646 research outputs found
Correlation functions in a c=1 boundary conformal field theory
We obtain exact results for correlation functions of primary operators in the
two-dimensional conformal field theory of a scalar field interacting with a
critical periodic boundary potential. Amplitudes involving arbitrary bulk
discrete primary fields are given in terms of SU(2) rotation coefficients while
boundary amplitudes involving discrete boundary fields are independent of the
boundary interaction. Mixed amplitudes involving both bulk and boundary
discrete fields can also be obtained explicitly. Two- and three-point boundary
amplitudes involving fields at generic momentum are determined, up to
multiplicative constants, by the band spectrum in the open-string sector of the
theory.Comment: 33 pages, 6 figure
Trends in prevalence of substance use among Icelandic adolescents, 1995–2006
BACKGROUND: Adolescent substance use continues to be of great global public health concern in many countries with advanced economies. Previous research has shown that substance use among 15-16 year-old-youth has increased in many European countries in recent years. The aim of this study was to examine trends in prevalence of daily smoking, alcohol intoxication, and illicit substance use among Icelandic adolescents.
METHODS: Repeated-measures, population-based cross-sectional surveys of between 3,100 and 3,900 10th-grade students who participated in the annual Youth of Iceland studies were analyzed, with response rates of between 80% and 90%.
RESULTS: The prevalence of daily smoking, alcohol intoxication, and illicit substance use was at a peak in 1998, with almost 23% having reported daily smoking, 42% having reported becoming intoxicated at least once during the last 30 days, and over 17% having used hashish once or more often in their lifetime. By 2006, daily smoking had declined to 12%, having become intoxicated once or more often during the last 30 days to 25%, and having ever used hashish declined to 9%.
CONCLUSION: The prevalence of substance use among Icelandic 10th graders declined substantially from 1995 to 2006. Proportions of adolescents who smoke cigarettes, had become intoxicated during the last 30 days, as well as those admitting to hashish use all decreased to a great deal during the period under study. The decline in prevalence of adolescent substance use in Iceland is plausibly the result of local community collaboration where researchers, policy makers and practitioners who work with young people have combined their efforts
Cirrus Cloud Seeding has Potential to Cool Climate
Cirrus clouds, thin ice clouds in the upper troposphere, have a net warming effect on Earth s climate. Consequently, a reduction in cirrus cloud amount or optical thickness would cool the climate. Recent research indicates that by seeding cirrus clouds with particles that promote ice nucleation, their lifetimes and coverage could be reduced. We have tested this hypothesis in a global climate model with a state-of-the-art representation of cirrus clouds and find that cirrus cloud seeding has the potential to cancel the entire warming caused by human activity from pre-industrial times to present day. However, the desired effect is only obtained for seeding particle concentrations that lie within an optimal range. With lower than optimal particle concentrations, a seeding exercise would have no effect. Moreover, a higher than optimal concentration results in an over-seeding that could have the deleterious effect of prolonging cirrus lifetime and contributing to global warming
When is a randomised controlled trial health equity relevant? Development and validation of a conceptual framework
Background Randomised controlled trials can provide evidence relevant to assessing the equity impact of an intervention, but such information is often poorly reported. We describe a conceptual framework to identify health equity-relevant randomised trials with the aim of improving the design and reporting of such trials. Methods An interdisciplinary and international research team engaged in an iterative consensus building process to develop and refine the conceptual framework via face-to-face meetings, teleconferences and email correspondence, including findings from a validation exercise whereby two independent reviewers used the emerging framework to classify a sample of randomised trials. Results A randomised trial can usefully be classified as ‘health equity relevant’ if it assesses the effects of an intervention on the health or its determinants of either individuals or a population who experience ill health due to disadvantage defined across one or more social determinants of health. Health equity-relevant randomised trials can either exclusively focus on a single population or collect data potentially useful for assessing differential effects of the intervention across multiple populations experiencing different levels or types of social disadvantage. Trials that are not classified as ‘health equity relevant’ may nevertheless provide information that is indirectly relevant to assessing equity impact, including information about individual level variation unrelated to social disadvantage and potentially useful in secondary modelling studies. Conclusion The conceptual framework may be used to design and report randomised trials. The framework could also be used for other study designs to contribute to the evidence base for improved health equity
Social correlates of cigarette smoking among Icelandic adolescents: A population-based cross-sectional study
BACKGROUND: Previous research has shown that between 80 and 90 percent of adult
smokers report having started smoking before 18 years of age. Several studies
have revealed that multiple social factors influence the likelihood of smoking
during adolescence, the period during which the onset of smoking usually occurs.
