201 research outputs found
Critical Collapse in Einstein-Gauss-Bonnet Gravity in Five and Six Dimensions
Einstein-Gauss-Bonnet gravity (EGB) provides a natural higher dimensional and
higher order curvature generalization of Einstein gravity. It contains a new,
presumably microscopic, length scale that should affect short distance
properties of the dynamics, such as Choptuik scaling. We present the results of
a numerical analysis in generalized flat slice co-ordinates of self-gravitating
massless scalar spherical collapse in five and six dimensional EGB gravity near
the threshold of black hole formation. Remarkably, the behaviour is universal
(i.e. independent of initial data) but qualitatively different in five and six
dimensions. In five dimensions there is a minimum horizon radius, suggestive of
a first order transition between black hole and dispersive initial data. In six
dimensions no radius gap is evident. Instead, below the GB scale there is a
change in the critical exponent and echoing period.Comment: 21 pages, 39 figures, a couple of references and two new figures
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Hamiltonian Formulation of Scalar Field Collapse in Einstein Gauss Bonnet Gravity
We compute the Hamiltonian for spherically symmetric scalar field collapse in
Einstein-Gauss-Bonnet gravity in D dimensions using slicings that are regular
across future horizons. We first reduce the Lagrangian to two dimensions using
spherical symmetry. We then show that choosing the spatial coordinate to be a
function of the areal radius leads to a relatively simple Hamiltonian
constraint whose gravitational part is the gradient of the generalized mass
function. Next we complete the gauge fixing such that the metric is the
Einstein-Gauss-Bonnet generalization of non-static Painleve-Gullstrand
coordinates. Finally, we derive the resultant reduced equations of motion for
the scalar field. These equations are suitable for use in numerical simulations
of spherically symmetric scalar field collapse in Gauss-Bonnet gravity and can
readily be generalized to other matter fields minimally coupled to gravity.Comment: 14 pages, 0 figure
Social response to population change and migration: the impact of population change on individuals and institutions
Population change over the last 40 years has greatly redistributed Midwest populations. Migration has long been a social response to change—both to changing capacities in the agricultural system and to socio-cultural attractions and opportunities in the urban-industrial areas. Some of the surplus agricultural population has moved to cities and suburbs. The result is a selective dismembering of many communities and an inordinate growth of others.https://lib.dr.iastate.edu/specialreports/1037/thumbnail.jp
Modified general relativity as a model for quantum gravitational collapse
We study a class of Hamiltonian deformations of the massless
Einstein-Klein-Gordon system in spherical symmetry for which the Dirac
constraint algebra closes. The system may be regarded as providing effective
equations for quantum gravitational collapse. Guided by the observation that
scalar field fluxes do not follow metric null directions due to the
deformation, we find that the equations take a simple form in characteristic
coordinates. We analyse these equations by a unique combination of numerical
methods and find that Choptuik's mass scaling law is modified by a mass gap as
well as jagged oscillations. Furthermore, the results are universal with
respect to different initial data profiles and robust under changes of the
deformation.Comment: 22 pages, 4 figure
A New Family of Covariate-Adjusted Response Adaptive Designs and their Asymptotic Properties
It is often important to incorporating covariate information in the design of
clinical trials. In literature, there are many designs of using stratification
and covariate-adaptive randomization to balance on certain known covariate.
Recently Zhang, Hu, Cheung and Chan (2007) have proposed a family of
covariate-adjusted response-adaptive (CARA) designs and studied their
asymptotic properties. However, these CARA designs often have high
variabilities. In this paper, we propose a new family of covariate-adjusted
response-adaptive (CARA) designs. We show that the new designs have smaller
variabilities and therefore more efficient
What Is the Role of Astrocyte Calcium in Neurophysiology?
Astrocytes comprise approximately half of the volume of the adult mammalian brain and are the primary neuronal structural and trophic supportive elements. Astrocytes are organized into distinct nonoverlapping domains and extend elaborate and dense fine processes that interact intimately with synapses and cerebrovasculature. The recognition in the mid 1990s that astrocytes undergo elevations in intracellular calcium concentration following activation of G protein-coupled receptors by synaptically released neurotransmitters demonstrated not only that astrocytes display a form of excitability but also that astrocytes may be active participants in brain information processing. The roles that astrocytic calcium elevations play in neurophysiology and especially in modulation of neuronal activity have been intensely researched in recent years. This review will summarize the current understanding of the function of astrocytic calcium signaling in neurophysiological processes and discuss areas where the role of astrocytes remains controversial and will therefore benefit from further study
Statistical issues in the use of dynamic allocation methods for balancing baseline covariates
Social and reproductive physiology and behavior of the Neotropical cichlid fish Cichlasoma dimerus under laboratory conditions
‘We are not calling her Italian’: narratives and images of ethnic incorporation in Isa Miranda’s American persona.
This article discusses the impact of Hollywood standards of homogenisation on Isa Miranda’s cosmopolitan appeal during her work for Paramount. The article suggests that Miranda’s American image was constructed in an attempt to establish her as a Marlene Dietrich type in order to defuse the potential threat represented by her ethnic Otherness; meanwhile, the Italian actress was framed within film narratives that played out (albeit indirectly) an idealised conception of successful American assimilation of European immigrants. In the process, Miranda was
visually constructed through images that effectively ‘whitened’ her, and her Italianness was thus displaced onto an ideal of Northern European whiteness that bespoke a desire to reassert whiteness as the norm in 1930s America
Effect of exercise on fluoride metabolism in adult humans: a pilot study
An understanding of all aspects of fluoride metabolism is critical to identify its biological effects and avoid fluoride toxicity in humans. Fluoride metabolism and subsequently its body retention may be affected by physiological responses to acute exercise. This pilot study investigated the effect of exercise on plasma fluoride concentration, urinary fluoride excretion and fluoride renal clearance following no exercise and three exercise intensity conditions in nine healthy adults after taking a 1-mg Fluoride tablet. After no, light, moderate and vigorous exercise, respectively, the mean (SD) baseline-adjusted i) plasma fluoride concentration was 9.6(6.3), 11.4(6.3), 15.6(7.7) and 14.9(10.0) ng/ml; ii) rate of urinary fluoride excretion over 0–8 h was 46(15), 44(22), 34(17) and 36(17) μg/h; and iii) rate of fluoride renal clearance was 26.5(9.0), 27.2(30.4), 13.1(20.4) and 18.3(34.9) ml/min. The observed trend of a rise in plasma fluoride concentration and decline in rate of fluoride renal clearance with increasing exercise intensity needs to be investigated in a larger trial. This study, which provides the first data on the effect of exercise with different intensities on fluoride metabolism in humans, informs sample size planning for any subsequent definitive trial, by providing a robust estimate of the variability of the effect
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