probability distribution of a stochastic system. The first order
approximation for the one-dimensional system driven by noise in an anharmonic
potential is shown to agree well with the exact solution of the Fokker-Planck
equation. Even for a bistable system the whole period of evolution to
equilibrium is correctly described at various noise intensities.Comment: 12 pages, LATEX, 3 Postscript figures compressed an
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