224 research outputs found
Stability of Transparent Spherically Symmetric Thin Shells and Wormholes
The stability of transparent spherically symmetric thin shells (and
wormholes) to linearized spherically symmetric perturbations about static
equilibrium is examined. This work generalizes and systematizes previous
studies and explores the consequences of including the cosmological constant.
The approach shows how the existence (or not) of a domain wall dominates the
landscape of possible equilibrium configurations.Comment: 12 pages, 7 figures, revtex. Final form to appear in Phys. Rev.
Impact of calibration algorithms on hypoglycaemia detection in newborn infants using continuous glucose monitors
invited, 6-pagesNeonatal hypoglycaemia is a common condition that can cause seizures and serious brain injury in infants. It is diagnosed by blood glucose (BG) measurements, often taken several hours apart. Continuous glucose monitoring (CGM) devices can potentially improve hypoglycaemia detection, while reducing the number of BG measurements. Calibration algorithms convert the sensor signal into the CGM output. Thus, these algorithms can have a direct impact on measures used to quantify excursions from normal glycaemic levels. The aim of this study was to quantify the effects of calibration sensor error and non-linear filtering of CGM data on measures of hypoglycaemia (defined as BG < 2.6mmol/L) in neonates. CGM data was recalibrated using an algorithm that explicitly recognised the high accuracy of BG measurements available in this study. Median filtering was also implemented either before or after recalibration. Results for the entire cohort show an increase in the total number of hypoglycaemic events (161 to 193), duration of hypoglycaemia (2.2 to 2.6% of total data), and hypoglycaemic index (4.9 to 7.1µmol/L) after recalibration. With the addition of filtering, the number of hypoglycaemic events was reduced (193 to 131), with little or no change to the other metrics. These results show how reference sensor error and thus calibration algorithms play a significant role in quantifying hypoglycaemia. In particular, metrics such as counting the number of hypoglycaemic events were particularly sensitive to recalibration and filtering effects. While this conclusion might be expected, its potential impact is quantified here, in this case for at-risk neonates for whom hypoglycaemia carries potential long-term negative outcomes
The Dynamic Insulin Sensitivity and Secretion Test (DISST) - a novel measure of insulin sensitivity
Objective: To validate the methodology for the Dynamic Insulin Sensitivity and Secretion Test (DISST) and to demonstrate its potential in clinical and research settings.
Methods: 123 men and women had routine clinical and biochemical measurements, an oral glucose tolerance test and a DISST. For the DISST, participants were cannulated for blood sampling and bolus administration. Blood samples were drawn at t=0, 10, 15, 25 and 35 minutes for measurement of glucose, insulin and C-peptide. A 10g bolus of intravenous glucose at t=5 minutes and 1U of intravenous insulin immediately after the t=15 minute sample were given. Fifty participants also had a hyperinsulinaemic euglycaemic clamp. Relationships between DISST insulin sensitivity (SI) and the clamp, and both DISST SI and secretion and other metabolic variables were measured.
Results: A Bland-Altman plot showed little bias in the comparison of DISST with the clamp; with DISST underestimating the glucose clamp by 0.1·10-2·mg·l·kg-1·min-1·pmol-1 (90%CI -0.2 to 0). The correlation between SI as measured by DISST and the clamp was 0.82, the c unit for the ROC analysis for the two tests was 0.96. Metabolic variables showed significant correlations with DISST IS, and the second phase of insulin release. DISST also appears able to distinguish different insulin secretion patterns in individuals with identical SI values.
Conclusions: DISST is a simple, dynamic test that compares favourably with the clamp in assessing SI and allows simultaneous assessment of insulin secretion. DISST has the potential to provide even more information about the pathophysiology of diabetes than more complicated tests
Expanding and Collapsing Scalar Field Thin Shell
This paper deals with the dynamics of scalar field thin shell in the
Reissner-Nordstrm geometry. The Israel junction conditions between
Reissner-Nordstrm spacetimes are derived, which lead to the equation
of motion of scalar field shell and Klien-Gordon equation. These equations are
solved numerically by taking scalar field model with the quadratic scalar
potential. It is found that solution represents the expanding and collapsing
scalar field shell. For the better understanding of this problem, we
investigate the case of massless scalar field (by taking the scalar field
potential zero). Also, we evaluate the scalar field potential when is an
explicit function of . We conclude that both massless as well as massive
scalar field shell can expand to infinity at constant rate or collapse to zero
size forming a curvature singularity or bounce under suitable conditions.Comment: 15 pages, 11 figure
No hair for spherical black holes: charged and nonminimally coupled scalar field with self--interaction
We prove three theorems in general relativity which rule out classical scalar
hair of static, spherically symmetric, possibly electrically charged black
holes. We first generalize Bekenstein's no--hair theorem for a multiplet of
minimally coupled real scalar fields with not necessarily quadratic action to
the case of a charged black hole. We then use a conformal map of the geometry
to convert the problem of a charged (or neutral) black hole with hair in the
form of a neutral self--interacting scalar field nonminimally coupled to
gravity to the preceding problem, thus establishing a no--hair theorem for the
cases with nonminimal coupling parameter or . The
proof also makes use of a causality requirement on the field configuration.
