5,700 research outputs found
Two-point function for the Maxwell field in flat Robertson-Walker spacetimes
We obtain an explicit two-point function for the Maxwell field in flat
Roberson-Walker spaces, thanks to a new gauge condition which takes the scale
factor into account and assume a simple form. The two-point function is found
to have the short distance Hadamard behavior.Comment: 4 pages, Revte
Massive scalar field on (A)dS space from a massless conformal field in
We show how the equations for the scalar field (including the massive,
massless, minimally and conformally coupled cases) on de Sitter and Anti-de
Sitter spaces can be obtained from both the SO-invariant equation
in and two geometrical constraints defining
the (A)dS space. Apart from the equation in , the results only
follow from the geometry.Comment: Revtex 4.1, 6 pages. In v3: New material added (references, relation
with mass ladder operator), accepted in JM
Conformally covariant quantization of Maxwell field in de Sitter space
In this article, we quantize the Maxwell ("massless spin one") de Sitter
field in a conformally invariant gauge. This quantization is invariant under
the SO group and consequently under the de Sitter group. We obtain a
new de Sitter invariant two-points function which is very simple. Our method
relies on the one hand on a geometrical point of view which uses the
realization of Minkowski, de Sitter and anti-de Sitter spaces as intersections
of the null cone in \setR^6 and a moving plane, and on the other hand on a
canonical quantization scheme of the Gupta-Bleuler type.Comment: v2 is is the definitive (improved compare to v1) versio
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Maximizing the Strength of Fused-Deposition ABS Plastic Parts
Fused-Deposition (FD) creates parts using robotic extrusion of set.D.i-liquid .polymer
fiber, which molecularly bonds with neighboring fibers via thermal-dlffuslo.n bonding. T~e
strength ofthe. part depends on the bulk polymer strength, themesostructure ~flber layout, vOid
geom~try, extent of fiber bonding), and thefiber-t~-fiber ~ond strength. The ~nfluence of these
factors on the mechanical strength of FD-ABS.plasttc parts IS reported.along with the.FD process
variable settingsfor maximum strength. Substantial increases in transverse strength are achieved
at the optirnal settings and additional increases can be achieved by post..fabrication annealing.
Keywords: Stratasys, fused-deposition, ABSplastic, functional parts, strength, mesostructure, polymer diffusion.Mechanical Engineerin
Decoherence and entropy of primordial fluctuations. I: Formalism and interpretation
We propose an operational definition of the entropy of cosmological
perturbations based on a truncation of the hierarchy of Green functions. The
value of the entropy is unambiguous despite gauge invariance and the
renormalization procedure. At the first level of truncation, the reduced
density matrices are Gaussian and the entropy is the only intrinsic quantity.
In this case, the quantum-to-classical transition concerns the entanglement of
modes of opposite wave-vectors, and the threshold of classicality is that of
separability. The relations to other criteria of classicality are established.
We explain why, during inflation, most of these criteria are not intrinsic. We
complete our analysis by showing that all reduced density matrices can be
written as statistical mixtures of minimal states, the squeezed properties of
which are less constrained as the entropy increases. Pointer states therefore
appear not to be relevant to the discussion. The entropy is calculated for
various models in paper II.Comment: 23 page
Permutation Inference for Canonical Correlation Analysis
Canonical correlation analysis (CCA) has become a key tool for population
neuroimaging, allowing investigation of associations between many imaging and
non-imaging measurements. As other variables are often a source of variability
not of direct interest, previous work has used CCA on residuals from a model
that removes these effects, then proceeded directly to permutation inference.
We show that such a simple permutation test leads to inflated error rates. The
reason is that residualisation introduces dependencies among the observations
that violate the exchangeability assumption. Even in the absence of nuisance
variables, however, a simple permutation test for CCA also leads to excess
error rates for all canonical correlations other than the first. The reason is
that a simple permutation scheme does not ignore the variability already
explained by previous canonical variables. Here we propose solutions for both
problems: in the case of nuisance variables, we show that transforming the
residuals to a lower dimensional basis where exchangeability holds results in a
valid permutation test; for more general cases, with or without nuisance
variables, we propose estimating the canonical correlations in a stepwise
manner, removing at each iteration the variance already explained, while
dealing with different number of variables in both sides. We also discuss how
to address the multiplicity of tests, proposing an admissible test that is not
conservative, and provide a complete algorithm for permutation inference for
CCA.Comment: 49 pages, 2 figures, 10 tables, 3 algorithms, 119 reference
Examples of Berezin-Toeplitz Quantization: Finite sets and Unit Interval
We present a quantization scheme of an arbitrary measure space based on
overcomplete families of states and generalizing the Klauder and the
Berezin-Toeplitz approaches. This scheme could reveal itself as an efficient
tool for quantizing physical systems for which more traditional methods like
geometric quantization are uneasy to implement. The procedure is illustrated by
(mostly two-dimensional) elementary examples in which the measure space is a
-element set and the unit interval. Spaces of states for the -element set
and the unit interval are the 2-dimensional euclidean and hermitian
\C^2 planes
Modelling CO emission from hydrodynamic simulations of nearby spirals, starbursting mergers, and high-redshift galaxies
We model the intensity of emission lines from the CO molecule, based on
hydrodynamic simulations of spirals, mergers, and high-redshift galaxies with
very high resolutions (3pc and 10^3 Msun) and detailed models for the
phase-space structure of the interstellar gas including shock heating, stellar
feedback processes and galactic winds. The simulations are analyzed with a
Large Velocity Gradient (LVG) model to compute the local emission in various
molecular lines in each resolution element, radiation transfer and opacity
effects, and the intensity emerging from galaxies, to generate synthetic
spectra for various transitions of the CO molecule. This model reproduces the
known properties of CO spectra and CO-to-H2 conversion factors in nearby
spirals and starbursting major mergers. The high excitation of CO lines in
mergers is dominated by an excess of high-density gas, and the high turbulent
velocities and compression that create this dense gas excess result in broad
linewidths and low CO intensity-to-H2 mass ratios. When applied to
high-redshift gas-rich disks galaxies, the same model predicts that their
CO-to-H2 conversion factor is almost as high as in nearby spirals, and much
higher than in starbursting mergers. High-redshift disk galaxies contain giant
star-forming clumps that host a high-excitation component associated to gas
warmed by the spatially-concentrated stellar feedback sources, although CO(1-0)
to CO(3-2) emission is overall dominated by low-excitation gas around the
densest clumps. These results overall highlight a strong dependence of CO
excitation and the CO-to-H2 conversion factor on galaxy type, even at similar
star formation rates or densities. The underlying processes are driven by the
interstellar medium structure and turbulence and its response to stellar
feedback, which depend on global galaxy structure and in turn impact the CO
emission properties.Comment: A&A in pres
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