9,402 research outputs found
Systematic Errors in Cosmic Microwave Background Interferometry
Cosmic microwave background (CMB) polarization observations will require
superb control of systematic errors in order to achieve their full scientific
potential, particularly in the case of attempts to detect the B modes that may
provide a window on inflation. Interferometry may be a promising way to achieve
these goals. This paper presents a formalism for characterizing the effects of
a variety of systematic errors on interferometric CMB polarization
observations, with particular emphasis on estimates of the B-mode power
spectrum. The most severe errors are those that couple the temperature
anisotropy signal to polarization; such errors include cross-talk within
detectors, misalignment of polarizers, and cross-polarization. In a B mode
experiment, the next most serious category of errors are those that mix E and B
modes, such as gain fluctuations, pointing errors, and beam shape errors. The
paper also indicates which sources of error may cause circular polarization
(e.g., from foregrounds) to contaminate the cosmologically interesting linear
polarization channels, and conversely whether monitoring of the circular
polarization channels may yield useful information about the errors themselves.
For all the sources of error considered, estimates of the level of control that
will be required for both E and B mode experiments are provided. Both
experiments that interfere linear polarizations and those that interfere
circular polarizations are considered. The fact that circular experiments
simultaneously measure both linear polarization Stokes parameters in each
baseline mitigates some sources of error.Comment: 19 pages, 9 figures, submitted to Phys. Rev.
Mosaicking with cosmic microwave background interferometers
Measurements of cosmic microwave background (CMB) anisotropies by
interferometers offer several advantages over single-dish observations. The
formalism for analyzing interferometer CMB data is well developed in the
flat-sky approximation, valid for small fields of view. As the area of sky is
increased to obtain finer spectral resolution, this approximation needs to be
relaxed. We extend the formalism for CMB interferometry, including both
temperature and polarization, to mosaics of observations covering arbitrarily
large areas of the sky, with each individual pointing lying within the flat-sky
approximation. We present a method for computing the correlation between
visibilities with arbitrary pointing centers and baselines and illustrate the
effects of sky curvature on the l-space resolution that can be obtained from a
mosaic.Comment: 9 pages; submitted to Ap
Sommerfeld's image method in the calculation of van der Waals forces
We show how the image method can be used together with a recent method
developed by C. Eberlein and R. Zietal to obtain the dispersive van der Waals
interaction between an atom and a perfectly conducting surface of arbitrary
shape. We discuss in detail the case of an atom and a semi- infinite conducting
plane. In order to employ the above procedure to this problem it is necessary
to use the ingenious image method introduced by Sommerfeld more than one
century ago, which is a generalization of the standard procedure. Finally, we
briefly discuss other interesting situations that can also be treated by the
joint use of Sommerfeld's image technique and Eberlein-Zietal method.Comment: To appear in the proceedings of Conference on Quantum Field Theory
under the Influence of External Conditions (QFEXT11
Sensitivity of a Bolometric Interferometer to the CMB power spectrum
Context. The search for B-mode polarization fluctuations in the Cosmic
Microwave Background is one of the main challenges of modern cosmology. The
expected level of the B-mode signal is very low and therefore requires the
development of highly sensitive instruments with low systematic errors. An
appealing possibility is bolometric interferometry. Aims. We compare in this
article the sensitivity on the CMB angular power spectrum achieved with direct
imaging, heterodyne and bolometric interferometry. Methods. Using a simple
power spectrum estimator, we calculate its variance leading to the counterpart
for bolometric interferometry of the well known Knox formula for direct
imaging. Results. We find that bolometric interferometry is less sensitive than
direct imaging. However, as expected, it is finally more sensitive than
heterodyne interferometry due to the low noise of the bolometers. It therefore
appears as an alternative to direct imagers with different and possibly lower
systematic errors, mainly due to the absence of an optical setup in front of
the horns.Comment: 5 pages, 3 figures. This last version matches the published version
(Astronomy and Astrophysics 491 3 (2008) 923-927). Sensitivity of Heterodyne
Interferometers modified by a factor of tw
A Bayesian approach to discrete object detection in astronomical datasets
A Bayesian approach is presented for detecting and characterising the signal
from discrete objects embedded in a diffuse background. The approach centres
around the evaluation of the posterior distribution for the parameters of the
discrete objects, given the observed data, and defines the
theoretically-optimal procedure for parametrised object detection. Two
alternative strategies are investigated: the simultaneous detection of all the
discrete objects in the dataset, and the iterative detection of objects. In
both cases, the parameter space characterising the object(s) is explored using
Markov-Chain Monte-Carlo sampling. For the iterative detection of objects,
another approach is to locate the global maximum of the posterior at each
iteration using a simulated annealing downhill simplex algorithm. The
techniques are applied to a two-dimensional toy problem consisting of Gaussian
objects embedded in uncorrelated pixel noise. A cosmological illustration of