To better understand the social mechanisms that influence adolescent smoking, we
analyzed the relationship and relative importance of a broad spectrum of social
variables in adolescent smoking in Iceland, a Nordic country with high per-capita
income.
METHODS: We used cross-sectional data from 7,430 14- to 16 year-old
students (approximately 81% of all Icelanders in these age cohorts) in the 2006
Youth in Iceland study. The Youth in Iceland studies are designed to investigate
the role of several cognitive, behavioral, and social factors in the lives of
adolescents, and the data collected are used to inform the design,
implementation, and evaluation of substance use prevention programs that are
being developed by Icelandic social scientists, policy makers, and practitioners.
RESULTS: Our analysis revealed that friends' smoking behavior and attitude toward
smoking were strongly associated with adolescent smoking and other tobacco use,
as well as alcohol consumption during the previous 30 days. Main protective
factors were parent's perceived attitude toward smoking, the quantity of time
spent with parents, absence of serious verbal conflict between parents and
adolescents, and participation in physical activity. Family structure was related
to adolescent smoking to a small extent, but other background factors were not.
CONCLUSION: We conclude that multiple social factors are related to adolescent
smoking. Parents and other primary preventive agents need to be informed about
the complicated nature of the adolescent social world in order to maximize their
impact.Icelandic Alcohol and Drug Prevention CommitteeIcelandic Red CrossCity of ReykjavikSports and Recreational Committee of Reykjavik to the Icelandic Centre for Social Research and AnalysisIcelandic Centre for Research (RANNÍS)Publishe
q-Deformed de Sitter/Conformal Field Theory Correspondence
Unitary principal series representations of the conformal group appear in the
dS/CFT correspondence. These are infinite dimensional irreducible
representations, without highest weights. In earlier work of Guijosa and the
author it was shown for the case of two-dimensional de Sitter, there was a
natural q-deformation of the conformal group, with q a root of unity, where the
unitary principal series representations become finite-dimensional cyclic
unitary representations. Formulating a version of the dS/CFT correspondence
using these representations can lead to a description with a finite-dimensional
Hilbert space and unitary evolution. In the present work, we generalize to the
case of quantum-deformed three-dimensional de Sitter spacetime and compute the
entanglement entropy of a quantum field across the cosmological horizon.Comment: 18 pages, 2 figures, revtex, (v2 reference added
A biophysical model of prokaryotic diversity in geothermal hot springs
Recent field investigations of photosynthetic bacteria living in geothermal
hot spring environments have revealed surprisingly complex ecosystems, with an
unexpected level of genetic diversity. One case of particular interest involves
the distribution along hot spring thermal gradients of genetically distinct
bacterial strains that differ in their preferred temperatures for reproduction
and photosynthesis. In such systems, a single variable, temperature, defines
the relevant environmental variation. In spite of this, each region along the
thermal gradient exhibits multiple strains of photosynthetic bacteria adapted
to several distinct thermal optima, rather than the expected single thermal
strain adapted to the local environmental temperature. Here we analyze
microbiology data from several ecological studies to show that the thermal
distribution field data exhibit several universal features independent of
location and specific bacterial strain. These include the distribution of
optimal temperatures of different thermal strains and the functional dependence
of the net population density on temperature. Further, we present a simple
population dynamics model of these systems that is highly constrained by
biophysical data and by physical features of the environment. This model can
explain in detail the observed diversity of different strains of the
photosynthetic bacteria. It also reproduces the observed thermal population
distributions, as well as certain features of population dynamics observed in
laboratory studies of the same organisms
A new twist on dS/CFT
We stress that the dS/CFT correspondence should be formulated using unitary
principal series representations of the de Sitter isometry group/conformal
group, rather than highest-weight representations as originally proposed. These
representations, however, are infinite-dimensional, and so do not account for
the finite gravitational entropy of de Sitter space in a natural way. We then
propose to replace the classical isometry group by a q-deformed version. This
is carried out in detail for two-dimensional de Sitter and we find that the
unitary principal series representations deform to finite-dimensional unitary
representations of the quantum group. We believe this provides a promising
microscopic framework to account for the Bekenstein-Hawking entropy of de
Sitter space.Comment: 21 pages, revtex, v2 references adde
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