201,256 research outputs found
Coupling of sedimentation and liquid structure: influence on hard sphere nucleation
The discrepancy in nucleation rate densities between simulated and
experimental hard spheres remains staggering and unexplained. Suggestively,
more strongly sedimenting colloidal suspensions of hard spheres nucleate much
faster than weakly sedimenting systems. In this work we consider firstly the
effect of sedimentation on the structure of colloidal hard spheres, by tuning
the density mismatch between solvent and colloidal particles. In particular we
investigate the effect on the degree of five fold symmetry present. Secondly we
study the size of density fluctuations in these experimental systems in
comparison to simulations. The density fluctuations are measured by assigning
each particle a local density, which is related to the number of particles
within a distance of 3.25 particle diameters. The standard deviation of these
local densities gives an indication of the fluctuations present in the system.
Five fold symmetry is suppressed by a factor of two when sedimentation is
induced in our system. Density fluctuations are also increased by a factor of
two in experiments compared to simulations. The change in five fold symmetry
makes a difference to the expected nucleation rates, but we demonstrate that it
is ultimately too small to resolve the discrepancy between experiment and
simulation, while the fluctuations are shown to be an artefact of 3d particle
tracking.Comment: 8 page
Sample variance in N--body simulations and impact on tomographic shear predictions
We study the effects of sample variance in N--body simulations, as a function
of the size of the simulation box, namely in connection with predictions on
tomographic shear spectra. We make use of a set of 8 CDM simulations
in boxes of 128, 256, 512 Mpc aside, for a total of 24, differing just
by the initial seeds. Among the simulations with 128 and 512 Mpc aside,
we suitably select those closest and farthest from {\it average}. Numerical and
linear spectra are suitably connected at low so to evaluate the
effects of sample variance on shear spectra for 5 or 10
tomographic bands. We find that shear spectra obtained by using 128 Mpc
simulations can vary up to , just because of the seed. Sample
variance lowers to , when using 512 Mpc. These very
percentages could however slightly vary, if other sets of the same number of
realizations were considered. Accordingly, in order to match the
precision expected for data, if still using 8 boxes, we require a size -- Mpc for them.Comment: accepted by Ap
Cosmic Discordance: Are Planck CMB and CFHTLenS weak lensing measurements out of tune?
We examine the level of agreement between low redshift weak lensing data and
the CMB using measurements from the CFHTLenS and Planck+WMAP polarization. We
perform an independent analysis of the CFHTLenS six bin tomography results of
Heymans et al. (2013). We extend their systematics treatment and find the
cosmological constraints to be relatively robust to the choice of non-linear
modeling, extension to the intrinsic alignment model and inclusion of baryons.
We find that the 90% confidence contours of CFHTLenS and Planck+WP do not
overlap even in the full 6-dimensional parameter space of CDM, so the
two datasets are discrepant. Allowing a massive active neutrino or tensor modes
does not significantly resolve the disagreement in the full n-dimensional
parameter space. Our results differ from some in the literature because we use
the full tomographic information in the weak lensing data and marginalize over
systematics. We note that adding a sterile neutrino to CDM does bring
the 8-dimensional 64% contours to overlap, mainly due to the extra effective
number of neutrino species, which we find to be 0.84 0.35 (68%) greater
than standard on combining the datasets. We discuss why this is not a
completely satisfactory resolution, leaving open the possibility of other new
physics or observational systematics as contributing factors. We provide
updated cosmology fitting functions for the CFHTLenS constraints and discuss
the differences from ones used in the literature.Comment: 12 pages, 8 figures. We compare our findings with studies that
include other low redshift probes of structure. An interactive figure is
available at http://bit.ly/1oZH0KQ. This version is that accepted by MNRAS,
and so includes changes based on the referee's comments, and updates to the
analysis cod
A Refutation of Bell's Theorem
Bell's Theorem was developed on the basis of considerations involving a
linear combination of spin correlation functions, each of which has a distinct
pair of arguments. The simultaneous presence of these different pairs of
arguments in the same equation can be understood in two radically different
ways: either as `strongly objective,' that is, all correlation functions
pertain to the same set of particle pairs, or as `weakly objective,' that is,
each correlation function pertains to a different set of particle pairs.
It is demonstrated that once this meaning is determined, no discrepancy
appears between local realistic theories and quantum mechanics: the discrepancy
in Bell's Theorem is due only to a meaningless comparison between a local
realistic inequality written within the strongly objective interpretation (thus
relevant to a single set of particle pairs) and a quantum mechanical prediction
derived from a weakly objective interpretation (thus relevant to several
different sets of particle pairs).Comment: RevTex4, 9 pages. Extended and entirely revised version. A talk given
at the Vaxjo conference, Sweden; Nov. 2000. Submited to J. Math. Phy
A Monte Carlo algorithm for simulating fermions on Lefschetz thimbles
A possible solution of the notorious sign problem preventing direct Monte
Carlo calculations for systems with non-zero chemical potential is to deform
the integration region in the complex plane to a Lefschetz thimble. We
investigate this approach for a simple fermionic model. We introduce an easy to
implement Monte Carlo algorithm to sample the dominant thimble. Our algorithm
relies only on the integration of the gradient flow in the numerically stable
direction, which gives it a distinct advantage over the other proposed
algorithms. We demonstrate the stability and efficiency of the algorithm by
applying it to an exactly solvable fermionic model and compare our results with
the analytical ones. We report a very good agreement for a certain region in
the parameter space where the dominant contribution comes from a single
thimble, including a region where standard methods suffer from a severe sign
problem. However, we find that there are also regions in the parameter space
where the contribution from multiple thimbles is important, even in the
continuum limit.Comment: 16 pages, 7 figure
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