4,600 research outputs found
Degenerate Metric Phase Boundaries
The structure of boundaries between degenerate and nondegenerate solutions of
Ashtekar's canonical reformulation of Einstein's equations is studied. Several
examples are given of such "phase boundaries" in which the metric is degenerate
on one side of a null hypersurface and non-degenerate on the other side. These
include portions of flat space, Schwarzschild, and plane wave solutions joined
to degenerate regions. In the last case, the wave collides with a planar phase
boundary and continues on with the same curvature but degenerate triad, while
the phase boundary continues in the opposite direction. We conjecture that
degenerate phase boundaries are always null.Comment: 16 pages, 2 figures; erratum included in separate file: errors in
section 4, degenerate phase boundary is null without imposing field equation
Degenerate Sectors of the Ashtekar Gravity
This work completes the task of solving locally the Einstein-Ashtekar
equations for degenerate data. The two remaining degenerate sectors of the
classical 3+1 dimensional theory are considered. First, with all densitized
triad vectors linearly dependent and second, with only two independent ones. It
is shown how to solve the Einstein-Ashtekar equations completely by suitable
gauge fixing and choice of coordinates. Remarkably, the Hamiltonian weakly
Poisson commutes with the conditions defining the sectors. The summary of
degenerate solutions is given in the Appendix.Comment: 19 pages, late
Probing Fine-Scale Ionospheric Structure with the Very Large Array Radio Telescope
High resolution (~1 arcminute) astronomical imaging at low frequency (below
150 MHz) has only recently become practical with the development of new
calibration algorithms for removing ionospheric distortions. In addition to
opening a new window in observational astronomy, the process of calibrating the
ionospheric distortions also probes ionospheric structure in an unprecedented
way. Here we explore one aspect of this new type of ionospheric measurement,
the differential refraction of celestial source pairs as a function of their
angular separation. This measurement probes variations in the spatial gradient
of the line-of-sight total electron content (TEC) to 0.001 TECU/km accuracy
over spatial scales of under 10 km to over 100 km. We use data from the VLA
Low-frequency Sky Survey (VLSS; Cohen et al. 2007, AJ 134, 1245), a nearly
complete 74 MHz survey of the entire sky visible to the Very Large Array (VLA)
telescope in Socorro, New Mexico. These data comprise over 500 hours of
observations, all calibrated in a standard way. While ionospheric spatial
structure varies greatly from one observation to the next, when analyzed over
hundreds of hours, statistical patterns become apparent. We present a detailed
characterization of how the median differential refraction depends on source
pair separation, elevation and time of day. We find that elevation effects are
large, but geometrically predictable and can be "removed" analytically using a
"thin-shell" model of the ionosphere. We find significantly greater ionospheric
spatial variations during the day than at night. These diurnal variations
appear to affect the larger angular scales to a greater degree indicating that
they come from disturbances on relatively larger spatial scales (100s of km,
rather than 10s of km).Comment: Accepted for publication by The Astronomical Journa
High-precision Measurements of Ionospheric TEC Gradients with the Very Large Array VHF System
We have used a relatively long, contiguous VHF observation of a bright cosmic
radio source (Cygnus A) with the Very Large Array (VLA) to demonstrate the
capability of this instrument to study the ionosphere. This interferometer, and
others like it, can observe ionospheric total electron content (TEC)
fluctuations on a much wider range of scales than is possible with many other
instruments. We have shown that with a bright source, the VLA can measure
differential TEC values between pairs of antennas (delta-TEC) with an precision
of 0.0003 TECU. Here, we detail the data reduction and processing techniques
used to achieve this level of precision. In addition, we demonstrate techniques
for exploiting these high-precision delta-TEC measurements to compute the TEC
gradient observed by the array as well as small-scale fluctuations within the
TEC gradient surface. A companion paper details specialized spectral analysis
techniques used to characterize the properties of wave-like fluctuations within
this data.Comment: accepted for publication in Radio Scienc
Causal structure and degenerate phase boundaries
Timelike and null hypersurfaces in the degenerate space-times in the Ashtekar
theory are defined in the light of the degenerate causal structure proposed by
Matschull. Using the new definition of null hypersufaces, the conjecture that
the "phase boundary" separating the degenerate space-time region from the
non-degenerate one in Ashtekar's gravity is always null is proved under certain
