31,746 research outputs found
The shape of primordial non-Gaussianity and the CMB bispectrum
We present a set of formalisms for comparing, evolving and constraining
primordial non-Gaussian models through the CMB bispectrum. We describe improved
methods for efficient computation of the full CMB bispectrum for any general
(non-separable) primordial bispectrum, incorporating a flat sky approximation
and a new cubic interpolation. We review all the primordial non-Gaussian models
in the present literature and calculate the CMB bispectrum up to l <2000 for
each different model. This allows us to determine the observational
independence of these models by calculating the cross-correlation of their CMB
bispectra. We are able to identify several distinct classes of primordial
shapes - including equilateral, local, warm, flat and feature (non-scale
invariant) - which should be distinguishable given a significant detection of
CMB non-Gaussianity. We demonstrate that a simple shape correlator provides a
fast and reliable method for determining whether or not CMB shapes are well
correlated. We use an eigenmode decomposition of the primordial shape to
characterise and understand model independence. Finally, we advocate a
standardised normalisation method for based on the shape
autocorrelator, so that observational limits and errors can be consistently
compared for different models.Comment: 32 pages, 20 figure
Primordial non-Gaussianity and the CMB bispectrum
We present a new formalism, together with efficient numerical methods, to
directly calculate the CMB bispectrum today from a given primordial bispectrum
using the full linear radiation transfer functions. Unlike previous analyses
which have assumed simple separable ansatze for the bispectrum, this work
applies to a primordial bispectrum of almost arbitrary functional form, for
which there may have been both horizon-crossing and superhorizon contributions.
We employ adaptive methods on a hierarchical triangular grid and we establish
their accuracy by direct comparison with an exact analytic solution, valid on
large angular scales. We demonstrate that we can calculate the full CMB
bispectrum to greater than 1% precision out to multipoles l<1800 on reasonable
computational timescales. We plot the bispectrum for both the superhorizon
('local') and horizon-crossing ('equilateral') asymptotic limits, illustrating
its oscillatory nature which is analogous to the CMB power spectrum
Gamma-Ray Bursts observed by XMM-Newton
Analysis of observations with XMM-Newton have made a significant contribution
to the study of Gamma-ray Burst (GRB) X-ray afterglows. The effective area,
bandpass and resolution of the EPIC instrument permit the study of a wide
variety of spectral features. In particular, strong, time-dependent, soft X-ray
emission lines have been discovered in some bursts. The emission mechanism and
energy source for these lines pose major problems for the current generation of
GRB models. Other GRBs have intrinsic absorption, possibly related to the
environment around the progenitor, or possible iron emission lines similar to
those seen in GRBs observed with BeppoSAX. Further XMM-Newton observations of
GRBs discovered by the Swift satellite should help unlock the origin of the GRB
phenomenon over the next few years.Comment: To appear in proceedings of the "XMM-Newton EPIC Consortium meeting,
Palermo, 2003 October 14-16", published in Memorie della Societa Astronomica
Italian
Delayed soft X-ray emission lines in the afterglow of GRB 030227
Strong, delayed X-ray line emission is detected in the afterglow of GRB
030227, appearing near the end of the XMM-Newton observation, nearly twenty
hours after the burst. The observed flux in the lines, not simply the
equivalent width, sharply increases from an undetectable level (<1.7e-14
erg/cm^2/s, 3 sigma) to 4.1e-14 erg/cm^2/s in the final 9.7 ks. The line
emission alone has nearly twice as many detected photons as any previous
detection of X-ray lines. The lines correspond well to hydrogen and/or
helium-like emission from Mg, Si, S, Ar and Ca at a redshift z=1.39. There is
no evidence for Fe, Co or Ni--the ultimate iron abundance must be less than a
tenth that of the lighter metals. If the supernova and GRB events are nearly
simultaneous there must be continuing, sporadic power output after the GRB of a
luminosity >~5e46 erg/s, exceeding all but the most powerful quasars.Comment: Submitted to ApJL. 14 pages, 3 figures with AASLaTe
Curved planar quantum wires with Dirichlet and Neumann boundary conditions
We investigate the discrete spectrum of the Hamiltonian describing a quantum
particle living in the two-dimensional curved strip. We impose the Dirichlet
and Neumann boundary conditions on opposite sides of the strip. The existence
of the discrete eigenvalue below the essential spectrum threshold depends on
the sign of the total bending angle for the asymptotically straight strips.Comment: 7 page
Diagnosis of Helicobacter pylori by carbon-13 urea breath test using a portable mass spectrometer
