75 research outputs found
Quantum corrections to critical phenomena in gravitational collapse
We investigate conformally coupled quantum matter fields on spherically
symmetric, continuously self-similar backgrounds. By exploiting the symmetry
associated with the self-similarity the general structure of the renormalized
quantum stress-energy tensor can be derived. As an immediate application we
consider a combination of classical, and quantum perturbations about exactly
critical collapse. Generalizing the standard argument which explains the
scaling law for black hole mass, , we
demonstrate the existence of a quantum mass gap when the classical critical
exponent satisfies . When our argument is
inconclusive; the semi-classical approximation breaks down in the spacetime
region of interest.Comment: RevTeX, 6 pages, 3 figures included using psfi
Criticality and Bifurcation in the Gravitational Collapse of a Self-Coupled Scalar Field
We examine the gravitational collapse of a non-linear sigma model in
spherical symmetry. There exists a family of continuously self-similar
solutions parameterized by the coupling constant of the theory. These solutions
are calculated together with the critical exponents for black hole formation of
these collapse models. We also find that the sequence of solutions exhibits a
Hopf-type bifurcation as the continuously self-similar solutions become
unstable to perturbations away from self-similarity.Comment: 18 pages; one figure, uuencoded postscript; figure is also available
at http://www.physics.ucsb.edu/people/eric_hirschman
Dynamical formation and evolution of (2+1)-dimensional charged black holes
In this paper, we investigate the dynamical formation and evolution of 2 +
1-dimensional charged black holes. We numerically study dynamical collapses of
charged matter fields in an anti de Sitter background and note the formation of
black holes using the double-null formalism. Moreover, we include re-normalized
energy-momentum tensors assuming the S-wave approximation to determine
thermodynamical back-reactions to the internal structures. If there is no
semi-classical effects, the amount of charge determines the causal structures.
If the charge is sufficiently small, the causal structure has a space-like
singularity. However, as the charge increases, an inner Cauchy horizon appears.
If we have sufficient charge, we see a space-like outer horizon and a time-like
inner horizon, and if we give excessive charge, black hole horizons disappear.
We have some circumstantial evidences that weak cosmic censorship is still
satisfied, even for such excessive charge cases. Also, we confirm that there is
mass inflation along the inner horizon, although the properties are quite
different from those of four-dimensional cases. Semi-classical back-reactions
will not affect the outer horizon, but they will affect the inner horizon. Near
the center, there is a place where negative energy is concentrated. Thus,
charged black holes in three dimensions have two types of curvature
singularities in general: via mass inflation and via a concentration of
negative energy. Finally, we classify possible causal structures.Comment: 40 pages, 15 figure
The causal structure of dynamical charged black holes
We study the causal structure of dynamical charged black holes, with a
sufficient number of massless fields, using numerical simulations. Neglecting
Hawking radiation, the inner horizon is a null Cauchy horizon and a curvature
singularity due to mass inflation. When we include Hawking radiation, the inner
horizon becomes space-like and is separated from the Cauchy horizon, which is
parallel to the out-going null direction. Since a charged black hole must
eventually transit to a neutral black hole, we studied the neutralization of
the black hole and observed that the inner horizon evolves into a space-like
singularity, generating a Cauchy horizon which is parallel to the in-going null
direction. Since the mass function is finite around the inner horizon, the
inner horizon is regular and penetrable in a general relativistic sense.
However, since the curvature functions become trans-Planckian, we cannot
saymore about the region beyond the inner horizon, and it is natural to say
that there is a 'physical' space-like singularity. However, if we assume an
exponentially large number of massless scalar fields, our results can be
extended beyond the inner horizon. In this case, strong cosmic censorship and
black hole complementarity can be violated.Comment: 23 pages, 23 figure
When do colliding bubbles produce an expanding universe?