the iterative approach is also presented, in which the thermal and kinetic
Sunyaev-Zel'dovich effects from clusters of galaxies are detected in microwave
maps dominated by emission from primordial cosmic microwave background
anisotropies.Comment: 20 pages, 12 figures, accepted by MNRAS; contains some additional
material in response to referee's comment
Detection of Cosmic Microwave Background Structure in a Second Field with the Cosmic Anisotropy Telescope
We describe observations at frequencies near 15 GHz of the second 2x2 degree
field imaged with the Cambridge Cosmic Anisotropy Telescope (CAT). After the
removal of discrete radio sources, structure is detected in the images on
characteristic scales of about half a degree, corresponding to spherical
harmonic multipoles in the approximate range l= 330--680. A Bayesian analysis
confirms that the signal arises predominantly from the cosmic microwave
background (CMB) radiation for multipoles in the lower half of this range; the
average broad-band power in a bin with centroid l=422 (theta = 51') is
estimated to be Delta_T/T=2.1^{+0.4}_{-0.5} x 10^{-5}. For multipoles centred
on l=615 (theta =35'), we find contamination from Galactic emission is
significant, and constrain the CMB contribution to the measured power in this
bin to be Delta_T/T <2.0 x 10^{-5} (1-sigma upper limit). These new results are
consistent with the first detection made by CAT in a completely different area
of sky. Together with data from other experiments, this new CAT detection adds
weight to earlier evidence from CAT for a downturn in the CMB power spectrum on
scales smaller than 1 degree. Improved limits on the values of H_0 and Omega
are determined using the new CAT data.Comment: 5 pages, 5 figures (gif), submitted to MNRA
Phase diagram of KHF2 and non-equilibrium effects
The equilibrium diagram for the KHF2-H2O system was constructed from cooling and heating curves for the compositions between 5 wt% and 40 wt% KHF2 and the results are shown. The phase diagrams shown is typical of that of a two component system with miscible liquid phases and whole solid phases consist of pure components. A eutectic point was found at approximately 15% KHF2 which remains completely liquid down to a temperature of -9.0 C. No hydrate formation was observed and no anomalous behavior such as the occurrence of solid transitions or metastable states was observed. The effect of rapid freezing on the equilibrium diagram did not appear, and cooling curves exhibited only one halt. Also, at rapid freezing rates, the supercooling of the solutions was smaller than those observed at the slow cooling rates. The existence of a eutectic composition and the slow rate of dissolution of the salt are used to interpret heat absorption behavior in practical applications of the KHF2-H2O system
First Observational Tests of Eternal Inflation: Analysis Methods and WMAP 7-Year Results
In the picture of eternal inflation, our observable universe resides inside a
single bubble nucleated from an inflating false vacuum. Many of the theories
giving rise to eternal inflation predict that we have causal access to
collisions with other bubble universes, providing an opportunity to confront
these theories with observation. We present the results from the first
observational search for the effects of bubble collisions, using cosmic
microwave background data from the WMAP satellite. Our search targets a generic
set of properties associated with a bubble collision spacetime, which we
describe in detail. We use a modular algorithm that is designed to avoid a
posteriori selection effects, automatically picking out the most promising
signals, performing a search for causal boundaries, and conducting a full
Bayesian parameter estimation and model selection analysis. We outline each
component of this algorithm, describing its response to simulated CMB skies
with and without bubble collisions. Comparing the results for simulated bubble
collisions to the results from an analysis of the WMAP 7-year data, we rule out
bubble collisions over a range of parameter space. Our model selection results
based on WMAP 7-year data do not warrant augmenting LCDM with bubble
collisions. Data from the Planck satellite can be used to more definitively
test the bubble collision hypothesis.Comment: Companion to arXiv:1012.1995. 41 pages, 23 figures. v2: replaced with
version accepted by PRD. Significant extensions to the Bayesian pipeline to
do the full-sky non-Gaussian source detection problem (previously restricted
to patches). Note that this has changed the normalization of evidence values
reported previously, as full-sky priors are now employed, but the conclusions
remain unchange
Faecal incontinence persisting after childbirth : a 12 year longitudinal study
© 2012 The Authors BJOG An International Journal of Obstetrics and Gynaecology © 2012 RCOG.Peer reviewedPostprin
The physical significance of the Babak-Grishchuk gravitational energy-momentum tensor
We examine the claim of Babak and Grishchuk [1] to have solved the problem of
localising the energy and momentum of the gravitational field. After
summarising Grishchuk's flat-space formulation of gravity, we demonstrate its
equivalence to General Relativity at the level of the action. Two important
transformations are described (diffeomorphisms applied to all fields, and
diffeomorphisms applied to the flat-space metric alone) and we argue that both
should be considered gauge transformations: they alter the mathematical
representation of a physical system, but not the system itself. By examining
the transformation properties of the Babak-Grishchuk gravitational
energy-momentum tensor under these gauge transformations (infinitesimal and
finite) we conclude that this object has no physical significance.Comment: 10 pages. Submitted to Phys. Rev. D; acknowledgements adjuste
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