645 research outputs found
FRW Universe Models in Conformally Flat Spacetime Coordinates. II: Universe models with negative and vanishing spatial curvature
We deduce general expressions for the line element of universe models with
negative and vanishing spatial curvature described by conformally flat
spacetime coordinates. The empty Milne universe model and models with dust,
radiation and vacuum energy are exhibited. Discussing the existence of particle
horizons we show that there is continual creation of space, matter and energy
when conformal time is used in Friedmann-Robertson-Walker models with negative
spatial curvature.Comment: 25 pages, 12 figure
Weak field limit of Reissner-Nordstrom black hole lensing
We study gravitational lensing by a Reissner-Nordstrom (RN) black hole in the
weak field limit. We obtain the basic equations for the deflection angle and
time delay and find analytical expressions for the positions and amplifications
of the primary and secondary images. Due to a net positive charge, the
separation between images increases, but no change in the total magnification
occurs.Comment: 8 pages, to appear in Phys. Rev.
Computational advances in gravitational microlensing: a comparison of CPU, GPU, and parallel, large data codes
To assess how future progress in gravitational microlensing computation at
high optical depth will rely on both hardware and software solutions, we
compare a direct inverse ray-shooting code implemented on a graphics processing
unit (GPU) with both a widely-used hierarchical tree code on a single-core CPU,
and a recent implementation of a parallel tree code suitable for a CPU-based
cluster supercomputer. We examine the accuracy of the tree codes through
comparison with a direct code over a much wider range of parameter space than
has been feasible before. We demonstrate that all three codes present
comparable accuracy, and choice of approach depends on considerations relating
to the scale and nature of the microlensing problem under investigation. On
current hardware, there is little difference in the processing speed of the
single-core CPU tree code and the GPU direct code, however the recent plateau
in single-core CPU speeds means the existing tree code is no longer able to
take advantage of Moore's law-like increases in processing speed. Instead, we
anticipate a rapid increase in GPU capabilities in the next few years, which is
advantageous to the direct code. We suggest that progress in other areas of
astrophysical computation may benefit from a transition to GPUs through the use
of "brute force" algorithms, rather than attempting to port the current best
solution directly to a GPU language -- for certain classes of problems, the
simple implementation on GPUs may already be no worse than an optimised
single-core CPU version.Comment: 11 pages, 4 figures, accepted for publication in New Astronom
PAPPA: Primordial Anisotropy Polarization Pathfinder Array
The Primordial Anisotropy Polarization Pathfinder Array (PAPPA) is a
balloon-based instrument to measure the polarization of the cosmic microwave
background and search for the signal from gravity waves excited during an
inflationary epoch in the early universe. PAPPA will survey a 20 x 20 deg patch
at the North Celestial Pole using 32 pixels in 3 passbands centered at 89, 212,
and 302 GHz. Each pixel uses MEMS switches in a superconducting microstrip
transmission line to combine the phase modulation techniques used in radio
astronomy with the sensitivity of transition-edge superconducting bolometers.
Each switched circuit modulates the incident polarization on a single detector,
allowing nearly instantaneous characterization of the Stokes I, Q, and U
parameters. We describe the instrument design and status.Comment: 12 pages, 9 figures. Proceedings of the Fundamental Physics With CMB
workshop, UC Irvine, March 23-25, 2006, to be published in New Astronomy
Review
Strong Gravitational Lensing and Dark Energy Complementarity
In the search for the nature of dark energy most cosmological probes measure
simple functions of the expansion rate. While powerful, these all involve
roughly the same dependence on the dark energy equation of state parameters,
with anticorrelation between its present value w_0 and time variation w_a.
Quantities that have instead positive correlation and so a sensitivity
direction largely orthogonal to, e.g., distance probes offer the hope of
achieving tight constraints through complementarity. Such quantities are found
in strong gravitational lensing observations of image separations and time
delays. While degeneracy between cosmological parameters prevents full
complementarity, strong lensing measurements to 1% accuracy can improve
equation of state characterization by 15-50%. Next generation surveys should
provide data on roughly 10^5 lens systems, though systematic errors will remain
challenging.Comment: 7 pages, 5 figure
Scaling algebras and pointlike fields: A nonperturbative approach to renormalization
We present a method of short-distance analysis in quantum field theory that
does not require choosing a renormalization prescription a priori. We set out
from a local net of algebras with associated pointlike quantum fields. The net
has a naturally defined scaling limit in the sense of Buchholz and Verch; we
investigate the effect of this limit on the pointlike fields. Both for the
fields and their operator product expansions, a well-defined limit procedure
can be established. This can always be interpreted in the usual sense of
multiplicative renormalization, where the renormalization factors are
determined by our analysis. We also consider the limits of symmetry actions. In
particular, for suitable limit states, the group of scaling transformations
induces a dilation symmetry in the limit theory.Comment: minor changes and clarifications; as to appear in Commun. Math.
Phys.; 37 page
Gene expression time delays & Turing pattern formation systems
The incorporation of time delays can greatly affect the behaviour of partial differential equations and dynamical systems. In addition, there is evidence that time delays in gene expression due to transcription and translation play an important role in the dynamics of cellular systems. In this paper, we investigate the effects of incorporating gene expression time delays into a one-dimensional putative reaction diffusion pattern formation mechanism on both stationary domains and domains with spatially uniform exponential growth. While oscillatory behaviour is rare, we find that the time taken to initiate and stabilise patterns increases dramatically as the time delay is increased. In addition, we observe that on rapidly growing domains the time delay can induce a failure of the Turing instability which cannot be predicted by a naive linear analysis of the underlying equations about the homogeneous steady state. The dramatic lag in the induction of patterning, or even its complete absence on occasions, highlights the importance of considering explicit gene expression time delays in models for cellular reaction diffusion patterning
Zanamivir susceptibility monitoring and characterization of influenza virus clinical isolates obtained during phase II clinical efficacy studies
Zanamivir is a highly selective neuraminidase (NA) inhibitor with
demonstrated clinical efficacy against influenza A and B virus infections.
In phase II clinical efficacy trials (NAIB2005 and NAIB2008), virological
substudies showed mean reductions in virus shedding after 24 h of
treatment of 1.5 to 2.0 log(10) 50% tissue culture infective doses
compared to a placebo, with no reemergence of virus after the completion
of therapy. Paired isolates (n = 41) obtained before and during therapy
with zanamivir demonstrated no shifts in susceptibility to zanamivir when
measured by NA assays, although for a few isolates NA activity was too low
to evaluate. In plaque reduction assays in MDCK cells, the susceptibility
of isolates to zanamivir was extremely variable even at baseline and did
not correlate with the speed of resolution of virus shedding. Isolates
with apparent limited susceptibility to zanamivir by plaque reduction
proved highly susceptible in vivo in the ferret model. Further sequence
analysis of paired isolates revealed no changes in the hemagglutinin and
NA genes in the majority of isolates. The few changes observed were all
natural variants. No amino acid changes that had previously been
identified in vitro as being involved with reduced susceptibility to
zanamivir were observed. These studies highlighted problems associated
with monitoring susceptibility to NA inhibitors in the clinic, in that no
reliable cell-based assay is available. At present the NA assay is the
best available predictor of susceptibility to NA inhibitors in vivo, as
measured in the validated ferret model of infection
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