It is intriguing to consider the possibility that the Big Bang of the
standard (3+1) dimensional cosmology originated from the collision of two
branes within a higher dimensional spacetime, leading to the production of a
large amount of entropy. In this paper we study, subject to certain
well-defined assumptions, under what conditions such a collision leads to an
expanding universe. We assume the absence of novel physics, so that ordinary
(4+1) -dimensional Einstein gravity remains a valid approximation. It is
necessary that the fifth dimension not become degenerate at the moment of
collision. First the case of a symmetric collision of infinitely thin branes
having a hyperbolic or flat spatial geometry is considered. We find that a
symmetric collision results in a collapsing universe on the final brane unless
the pre-existing expansion rate in the bulk just prior to the collision is
sufficiently large in comparison to the momentum transfer in the fifth
dimension. Such prior expansion may either result from negative spatial
curvature or from a positive five-dimensional cosmological constant. The
relevance of these findings to the Colliding Bubble Braneworld Universe
scenario is discussed. Finally, results from a numerical study of colliding
thick-wall branes is presented, which confirm the results of the thin-wall
approximation.Comment: 24 pages, 13 figures. Minor changes and references include
Dimensional Dependence of Black Hole Formation in Self-Similar Collapse of Scalar Field
We study classical and quantum self-similar collapses of a massless scalar
field in higher dimensions, and examine how the increase in the number of
dimensions affects gravitational collapse and black hole formation. Higher
dimensions seem to favor formation of black hole rather than other final
states, in that the initial data space for black hole formation enlarges as
dimension increases. On the other hand, the quantum gravity effect on the
collapse lessens as dimension increases. We also discuss the gravitational
collapse in a brane world with large but compact extra dimensions.Comment: Improved a few arguments and added a figur
Numerical Relativity: A review
Computer simulations are enabling researchers to investigate systems which
are extremely difficult to handle analytically. In the particular case of
General Relativity, numerical models have proved extremely valuable for
investigations of strong field scenarios and been crucial to reveal unexpected
phenomena. Considerable efforts are being spent to simulate astrophysically
relevant simulations, understand different aspects of the theory and even
provide insights in the search for a quantum theory of gravity. In the present
article I review the present status of the field of Numerical Relativity,
describe the techniques most commonly used and discuss open problems and (some)
future prospects.Comment: 2 References added; 1 corrected. 67 pages. To appear in Classical and
Quantum Gravity. (uses iopart.cls
The use of formative research to inform the design of a seasonal malaria chemoprevention intervention in northern Nigeria
BACKGROUND:Experience of seasonal malaria chemoprevention (SMC) is growing in the Sahel sub-region of Africa, though there remains insufficient evidence to recommend a standard deployment strategy. In 2012, a project was initiated in Katsina state, northern Nigeria, to design an appropriate and effective community-based delivery approach for SMC, in consultation with local stakeholders. Formative research (FR) was conducted locally to explore the potential feasibility and acceptability of SMC and to highlight information gaps and practical considerations to inform the intervention design.
METHODS:The FR adopted qualitative methods; 36 in-depth interviews and 18 focus group discussions were conducted across 13 target groups active across the health system and within the community. Analysis followed the 'framework' approach. The process for incorporating the FR results into the project design was iterative which was initiated by a week-long 'intervention design' workshop with relevant stakeholders.
RESULTS:The FR highlighted both supportive and hindering factors to be considered in the intervention design. Malaria control was identified as a community priority, the community health workers were a trusted resource and the local leadership exerted strong influence over household decisions. However, there were perceived challenges with quality of care at both community and health facility levels, referral linkage and supportive supervision were weak, literacy levels lower than anticipated and there was the potential for suspicion of 'outside' interventions. There was broad consensus across target groups that community-based SMC drug delivery would better enable a high coverage of beneficiaries and potentially garner wider community support. A mixed approach was recommended, including both community fixed-point and household-to-household SMC delivery. The FR findings were used to inform the overall distribution strategy, mechanisms for integration into the health system, capacity building and training approaches, supportive interventions to strengthen the health system, and the social mobilization strategy.
