107 research outputs found
Localization of gravity on a de Sitter thick braneworld without scalar fields
In this work we present a simple thick braneworld model that is generated by
an intriguing interplay between a 5D cosmological constant with a de Sitter
metric induced in the 3-brane without the inclusion of scalar fields. We show
that 4D gravity is localized on this brane, provide analytic expressions for
the massive Kaluza-Klein (KK) fluctuation modes and also show that the spectrum
of metric excitations displays a mass gap. We finally present the corrections
to Newton's law due to these massive modes. This model has no naked
singularities along the fifth dimension despite the existence of a mass gap in
the graviton spectrum as it happens in thick branes with 4D Poincare symmetry,
providing a simple model with very good features: the curvature is completely
smooth along the fifth dimension, it localizes 4D gravity and the spectrum of
gravity fluctuations presents a mass gap, a fact that rules out the existence
of phenomenologically dangerous ultralight KK excitations in the model. We
finally present our solution as a limit of scalar thick branes.Comment: 11 pages in latex, no figures, title and abstract changed, a new
section and some references adde
Stochastic Gravity: Theory and Applications
Whereas semiclassical gravity is based on the semiclassical Einstein equation
with sources given by the expectation value of the stress-energy tensor of
quantum fields, stochastic semiclassical gravity is based on the
Einstein-Langevin equation, which has in addition sources due to the noise
kernel.In the first part, we describe the fundamentals of this new theory via
two approaches: the axiomatic and the functional. In the second part, we
describe three applications of stochastic gravity theory. First, we consider
metric perturbations in a Minkowski spacetime: we compute the two-point
correlation functions for the linearized Einstein tensor and for the metric
perturbations. Second, we discuss structure formation from the stochastic
gravity viewpoint. Third, we discuss the backreaction of Hawking radiation in
the gravitational background of a quasi-static black hole.Comment: 75 pages, no figures, submitted to Living Reviews in Relativit
Stochastic Gravity: Theory and Applications
Whereas semiclassical gravity is based on the semiclassical Einstein equation
with sources given by the expectation value of the stress-energy tensor of
quantum fields, stochastic semiclassical gravity is based on the
Einstein-Langevin equation, which has in addition sources due to the noise
kernel. In the first part, we describe the fundamentals of this new theory via
two approaches: the axiomatic and the functional. In the second part, we
describe three applications of stochastic gravity theory. First, we consider
metric perturbations in a Minkowski spacetime, compute the two-point
correlation functions of these perturbations and prove that Minkowski spacetime
is a stable solution of semiclassical gravity. Second, we discuss structure
formation from the stochastic gravity viewpoint. Third, we discuss the
backreaction of Hawking radiation in the gravitational background of a black
hole and describe the metric fluctuations near the event horizon of an
evaporating black holeComment: 100 pages, no figures; an update of the 2003 review in Living Reviews
in Relativity gr-qc/0307032 ; it includes new sections on the Validity of
Semiclassical Gravity, the Stability of Minkowski Spacetime, and the Metric
Fluctuations of an Evaporating Black Hol
Alternative methods to analyse the impact of HIV mutations on virological response to antiviral therapy
<p>Abstract</p> <p>Background</p> <p>Principal component analysis (PCA) and partial least square (PLS) regression may be useful to summarize the HIV genotypic information. Without pre-selection each mutation presented in at least one patient is considered with a different weight. We compared these two strategies with the construction of a usual genotypic score.</p> <p>Methods</p> <p>We used data from the ANRS-CO3 Aquitaine Cohort Zephir sub-study. We used a subset of 87 patients with a complete baseline genotype and plasma HIV-1 RNA available at baseline and at week 12. PCA and PLS components were determined with all mutations that had prevalences >0. For the genotypic score, mutations were selected in two steps: 1) p-value < 0.01 in univariable analysis and prevalences between 10% and 90% and 2) backwards selection procedure based on the Cochran-Armitage Test. The predictive performances were compared by means of the cross-validated area under the receiver operating curve (AUC).</p> <p>Results</p> <p>Virological failure was observed in 46 (53%) patients at week 12. Principal components and PLS components showed a good performance for the prediction of virological response in HIV infected patients. The cross-validated AUCs for the PCA, PLS and genotypic score were 0.880, 0.868 and 0.863, respectively. The strength of the effect of each mutation could be considered through PCA and PLS components. In contrast, each selected mutation contributes with the same weight for the calculation of the genotypic score. Furthermore, PCA and PLS regression helped to describe mutation clusters (e.g. 10, 46, 90).</p> <p>Conclusion</p> <p>In this dataset, PCA and PLS showed a good performance but their predictive ability was not clinically superior to that of the genotypic score.</p
Brane-World Gravity
The observable universe could be a 1+3-surface (the "brane") embedded in a
1+3+\textit{d}-dimensional spacetime (the "bulk"), with Standard Model
particles and fields trapped on the brane while gravity is free to access the
bulk. At least one of the \textit{d} extra spatial dimensions could be very
large relative to the Planck scale, which lowers the fundamental gravity scale,
possibly even down to the electroweak ( TeV) level. This revolutionary
picture arises in the framework of recent developments in M theory. The
1+10-dimensional M theory encompasses the known 1+9-dimensional superstring
theories, and is widely considered to be a promising potential route to quantum
gravity. At low energies, gravity is localized at the brane and general
relativity is recovered, but at high energies gravity "leaks" into the bulk,
behaving in a truly higher-dimensional way. This introduces significant changes
to gravitational dynamics and perturbations, with interesting and potentially
testable implications for high-energy astrophysics, black holes, and cosmology.
