6,043 research outputs found
Cosmic String Power Spectrum, Bispectrum and Trispectrum
We use analytic calculations of the post-recombination gravitational effects
of cosmic strings to estimate the resulting CMB power spectrum, bispectrum and
trispectrum. We place a particular emphasis on multipole regimes relevant for
forthcoming CMB experiments, notably the Planck satellite. These calculations
use a flat sky approximation, generalising previous work by integrating string
contributions from last scattering to the present day, finding the dominant
contributions to the correlators for multipoles l > 50. We find a well-behaved
shape for the string bispectrum (without divergences) which is easily
distinguishable from the inflationary bispectra which possess significant
acoustic peaks. We estimate that the nonlinearity parameter characterising the
bispectrum is approximately f_NL \sim -20 (given present string constraints
from the CMB power spectrum. We also apply these unequal time correlator
methods to calculate the trispectrum for parrallelogram configurations, again
valid over a large range of angular scales relevant for WMAP and Planck, as
well as on very small angular scales. We find that, unlike the bispectrum which
is suppressed by symmetry considerations, the trispectrum for cosmic strings is
large. Our current estimate for the trispectrum parameter is tau_NL \sim 10^5,
which may provide one of the strongest constraints on the string model as
estimators for the trispectrum are developed
General CMB and Primordial Trispectrum Estimation
We present trispectrum estimation methods which can be applied to general
non-separable primordial and CMB trispectra. We present a general optimal
estimator for the connected part of the trispectrum, for which we derive a
quadratic term to incorporate the effects of inhomogeneous noise and masking.
We describe a general algorithm for creating simulated maps with given
arbitrary (and independent) power spectra, bispectra and trispectra. We propose
a universal definition of the trispectrum parameter , so that the
integrated bispectrum on the observational domain can be consistently compared
between theoretical models. We define a shape function for the primordial
trispectrum, together with a shape correlator and a useful parametrisation for
visualizing the trispectrum. We derive separable analytic CMB solutions in the
large-angle limit for constant and local models. We present separable mode
decompositions which can be used to describe any primordial or CMB bispectra on
their respective wavenumber or multipole domains. By extracting coefficients of
these separable basis functions from an observational map, we are able to
present an efficient estimator for any given theoretical model with a
nonseparable trispectrum. The estimator has two manifestations, comparing the
theoretical and observed coefficients at either primordial or late times. These
mode decomposition methods are numerically tractable with order
operations for the CMB estimator and approximately order for the general
primordial estimator (reducing to order in both cases for a special class
of models). We also demonstrate how the trispectrum can be reconstructed from
observational maps using these methods.Comment: 38 pages, 9 figures. In v2 Figures 4-7 are altered slightly and some
extra references are included in the bibliography. v3 matches version
submitted to journal. Includes discussion of special case
Universal Non-Gaussian Initial Conditions for N-body Simulations
In this paper we present the implementation of an efficient formalism for the
generation of arbitrary non-Gaussian initial conditions for use in N-body
simulations. The methodology involves the use of a separable modal approach for
decomposing a primordial bispectrum or trispectrum. This approach allows for
the far more efficient generation of the non-Gaussian initial conditions
already described in the literature, as well as the generation for the first
time of non-separable bispectra and the special class of diagonal-free
trispectra. The modal approach also allows for the reconstruction of the
spectra from given realisations, a fact which is exploited to provide an
accurate consistency check of the simulations.Comment: 7 pages, 3 figure
Rapid Separable Analysis of Higher Order Correlators in Large Scale Structure
We present an efficient separable approach to the estimation and
reconstruction of the bispectrum and the trispectrum from observational (or
simulated) large scale structure data. This is developed from general CMB
(poly-)spectra methods which exploit the fact that the bispectrum and
trispectrum in the literature can be represented by a separable mode expansion
which converges rapidly (with terms). With an
effective grid resolution (number of particles/grid points
), we present a bispectrum estimator which requires only
operations, along with a
corresponding method for direct bispectrum reconstruction. This method is
extended to the trispectrum revealing an estimator which requires only operations. The complexity in
calculating the trispectrum in this method is now involved in the original
decomposition and orthogonalisation process which need only be performed once
for each model. However, for non-diagonal trispectra these processes present
little extra difficulty and may be performed in
operations. A discussion of how the methodology may be applied to the
quadspectrum is also given. An efficient algorithm for the generation of
arbitrary nonGaussian initial conditions for use in N-body codes using this
separable approach is described. This prescription allows for the production of
nonGaussian initial conditions for arbitrary bispectra and trispectra. A brief
outline of the key issues involved in parameter estimation, particularly in the
non-linear regime, is also given
Vegetarianism
Ethical vegetarians maintain that vegetarianism is morally required. The principal reasons offered in support of ethical vegetarianism are: (i) concern for the welfare and well-being of the animals being eaten, (ii) concern for the environment, (iii) concern over global food scarcity and the just distribution of resources, and (iv) concern for future generations. Each of these reasons is explored in turn, starting with a historical look at ethical vegetarianism and the moral status of animals
Microscopic Surface Structure of Liquid Alkali Metals
We report an x-ray scattering study of the microscopic structure of the
surface of a liquid alkali metal. The bulk liquid structure factor of the
eutectic K67Na33 alloy is characteristic of an ideal mixture, and so shares the
properties of an elemental liquid alkali metal. Analysis of off-specular
diffuse scattering and specular x-ray reflectivity shows that the surface
roughness of the K-Na alloy follows simple capillary wave behavior with a
surface structure factor indicative of surface induced layering. Comparison of
thelow-angle tail of the K67Na33 surface structure factor with the one measured
for liquid Ga and In previously suggests that layering is less pronounced in
alkali metals. Controlled exposure of the liquid to H2 and O2 gas does not
affect the surface structure, indicating that oxide and hydride are not stable
at the liquid surface under these experimental conditions.Comment: 12 pages, 3 figures, published in Phys. Rev.