circumstances.Comment: 13 pages, Revte
Algebraic Approach to Interacting Quantum Systems
We present an algebraic framework for interacting extended quantum systems to
study complex phenomena characterized by the coexistence and competition of
different states of matter. We start by showing how to connect different
(spin-particle-gauge) {\it languages} by means of exact mappings (isomorphisms)
that we name {\it dictionaries} and prove a fundamental theorem establishing
when two arbitrary languages can be connected. These mappings serve to unravel
symmetries which are hidden in one representation but become manifest in
another. In addition, we establish a formal link between seemingly unrelated
physical phenomena by changing the language of our model description. This link
leads to the idea of {\it universality} or equivalence. Moreover, we introduce
the novel concept of {\it emergent symmetry} as another symmetry guiding
principle. By introducing the notion of {\it hierarchical languages}, we
determine the quantum phase diagram of lattice models (previously unsolved) and
unveil hidden order parameters to explore new states of matter. Hierarchical
languages also constitute an essential tool to provide a unified description of
phases which compete and coexist. Overall, our framework provides a simple and
systematic methodology to predict and discover new kinds of orders. Another
aspect exploited by the present formalism is the relation between condensed
matter and lattice gauge theories through quantum link models. We conclude
discussing applications of these dictionaries to the area of quantum
information and computation with emphasis in building new models of computation
and quantum programming languages.Comment: 44 pages, 14 psfigures. Advances in Physics 53, 1 (2004
The prevalence and risk of immune restoration disease in HIV-infected patients treated with highly active antiretroviral therapy
Background It is becoming increasingly clear that, during successful highly active antiretroviral therapy (HAART), a proportion of treated patients develop opportunistic infections (OIs), referred to in this setting as immune restoration disease (IRD). We examined the risk of developing IRD in HAART-treated HIV-infected patients. Methods A retrospective study of a cohort including all 389 patients treated with HAART between I January 1998 and 31 May 2004 in our HIV unit was performed to evaluate the occurrence of and risk factors for IRD during HAART. Baseline and follow-up values of CD4 T-cell counts and plasma viral loads (pVLs) were compared to assess the success of HAART. Results During successful HAART (significant increase in CD4 T-cell counts and decrease in pVL), at least one IRD episode occurred in 65 patients (16.7%). The median time to IRD was 4.6 months (range 212 months). IRDs included dermatomal herpes zoster (26 patients), pulmonary tuberculosis (four patients), tuberculous exudative pericarditis (two patients), tuberculous lymphadenitis (two patients), cerebral toxoplasmosis (one patient), progressive multifocal leucoencephalopathy (PML) (one patient), inflamed molluscum (one patient), inflamed Candida albicans angular cheilitis (three patients), genital herpes simplex (two patients), tinea corporis (two patients), cytomegalovirus (CMV) retinitis (two patients), CMV vitritis (one patient) and hepatitis B (three patients) or C (fifteen patients). A baseline CD4 T-cell count below 100 cells/mu L was shown to be the single predictor [odds ratio (OR) 2.5, 95% confidence interval (CI) 0.9-6.4] of IRD, while a CD4 T-cell count increase to gt 400 cells/mu L, but not undetectable pVL, was a negative predictor of IRD (OR 0.3, 95% CI 0.1-0.8). Conclusions To avoid IRD in advanced patients, HAART should be initiated before the CD4 T-cell count falls below 100 cells/mu L
Experimental modulation of capsule size in Cryptococcus neoformans
Experimental modulation of capsule size is an important technique for the study of the virulence of the encapsulated pathogen Cryptococcus neoformans. In this paper, we summarize the techniques available for experimental modulation of capsule size in this yeast and describe improved methods to induce capsule size changes. The response of the yeast to the various stimuli is highly dependent on the cryptococcal strain. A high CO(2) atmosphere and a low iron concentration have been used classically to increase capsule size. Unfortunately, these stimuli are not reliable for inducing capsular enlargement in all strains. Recently we have identified new and simpler conditions for inducing capsule enlargement that consistently elicited this effect. Specifically, we noted that mammalian serum or diluted Sabouraud broth in MOPS buffer pH 7.3 efficiently induced capsule growth. Media that slowed the growth rate of the yeast correlated with an increase in capsule size. Finally, we summarize the most commonly used media that induce capsule growth in C. neoformans
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