Brane-world models offer a phenomenological way to test some of the novel
predictions and corrections to general relativity that are implied by M theory.
This review analyzes the geometry, dynamics and perturbations of simple
brane-world models for cosmology and astrophysics, mainly focusing on warped
5-dimensional brane-worlds based on the Randall--Sundrum models. We also cover
the simplest brane-world models in which 4-dimensional gravity on the brane is
modified at \emph{low} energies -- the 5-dimensional Dvali--Gabadadze--Porrati
models. Then we discuss co-dimension two branes in 6-dimensional models.Comment: A major update of Living Reviews in Relativity 7:7 (2004)
"Brane-World Gravity", 119 pages, 28 figures, the update contains new
material on RS perturbations, including full numerical solutions of
gravitational waves and scalar perturbations, on DGP models, and also on 6D
models. A published version in Living Reviews in Relativit
Therapeutic implications of selecting the SCORE (European) versus the D'AGOSTINO (American) risk charts for cardiovascular risk assessment in hypertensive patients
Background: No comparisons have been made of scales estimating cardiovascular mortality and overall cardiovascular morbidity and mortality. The study objectives were to assess the agreement between the Framingham-D'Agostino cardiovascular risk (CVR) scale and the chart currently recommended in Europe (SCORE) with regard to identification of patients with high CVR, and to describe the discrepancies between them and the attendant implications for the treatment of hypertension and hyperlipidaemia. Methods: A total of 474 hypertensive patients aged 40-65 years monitored in primary care were enrolled into the study. CVR was assessed using the Framingham-D'Agostino scale, which estimates the overall cardiovascular morbidity and mortality risk, and the SCORE chart, which estimates the cardiovascular mortality risk. Cardiovascular risk was considered to be high for values ≥ 20% and ≥ 5% according to the Framingham-D'Agostino and SCORE charts respectively. Kappa statistics was estimated for agreement in classification of patients with high CVR. The therapeutic recommendations in the 2007 European Guidelines on Cardiovascular Disease Prevention were followed. Results
A community-based physical activity intervention to prevent mobility-related disability for retired older people (REtirement in ACTion (REACT)): Study protocol for a randomised controlled trial
© 2018 The Author(s). Background: The REtirement in ACTion (REACT) study is a multi-centre, pragmatic, two-arm, parallel-group randomised controlled trial (RCT) with an internal pilot phase. It aims to test the effectiveness and cost-effectiveness of a community, group-based physical activity intervention for reducing, or reversing, the progression of functional limitations in older people who are at high risk of mobility-related disability. Methods/design: A sample of 768 sedentary, community-dwelling, older people aged 65 years and over with functional limitations, but who are still ambulatory (scores between 4 and 9 out of 12 in the Short Physical Performance Battery test (SPPB)) will be randomised to receive either the REACT intervention, delivered over a period of 12 months by trained facilitators, or a minimal control intervention. The REACT study incorporates comprehensive process and economic evaluation and a nested sub-study which will test the hypothesis that the REACT intervention will slow the rate of brain atrophy and of decline in cognitive function assessed using magnetic resonance imaging (MRI). Outcome data will be collected at baseline, 6, 12 and 24 months for the main study, with MRI sub-study data collected at baseline, 6 and 12 months. The primary outcome analysis (SPPB score at 24 months) will be undertaken blinded to group allocation. Primary comparative analyses will be on an intention-to-treat (ITT) basis with due emphasis placed on confidence intervals. Discussion: REACT represents the first large-scale, pragmatic, community-based trial in the UK to target the non-disabled but high-risk segment of the older population with an intervention to reduce mobility-related disability. A programme that can successfully engage this population in sufficient activity to improve strength, aerobic capacity, coordination and balance would have a major impact on sustaining health and independence. REACT is also the first study of its kind to conduct a full economic and comprehensive process evaluation alongside the RCT. If effective and cost-effective, the REACT intervention has strong potential to be implemented widely in the UK and elsewhere