Inner Molecular Rings in Barred Galaxies: BIMA SONG CO Observations
Although inner star-forming rings are common in optical images of barred
spiral galaxies, observational evidence for the accompanying molecular gas has
been scarce. In this paper we present images of molecular inner rings, traced
using the CO (1-0) emission line, from the
Berkeley-Illinois-Maryland-Association Survey of Nearby Galaxies (BIMA SONG).
We detect inner ring CO emission from all five SONG barred galaxies classified
as inner ring (type (r)). We also examine the seven SONG barred galaxies
classified as inner spiral (type (s)); in one of these, NGC 3627, we find
morphological and kinematic evidence for a molecular inner ring. Inner ring
galaxies have been classified as such based on optical images, which emphasize
recent star formation. We consider the possibility that there may exist inner
rings in which star formation efficiency is not enhanced. However, we find that
in NGC 3627 the inner ring star formation efficiency is enhanced relative to
most other regions in that galaxy. We note that the SONG (r) galaxies have a
paucity of CO and H alpha emission interior to the inner ring (except near the
nucleus), while NGC 3627 has relatively bright bar CO and H alpha emission; we
suggest that galaxies with inner rings such as NGC 3627 may be misclassified if
there are significant amounts of gas and star formation in the bar.Comment: To be published in the Astrophysical Journal, July 2002 A version of
the paper with full resolution figures is available at:
http://www.astro.umd.edu/~mregan/ms.ps.g
The suppression of superconductivity in MgCNi3 by Ni-site doping
The effects of partial substitution of Cu and Co for Ni in the intermetallic
perovskite superconductor MgCNi3 are reported. Calculation of the expected
electronic density of states suggests that electron (Cu) and hole (Co) doping
should have different effects. For MgCNi3-xCux, solubility of Cu is limited to
approximately 3% (x = 0.1), and Tc decreases systematically from 7K to 6K. For
MgCNi3-xCox, solubility of Co is much more extensive, but bulk
superconductivity disappears for Co doping of 1% (x = 0.03). No signature of
long range magnetic ordering is observed in the magnetic susceptibility of the
Co doped material.Comment: submitted, Solid State Communication
Oral contraceptives, hormone replacement therapy, thrombophilias and risk of venous thromboembolism: a systematic review The Thrombosis: Risk and Economic Assessment of Thrombophilia Screening (TREATS) Study
Combined oral contraceptives,oral hormone replacement therapy and thrombophilias are recognised risk factors for venous thromboembolism in women.The objective of this study was to assess the risk of thromboembolism among women with thrombophilia who are taking oral contraceptives or hormone replacement therapy, conducting a systematic review and metaanalysis. Of 201 studies identified, only nine met the inclusion criteria. Seven studies included pre-menopausal women on oral contraceptives and two studies included peri-menopausal women on hormone replacement therapy. For oral contraceptive use, significant associations of the risk of venous thromboembolism were found in women with factor V Leiden (OR 15.62; 95%CI 8.66 to 28.15); deficiencies of antithrombin (OR 12.60; 95%CI 1.37 to 115.79), protein C (OR 6.33; 95%CI 1.68 to 23.87), or protein S (OR 4.88; 95%CI 1.39 to 17.10), elevated levels of factor VIIIc (OR 8.80; 95%CI 4.13 to 18.75); and factor V Leiden and prothrombin G20210A (OR 7.85; 95%CI 1.65 to 37.41). For hormone replacement therapy, a significant association was found in women with factor V Leiden (OR 13.16; 95%CI 4.28 to 40.47).Although limited by the small number of studies, the findings of this study support the presence of interaction between thrombophilia and venous thromboembolism among women taking oral contraceptives. However, further studies are required to establish with greater confidence the associations of these, and other, thrombophilias with venous thromboembolism among hormone users
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