CONCLUSIONS:Formative research played a valuable role in exploring local socio-cultural contexts and health system realities. Both opportunities and challenges for the introduction of SMC delivery were highlighted, which were appropriately considered in the design of the project
AMP-activated protein kinase deficiency reduces ozone-induced lung injury and oxidative stress in mice
<p>Abstract</p> <p>Background</p> <p>Acute ozone exposure causes lung oxidative stress and inflammation leading to lung injury. At least one mechanism underlying the lung toxicity of ozone involves excessive production of reactive oxygen and nitrogen intermediates such as peroxynitrite. In addition and beyond its major prooxidant properties, peroxynitrite may nitrate tyrosine residues altering phosphorylation of many protein kinases involved in cell signalling. It was recently proposed that peroxynitrite activates 5'-AMP-activated kinase (AMPK), which regulates metabolic pathways and the response to cell stress. AMPK activation as a consequence of ozone exposure has not been previously evaluated. First, we tested whether acute ozone exposure in mice would impair alveolar fluid clearance, increase lung tissue peroxynitrite production and activate AMPK. Second, we tested whether loss of AMP-activated protein kinase alpha1 subunit in mouse would prevent enhanced oxidative stress and lung injury induced by ozone exposure.</p> <p>Methods</p> <p>Control and AMPKα1 deficient mice were exposed to ozone at a concentration of 2.0 ppm for 3 h in glass cages. Evaluation was performed 24 h after ozone exposure. Alveolar fluid clearance (AFC) was evaluated using fluorescein isothiocyanate tagged albumin. Differential cell counts, total protein levels, cytokine concentrations, myeloperoxidase activity and markers of oxidative stress, i.e. malondialdehyde and peroxynitrite, were determined in bronchoalveolar lavage (BAL) and lung homogenates (LH). Levels of AMPK-Thr<sup>172 </sup>phosphorylation and basolateral membrane Na(+)-K(+)-ATPase abundance were determined by Western blot.</p> <p>Results</p> <p>In control mice, ozone exposure induced lung inflammation as evidence by increased leukocyte count, protein concentration in BAL and myeloperoxidase activity, pro-inflammatory cytokine levels in LH. Increases in peroxynitrite levels (3 vs 4.4 nM, p = 0.02) and malondialdehyde concentrations (110 vs 230 μmole/g wet tissue) were detected in LH obtained from ozone-exposed control mice. Ozone exposure consistently increased phosphorylated AMPK-Thr<sup>172 </sup>to total AMPK ratio by 80% in control mice. Ozone exposure causes increases in AFC and basolateral membrane Na(+)-K(+)-ATPase abundance in control mice which did not occur in AMPKα1 deficient mice.</p> <p>Conclusions</p> <p>Our results collectively suggest that AMPK activation participates in ozone-induced increases in AFC, inflammation and oxidative stress. Further studies are needed to understand how the AMPK pathway may provide a novel approach for the prevention of ozone-induced lung injury.</p
Assessing the Quality of Care for Pneumonia in Integrated Community Case Management: A Cross-Sectional Mixed Methods Study
Background Pneumonia is the leading infectious cause of mortality in children under five worldwide. Community-level interventions, such as integrated community case management, have great potential to reduce the burden of pneumonia, as well as other diseases, especially in remote populations. However, there are still questions as to whether community health workers (CHW) are able to accurately assess symptoms of pneumonia and prescribe appropriate treatment. This research addresses limitations of previous studies using innovative methodology to assess the accuracy of respiratory rate measurement by CHWs and provides new evidence on the quality of care given for children with symptoms of pneumonia. It is one of few that assesses CHW performance in their usual setting, with independent re-examination by experts, following a considerable period of time post-training of CHWs. Methods In this cross-sectional mixed methods study, 1,497 CHW consultations, conducted by 90 CHWs in two districts of Luapula province, Zambia, were directly observed, with measurement of respiratory rate for children with suspected pneumonia recorded by video. Using the video footage, a retrospective reference standard assessment of respiratory rate was conducted by experts. Counts taken by CHWs were compared against the reference standard and appropriateness of the treatment prescribed by CHWs was assessed. To supplement observational findings, three focus group discussions and nine in depth interviews with CHWs were conducted. Results and Conclusion The findings support existing literature that CHWs are capable of measuring respiratory rates and providing appropriate treatment, with 81% and 78% agreement, respectively, between CHWs and experts. Accuracy in diagnosis could be strengthened through further training and the development of improved diagnostic tools appropriate for resource-poor settings
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