Limited redundancy in genes regulated by Cyclin T2 and Cyclin T1
<p>Abstract</p> <p>Background</p> <p>The elongation phase, like other steps of transcription by RNA Polymerase II, is subject to regulation. The positive transcription elongation factor b (P-TEFb) complex allows for the transition of mRNA synthesis to the productive elongation phase. P-TEFb contains Cdk9 (Cyclin-dependent kinase 9) as its catalytic subunit and is regulated by its Cyclin partners, Cyclin T1 and Cyclin T2. The HIV-1 Tat transactivator protein enhances viral gene expression by exclusively recruiting the Cdk9-Cyclin T1 P-TEFb complex to a RNA element in nascent viral transcripts called TAR. The expression patterns of Cyclin T1 and Cyclin T2 in primary monocytes and CD4<sup>+ </sup>T cells suggests that Cyclin T2 may be generally involved in expression of constitutively expressed genes in quiescent cells, while Cyclin T1 may be involved in expression of genes up-regulated during macrophage differentiation, T cell activation, and conditions of increased metabolic activity To investigate this issue, we wished to identify the sets of genes whose levels are regulated by either Cyclin T2 or Cyclin T1.</p> <p>Findings</p> <p>We used shRNA lentiviral vectors to stably deplete either Cyclin T2 or Cyclin T1 in HeLa cells. Total RNA extracted from these cells was subjected to cDNA microarray analysis. We found that 292 genes were down- regulated by depletion of Cyclin T2 and 631 genes were down-regulated by depletion of Cyclin T1 compared to cells transduced with a control lentivirus. Expression of 100 genes was commonly reduced in either knockdown. Additionally, 111 and 287 genes were up-regulated when either Cyclin T2 or Cyclin T1 was depleted, respectively, with 45 genes in common.</p> <p>Conclusions</p> <p>These results suggest that there is limited redundancy in genes regulated by Cyclin T1 or Cyclin T2.</p
Prevention of depression and sleep disturbances in elderly with memory-problems by activation of the biological clock with light - a randomized clinical trial
<p>Abstract</p> <p>Background</p> <p>Depression frequently occurs in the elderly and in patients suffering from dementia. Its cause is largely unknown, but several studies point to a possible contribution of circadian rhythm disturbances. Post-mortem studies on aging, dementia and depression show impaired functioning of the suprachiasmatic nucleus (SCN) which is thought to be involved in the increased prevalence of day-night rhythm perturbations in these conditions. Bright light enhances neuronal activity in the SCN. Bright light therapy has beneficial effects on rhythms and mood in institutionalized moderate to advanced demented elderly. In spite of the fact that this is a potentially safe and inexpensive treatment option, no previous clinical trial evaluated the use of long-term daily light therapy to prevent worsening of sleep-wake rhythms and depressive symptoms in early to moderately demented home-dwelling elderly.</p> <p>Methods/Design</p> <p>This study investigates whether long-term daily bright light prevents worsening of sleep-wake rhythms and depressive symptoms in elderly people with memory complaints. Patients with early Alzheimer's Disease (AD), Mild Cognitive Impairment (MCI) and Subjective Memory Complaints (SMC), between the ages of 50 and 75, are included in a randomized double-blind placebo-controlled trial. For the duration of two years, patients are exposed to ~10,000 lux in the active condition or ~300 lux in the placebo condition, daily, for two half-hour sessions at fixed times in the morning and evening. Neuropsychological, behavioral, physiological and endocrine measures are assessed at baseline and follow-up every five to six months.</p> <p>Discussion</p> <p>If bright light therapy attenuates the worsening of sleep-wake rhythms and depressive symptoms, it will provide a measure that is easy to implement in the homes of elderly people with memory complaints, to complement treatments with cholinesterase inhibitors, sleep medication or anti-depressants or as a stand-alone treatment.</p> <p>Trial registration</p> <p>ISRCTN29863753</p
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