Finally, from the required behavior of the fields at the horizon and infinity
we exclude hair of a charged black hole in the form of a charged
self--interacting scalar field nonminimally coupled to gravity for any .Comment: 30 pages, RevTeX. Sec.IV corrected, simplified and shortened.
Corrections to Sec.IIA between Eqs. 2.7 and Eq.2.1. First two paragraphs of
Sec. VC new. To appear Phys. Rev. D, Oct. 15, 199
Popular attitudes to memory, the body, and social identity : the rise of external commemoration in Britain, Ireland, and New England
A comparative analysis of samples of external memorials from burial grounds in Britain, Ireland and New England reveals a widespread pattern of change in monument style and content, and exponential growth in the number of permanent memorials from the 18th century onwards. Although manifested in regionally distinctive styles on which most academic attention has so far been directed, the expansion reflects global changes in social relationships and concepts of memory and the body. An archaeological perspective reveals the importance of external memorials in articulating these changing attitudes in a world of increasing material consumption
A New Class of Exact Hairy Black Hole Solutions
We present a new class of black hole solutions with minimally coupled scalar
field in the presence of a negative cosmological constant. We consider a
one-parameter family of self-interaction potentials parametrized by a
dimensionless parameter . When , we recover the conformally invariant
solution of the Martinez-Troncoso-Zanelli (MTZ) black hole. A non-vanishing
signals the departure from conformal invariance. All solutions are
perturbatively stable for negative black hole mass and they may develop
instabilities for positive mass. Thermodynamically, there is a critical
temperature at vanishing black hole mass, where a higher-order phase transition
occurs, as in the case of the MTZ black hole. Additionally, we obtain a branch
of hairy solutions which undergo a first-order phase transition at a second
critical temperature which depends on and it is higher than the MTZ
critical temperature. As , this second critical temperature diverges.Comment: 18 pages, 6 figures, minor changes, references added, published
versio
Scalar hairy black holes and solitons in asymptotically flat spacetimes
A numerical analysis shows that a class of scalar-tensor theories of gravity
with a scalar field minimally and nonminimally coupled to the curvature allows
static and spherically symmetric black hole solutions with scalar-field hair in
asymptotically flat spacetimes. In the limit when the horizon radius of the
black hole tends to zero, regular scalar solitons are found. The asymptotically
flat solutions are obtained provided that the scalar potential of the
theory is not positive semidefinite and such that its local minimum is also a
zero of the potential, the scalar field settling asymptotically at that
minimum. The configurations for the minimal coupling case, although unstable
under spherically symmetric linear perturbations, are regular and thus can
serve as counterexamples to the no-scalar-hair conjecture. For the nonminimal
coupling case, the stability will be analyzed in a forthcoming paper.Comment: 7 pages, 10 postscript figures, file tex, new postscript figs. and
references added, stability analysis revisite
Three-Dimensional Gravity with Conformal Scalar and Asymptotic Virasoro Algebra
Strominger has derived the Bekenstein-Hawking entropy of the BTZ black hole
using asymptotic Virasoro algebra. We apply Strominger's method to a black hole
solution found by Martinez and Zanelli (MZ). This is a solution of
three-dimensional gravity with a conformal scalar field. The solution is not
, but it is asymptotically ; therefore, it has the asymptotic
Virasoro algebra. We compute the central charge for the theory and compares
Cardy's formula with the Bekenstein-Hawking entropy. It turns out that the
functional form does agree, but the overall numerical coefficient does not.
This is because this approach gives the "maximum possible entropy" for the
numerical coefficient.Comment: 26 pages, LaTeX; v2: minor correction
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