3,807 research outputs found
Improving the precision matrix for precision cosmology
The estimation of cosmological constraints from observations of the large
scale structure of the Universe, such as the power spectrum or the correlation
function, requires the knowledge of the inverse of the associated covariance
matrix, namely the precision matrix, . In most analyses,
is estimated from a limited set of mock catalogues. Depending
on how many mocks are used, this estimation has an associated error which must
be propagated into the final cosmological constraints. For future surveys such
as Euclid and DESI, the control of this additional uncertainty requires a
prohibitively large number of mock catalogues. In this work we test a novel
technique for the estimation of the precision matrix, the covariance tapering
method, in the context of baryon acoustic oscillation measurements. Even though
this technique was originally devised as a way to speed up maximum likelihood
estimations, our results show that it also reduces the impact of noisy
precision matrix estimates on the derived confidence intervals, without
introducing biases on the target parameters. The application of this technique
can help future surveys to reach their true constraining power using a
significantly smaller number of mock catalogues.Comment: 9 pages, 7 figures, minor changes to match version accepted by MNRA
Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces: Surfactant Mixtures
The adsorption at the interface between an aqueous solution of several
surface-active agents and another fluid (air or oil) phase is addressed
theoretically. We derive the kinetic equations from a variation of the
interfacial free energy, solve them numerically and provide an analytic
solution for the simple case of a linear adsorption isotherm. Calculating
asymptotic solutions analytically, we find the characteristic time scales of
the adsorption process and observe the behavior of the system at various
temporal stages. In particular, we relate the kinetic behavior of the mixture
to the properties of its individual constituents and find good agreement with
experiments. In the case of kinetically limited adsorption, the mixture
kinetics is found to be considerably different from that of the
single-surfactant solutions because of strong coupling between the species.Comment: 19 pages, 7 figures, to be published in Langmui
Classical Signature Change in the Black Hole Topology
Investigations of classical signature change have generally envisaged
applications to cosmological models, usually a
Friedmann-Lemaitre-Robertson-Walker model. The purpose has been to avoid the
inevitable singularity of models with purely Lorentzian signature, replacing
the neighbourhood of the big bang with an initial, singularity free region of
Euclidean signture, and a signature change. We here show that signature change
can also avoid the singularity of gravitational collapse. We investigate the
process of re-birth of Schwarzschild type black holes, modelling it as a double
signature change, joining two universes of Lorentzian signature through a
Euclidean region which provides a `bounce'. We show that this process is viable
both with and without matter present, but realistic models -- which have the
signature change surfaces hidden inside the horizons -- require non-zero
density. In fact the most realistic models are those that start as a finite
cloud of collapsing matter, surrounded by vacuum. We consider how geodesics may
be matched across a signature change surface, and conclude that the particle
`masses' must jump in value. This scenario may be relevant to Smolin's recent
proposal that a form of natural selection operates on the level of universes,
which favours the type of universe we live in.Comment: LaTeX, 19 pages, 11 Figures. Replacement - only change is following
comment: For a pdf version with the figures embedded, see
http://www.mth.uct.ac.za/~cwh/mypub.htm
On the use of bianisotropic huygens' metasurfaces to build leaky-wave antennas
The Electromagnetics AcademyHuygens' metasurfaces are considered a powerful tool to achieve anomalous electromagnetic field transformations. They consist of an artifcial surface built of pairs of collocated electric and magetic dipoles that force the boundary conditions for the desired transformation to be ful lled [1]. Despite their possibilities, the achievable transformations must ful l some conditions. In [2] it was
shown that Huygens' metasurfaces with passive and lossless particles can achieve an arbitrary field transformation provided that the power is conserved at each point of the metasurface and there is wave impedance matching. However, it was shown in [3], that by introducing bianisotropy of the omega-type, the matching condition can be suppressed, which allows the control of both the transmission and rejection coe cients on the metasurface.Universidad de Málaga. Campus de Excelencia Internacional AndalucÃa Tech
Polyelectrolyte Persistence Length: Attractive Effect of Counterion Correlations and Fluctuations
The persistence length of a single, strongly charged, stiff polyelectrolyte
chain is investigated theoretically. Path integral formulation is used to
obtain the effective electrostatic interaction between the monomers. We find
significant deviations from the classical Odijk, Skolnick and Fixman (OSF)
result. An induced attraction between monomers is due to thermal fluctuations
and correlations between bound counterions. The electrostatic persistence
length is found to be smaller than the OSF value and indicates a possible
mechanical instability (collapse) for highly charged polyelectrolytes with
multivalent counterions. In addition, we calculate the amount of condensed
counterions on a slightly bent polyelectrolyte. More counterions are found to
be adsorbed as compared to the Manning condensation on a cylinder.Comment: 5 pages, 1 ps figur
Progress toward scalable tomography of quantum maps using twirling-based methods and information hierarchies
We present in a unified manner the existing methods for scalable partial
quantum process tomography. We focus on two main approaches: the one presented
in Bendersky et al. [Phys. Rev. Lett. 100, 190403 (2008)], and the ones
described, respectively, in Emerson et al. [Science 317, 1893 (2007)] and
L\'{o}pez et al. [Phys. Rev. A 79, 042328 (2009)], which can be combined
together. The methods share an essential feature: They are based on the idea
that the tomography of a quantum map can be efficiently performed by studying
certain properties of a twirling of such a map. From this perspective, in this
paper we present extensions, improvements and comparative analyses of the
scalable methods for partial quantum process tomography. We also clarify the
significance of the extracted information, and we introduce interesting and
useful properties of the -matrix representation of quantum maps that can
be used to establish a clearer path toward achieving full tomography of quantum
processes in a scalable way.Comment: Replaced with published version (only minor changes respect to the
first version
GGPSA: A GRAPHICS USER INTERFACE TO FILTER MOVEMENT ANALYSIS DATA
Data filtering is a challenging procedure for the biomechanics scientists. It is however a fundamental part of biomechanical analysis when higher derivatives are needed to examine the movement. We present graphics user interface software specially designed to help biomechanics lecturers and students. It is openly designed to provide a powerful tool that could also be used for research purposes. Manual and automatic filtering procedures are provided along with single or batch processing of files. The program can process exported ASCII data files but also can be linked with the well-known APAS movement analysis program and read its binary files. Future work includes processing of many other binary files from other motion analysis systems
Arguments against using units in observational cosmology
It is common to express cosmological measurements in units of . Here, we review some of the complications that originate from this
practice. A crucial problem caused by these units is related to the
normalization of the matter power spectrum, which is commonly characterized in
terms of the linear-theory rms mass fluctuation in spheres of radius
, . This parameter does not correctly capture the
impact of on the amplitude of density fluctuations. We show that the use of
has caused critical misconceptions for both the so-called
tension regarding the consistency between low-redshift probes and cosmic
microwave background data, and the way in which growth-rate estimates inferred
from redshift-space distortions are commonly expressed. We propose to abandon
the use of units in cosmology and to characterize the
amplitude of the matter power spectrum in terms of , defined as
the mass fluctuation in spheres of radius , whose value is
similar to the standard for .Comment: 6 pages, 4 figures. Minor changes. Matches version published in PR
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