2,340 research outputs found
Beyond the Small-Angle Approximation For MBR Anisotropy from Seeds
In this paper we give a general expression for the energy shift of massless
particles travelling through the gravitational field of an arbitrary matter
distribution as calculated in the weak field limit in an asymptotically flat
space-time. It is {\it not} assumed that matter is non-relativistic. We
demonstrate the surprising result that if the matter is illuminated by a
uniform brightness background that the brightness pattern observed at a given
point in space-time (modulo a term dependent on the oberver's velocity) depends
only on the matter distribution on the observer's past light-cone. These
results apply directly to the cosmological MBR anisotropy pattern generated in
the immediate vicinity of of an object like a cosmic string or global texture.
We apply these results to cosmic strings, finding a correction to previously
published results for in the small-angle approximation. We also derive the
full-sky anisotropy pattern of a collapsing texture knot.Comment: 23 pages, FERMILAB-Pub-94/047-
A Causal Source which Mimics Inflation
How unique are the inflationary predictions for the cosmic microwave
anisotropy pattern? In this paper, it is asked whether an arbitrary causal
source for perturbations in the standard hot big bang could effectively mimic
the predictions of the simplest inflationary models. A surprisingly simple
example of a `scaling' causal source is found to closely reproduce the
inflationary predictions. This letter extends the work of a previous paper
(ref. 6) to a full computation of the anisotropy pattern, including the Sachs
Wolfe integral. I speculate on the possible physics behind such a source.Comment: 4 pages, RevTex, 3 figure
The Doppler Peaks from Cosmic Texture
We compute the angular power spectrum of temperature anisotropies on the
microwave sky in the cosmic texture theory, with standard recombination
assumed. The spectrum shows `Doppler' peaks analogous to those in scenarios
based on primordial adiabatic fluctuations such as `standard CDM', but at quite
different angular scales. There appear to be excellent prospects for using this
as a discriminant between inflationary and cosmic defect theories.Comment: 14 pages, latex, 3 figures, compressed and uuencoded, replaced
version has minor typographical correction
Cosmic Strings in an Open Universe with Baryonic and Non-Baryonic Dark Matter
We study the effects of cosmic strings on structure formation in open
universes. We calculate the power spectrum of density perturbations for two
class of models: one in which all the dark matter is non baryonic (CDM) and one
in which it is all baryonic (BDM). Our results are compared to the 1 in 6 IRAS
QDOT power spectrum. The best candidates are then used to estimate , the
energy per unit length of the string network. Some comments are made on
mechanisms by which structures are formed in the two theories.Comment: uu-encoded compressed tar of postscript files, Imperial/TP/94-95/0
Doppler peaks from active perturbations
We examine how the qualitative structure of the Doppler peaks in the angular
power spectrum of the cosmic microwave anisotropy depends on the fundamental
nature of the perturbations which produced them. The formalism of Hu and
Sugiyama is extended to treat models with cosmic defects. We discuss how
perturbations can be ``active'' or ``passive'' and ``incoherent'' or
``coherent'', and show how causality and scale invariance play rather different
roles in these various cases. We find that the existence of secondary Doppler
peaks and the rough placing of the primary peak unambiguously reflect these
basic properties.Comment: uufile, 8pages, 3 figures. Now available at
http://euclid.tp.ph/Papers/index.html; Changes: URL added, Eqn. (8) expanded,
grant numbers include
The growth of linear perturbations in generic defect models for structure formation
We study the growth of linear perturbations induced by a generic causal
scaling source as a function of the cosmological parameters ,
and . We show that for wavenumbers k \gsim 0.01 h/Mpc the
spectrum of density and velocity perturbations scale in a similar way to that
found in inflationary models with primordial perturbations. We show that this
result is independent of the more or less incoherent nature of the source, the
small scale power spectrum of the source and of deviations from scaling which
naturally occur at late times if .Comment: 4 pages, 2 figure
Studies on the host range of Piricularia oryzae Cav. causing blast disease of rice
This article does not have an abstract
Synthesis of α-glucan in mycobacteria involves a hetero-octameric complex of trehalose synthase TreS and Maltokinase Pep2
Recent evidence established that the cell envelope of Mycobacterium tuberculosis, the bacillus causing tuberculosis (TB), is coated by an α-glucan-containing capsule that has been implicated in persistence in a mouse infection model. As one of three known metabolic routes to α-glucan in mycobacteria, the cytoplasmic GlgE-pathway converts trehalose to α(1 → 4),α(1 → 6)-linked glucan in 4 steps. Whether individual reaction steps, catalyzed by trehalose synthase TreS, maltokinase Pep2, and glycosyltransferases GlgE and GlgB, occur independently or in a coordinated fashion is not known. Here, we report the crystal structure of M. tuberculosis TreS, and show by small-angle X-ray scattering and analytical ultracentrifugation that TreS forms tetramers in solution. Together with Pep2, TreS forms a hetero-octameric complex, and we demonstrate that complex formation markedly accelerates maltokinase activity of Pep2. Thus, complex formation may act as part of a regulatory mechanism of the GlgE pathway, which overall must avoid accumulation of toxic pathway intermediates, such as maltose-1-phosphate, and optimize the use of scarce nutrients
Correlations in Cosmic String Networks
We investigate scaling and correlations of the energy and momentum in an
evolving network of cosmic strings in Minkowski space. These quantities are of
great interest, as they must be understood before accurate predictions for the
power spectra of the perturbations in the matter and radiation in the early
Universe can be made. We argue that Minkowski space provides a reasonable
approximation to a Friedmann background for string dynamics and we use our
results to construct a simple model of the network, in which it is considered
to consist of randomly placed segments moving with random velocities. This
model works well in accounting for features of the two-time correlation
functions, and even better for the power spectra.Comment: 20pp Plain LaTeX, 11 EPS figures, uses epsf.st
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Predicting the Impact of Vaccination on the Transmission Dynamics of Typhoid in South Asia: A Mathematical Modeling Study
Background: Modeling of the transmission dynamics of typhoid allows for an evaluation of the potential direct and indirect effects of vaccination; however, relevant typhoid models rooted in data have rarely been deployed. Methodology/Principal Findings We developed a parsimonious age-structured model describing the natural history and immunity to typhoid infection. The model was fit to data on culture-confirmed cases of typhoid fever presenting to Christian Medical College hospital in Vellore, India from 2000–2012. The model was then used to evaluate the potential impact of school-based vaccination strategies using live oral, Vi-polysaccharide, and Vi-conjugate vaccines. The model was able to reproduce the incidence and age distribution of typhoid cases in Vellore. The basic reproductive number (R0) of typhoid was estimated to be 2.8 in this setting. Vaccination was predicted to confer substantial indirect protection leading to a decrease in the incidence of typhoid in the short term, but (intuitively) typhoid incidence was predicted to rebound 5–15 years following a one-time campaign. Conclusions/Significance: We found that model predictions for the overall and indirect effects of vaccination depend strongly on the role of chronic carriers in transmission. Carrier transmissibility was tentatively estimated to be low, consistent with recent studies, but was identified as a pivotal area for future research. It is unlikely that typhoid can be eliminated from endemic settings through vaccination alone
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