CONTEXT: In the non-invasive detection of markers of disease, mass spectrometry is able to detect small quantities of volatile markers in exhaled air. However, the problem of size, expense and immobility of conventional mass spectrometry equipment has restricted its use. Now, a smaller, less expensive, portable quadrupole mass spectrometer system has been developed. Helicobacter pylori has been implicated in the development of chronic gastritis, gastric and duodenal ulcers and gastric cancer. OBJECTIVES: To compare the results obtained from the presence of H. pylori by a carbon-13 urea test using a portable quadrupole mass spectrometer system with those from a fixed mass spectrometer in a hospital-based clinical trial. METHODS: Following ethical approval, 45 patients attending a gastroenterology clinic at the Royal Liverpool University Hospital exhaled a breath sample into a Tedlar gas sampling bag. They then drank an orange juice containing urea radiolabelled with carbon and 30 min later gave a second breath sample. The carbon-13 content of both samples was measured using both quadrupole mass spectrometer systems. If the post-drink level exceeded the pre-drink level by 3% or more, a positive diagnosis for the presence of H. pylori was made. RESULTS: The findings were compared to the results using conventional isotope ratio mass spectrometry using a laboratory-based magnetic sector instrument off-site. The results showed agreement in 39 of the 45 patients. CONCLUSIONS: This study suggests that a portable quadrupole mass spectrometer is a potential alternative to the conventional centralised testing equipment. Future development of the portable quadrupole mass spectrometer to reduce further its size and cost is indicated, together with further work to validate this new equipment and to enhance its use in mass spectrometry diagnosis of other medical conditions
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
Full-analytic frequency-domain 1pN-accurate gravitational wave forms from eccentric compact binaries
The article provides ready-to-use 1pN-accurate frequency-domain gravitational
wave forms for eccentric nonspinning compact binaries of arbitrary mass ratio
including the first post-Newtonian (1pN) point particle corrections to the
far-zone gravitational wave amplitude, given in terms of tensor spherical
harmonics. The averaged equations for the decay of the eccentricity and growth
of radial frequency due to radiation reaction are used to provide stationary
phase approximations to the frequency-domain wave forms.Comment: 28 pages, submitted to PR
The gravitational S-matrix
We investigate the hypothesized existence of an S-matrix for gravity, and
some of its expected general properties. We first discuss basic questions
regarding existence of such a matrix, including those of infrared divergences
and description of asymptotic states. Distinct scattering behavior occurs in
the Born, eikonal, and strong gravity regimes, and we describe aspects of both
the partial wave and momentum space amplitudes, and their analytic properties,
from these regimes. Classically the strong gravity region would be dominated by
formation of black holes, and we assume its unitary quantum dynamics is
described by corresponding resonances. Masslessness limits some powerful
methods and results that apply to massive theories, though a continuation path
implying crossing symmetry plausibly still exists. Physical properties of
gravity suggest nonpolynomial amplitudes, although crossing and causality
constrain (with modest assumptions) this nonpolynomial behavior, particularly
requiring a polynomial bound in complex s at fixed physical momentum transfer.
We explore the hypothesis that such behavior corresponds to a nonlocality
intrinsic to gravity, but consistent with unitarity, analyticity, crossing, and
causality.Comment: 46 pages, 10 figure
A Minimum-Labeling Approach for Reconstructing Protein Networks across Multiple Conditions
The sheer amounts of biological data that are generated in recent years have
driven the development of network analysis tools to facilitate the
interpretation and representation of these data. A fundamental challenge in
this domain is the reconstruction of a protein-protein subnetwork that
underlies a process of interest from a genome-wide screen of associated genes.
Despite intense work in this area, current algorithmic approaches are largely
limited to analyzing a single screen and are, thus, unable to account for
information on condition-specific genes, or reveal the dynamics (over time or
condition) of the process in question. Here we propose a novel formulation for
network reconstruction from multiple-condition data and devise an efficient
integer program solution for it. We apply our algorithm to analyze the response
to influenza infection in humans over time as well as to analyze a pair of ER
export related screens in humans. By comparing to an extant, single-condition
tool we demonstrate the power of our new approach in integrating data from
multiple conditions in a compact and coherent manner, capturing the dynamics of
the underlying processes.Comment: Peer-reviewed and presented as part of the 13th Workshop on
Algorithms in Bioinformatics (WABI2013
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