6,980 research outputs found
The effect of point sources on satellite observations of the cosmic microwave background
We study the effect of extragalactic point sources on satellite observations
of the cosmic microwave background (CMB). In order to separate the
contributions due to different foreground components, a maximum-entropy method
is applied to simulated observations by the Planck Surveyor satellite. In
addition to point sources, the simulations include emission from the CMB and
the kinetic and thermal Sunyaev-Zel'dovich (SZ) effects from galaxy clusters,
as well as Galactic dust, free-free and synchrotron emission. We find that the
main input components are faithfully recovered and, in particular, that the
quality of the CMB reconstruction is only slightly reduced by the presence of
point sources. In addition, we find that it is possible to recover accurate
point source catalogues at each of the Planck Surveyor observing frequencies.Comment: 12 pages, 9 figures, submitted to MNRA
Non-parametric comparison of histogrammed two-dimensional data distributions using the Energy Test
When monitoring complex experiments, comparison is often made between regularly acquired histograms of data and reference histograms which represent the ideal state of the equipment. With the larger HEP experiments now ramping up, there is a need for automation of this task since the volume of comparisons could overwhelm human operators. However, the two-dimensional histogram comparison tools available in ROOT have been noted in the past to exhibit shortcomings. We discuss a newer comparison test for two-dimensional histograms, based on the Energy Test of Aslan and Zech, which provides more conclusive
discrimination between histograms of data coming from different distributions than methods provided in a recent ROOT release.The Science and Technology Facilities Council, U
Scalar field quantization on the 2+1 dimensional black hole background
The quantization of a massless conformally coupled scalar field on the 2+1
dimensional Anti de Sitter black hole background is presented. The Green's
function is calculated, using the fact that the black hole is Anti de Sitter
space with points identified, and taking into account the fact that the black
hole spacetime is not globally hyperbolic. It is shown that the Green's
function calculated in this way is the Hartle-Hawking Green's function. The
Green's function is used to compute , which is
regular on the black hole horizon, and diverges at the singularity. A particle
detector response function outside the horizon is also calculated and shown to
be a fermi type distribution. The back-reaction from is calculated exactly and is shown to give rise to a curvature
singularity at and to shift the horizon outwards. For a horizon
develops, shielding the singularity. Some speculations about the endpoint of
evaporation are discussed.Comment: CTP 2243, 24 pages, RevTex. (The backreaction section is extended,
and some confusing notation has been changed
Visualizing Spacetime Curvature via Gradient Flows I: Introduction
Traditional approaches to the study of the dynamics of spacetime curvature in
a very real sense hide the intricacies of the nonlinear regime. Whether it be
huge formulae, or mountains of numerical data, standard methods of presentation
make little use of our remarkable skill, as humans, at pattern recognition.
Here we introduce a new approach to the visualization of spacetime curvature.
We examine the flows associated with the gradient fields of invariants derived
from the spacetime. These flows reveal a remarkably rich structure, and offer
fresh insights even for well known analytical solutions to Einstein's
equations. This paper serves as an overview and as an introduction to this
approach.Comment: 10 pages twocolumn revtex 4-1 two figures. Final form to appear in
Phys Rev
Scientific optimization of a ground-based CMB polarization experiment
We investigate the science goals achievable with the upcoming generation of
ground-based Cosmic Microwave Background polarization experiments and calculate
the optimal sky coverage for such an experiment including the effects of
foregrounds. We find that with current technology an E-mode measurement will be
sample-limited, while a B-mode measurement will be detector-noise-limited. We
conclude that a 300 sq deg survey is an optimal compromise for a two-year
experiment to measure both E and B-modes, and that ground-based polarization
experiments can make an important contribution to B-mode surveys. Focusing on
one particular experiment, QUaD, a proposed bolometric polarimeter operating
from the South Pole, we find that a ground-based experiment can make a high
significance measurement of the acoustic peaks in the E-mode spectrum, and will
be able to detect the gravitational lensing signal in the B-mode spectrum. Such
an experiment could also directly detect the gravitational wave component of
the B-mode spectrum if the amplitude of the signal is close to current upper
limits. We also investigate how a ground-based experiment can improve
constraints on the cosmological parameters. We estimate that by combining two
years of QUaD data with the four-year WMAP data, an optimized ground-based
polarization experiment can improve constraints on cosmological parameters by a
factor of two. If the foreground contamination can be reduced, the measurement
of the tensor-to-scalar ratio can be improved by up to a factor of six over
that obtainable from WMAP alone.Comment: 17 pages, 11 figures replaced with version accepted by MNRA
Complex Visibilities of Cosmic Microwave Background Anisotropies
We study the complex visibilities of the cosmic microwave background
anisotropies that are observables in interferometric observations of the cosmic
microwave background, using the multipole expansion methods commonly adopted in
analyzing single-dish experiments. This allows us to recover the properties of
the visibilities that is obscured in the flat-sky approximation. Discussions of
the window function, multipole resolution, instrumental noise, pixelization,
and polarization are given.Comment: 22 pages, 1 figure include
Making sense of the bizarre behaviour of horizons in the McVittie spacetime
The bizarre behaviour of the apparent (black hole and cosmological) horizons
of the McVittie spacetime is discussed using, as an analogy, the
Schwarzschild-de Sitter-Kottler spacetime (which is a special case of McVittie
anyway). For a dust-dominated "background" universe, a black hole cannot exist
at early times because its (apparent) horizon would be larger than the
cosmological(apparent) horizon. A phantom-dominated "background" universe
causes this situation, and the horizon behaviour, to be time-reversed.Comment: 8 pages, 3 figure
A well-separated pairs decomposition algorithm for k-d trees implemented on multi-core architectures
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Variations of k-d trees represent a fundamental data structure used in Computational Geometry with numerous applications in science. For example particle track tting in the software of the LHC experiments, and in simulations of N-body systems in the study of dynamics of interacting galaxies, particle beam physics, and molecular dynamics in biochemistry. The many-body tree methods devised by Barnes and Hutt in the 1980s and the Fast Multipole Method introduced in 1987 by Greengard and Rokhlin use variants of k-d trees to reduce the computation time upper bounds to O(n log n) and even O(n) from O(n2). We present an algorithm that uses the principle of well-separated pairs decomposition to always produce compressed trees in O(n log n) work. We present and evaluate parallel implementations for the algorithm that can take advantage of multi-core architectures.The Science and Technology Facilities Council, UK
Anticipated effects of abiotic environmental change on intraspecific social interactions
Peer reviewedPublisher PD
- …