15,406 research outputs found
Fundamental properties and applications of quasi-local black hole horizons
The traditional description of black holes in terms of event horizons is
inadequate for many physical applications, especially when studying black holes
in non-stationary spacetimes. In these cases, it is often more useful to use
the quasi-local notions of trapped and marginally trapped surfaces, which lead
naturally to the framework of trapping, isolated, and dynamical horizons. This
framework allows us to analyze diverse facets of black holes in a unified
manner and to significantly generalize several results in black hole physics.
It also leads to a number of applications in mathematical general relativity,
numerical relativity, astrophysics, and quantum gravity. In this review, I will
discuss the basic ideas and recent developments in this framework, and
summarize some of its applications with an emphasis on numerical relativity.Comment: 14 pages, 2 figures. Based on a talk presented at the 18th
International Conference on General Relativity and Gravitation, 8-13 July
2007, Sydney, Australi
Extremality conditions for isolated and dynamical horizons
A maximally rotating Kerr black hole is said to be extremal. In this paper we
introduce the corresponding restrictions for isolated and dynamical horizons.
These reduce to the standard notions for Kerr but in general do not require the
horizon to be either stationary or rotationally symmetric. We consider physical
implications and applications of these results. In particular we introduce a
parameter e which characterizes how close a horizon is to extremality and
should be calculable in numerical simulations.Comment: 13 pages, 4 figures, added reference; v3 appendix added with proof of
result from section IIID, some discussion and references added. Version to
appear in PR
Stationary untrapped boundary conditions in general relativity
A class of boundary conditions for canonical general relativity are proposed
and studied at the quasi-local level. It is shown that for untrapped or
marginal surfaces, fixing the area element on the 2-surface (rather than the
induced 2-metric) and the angular momentum surface density is enough to have a
functionally differentiable Hamiltonian, thus providing definition of conserved
quantities for the quasi-local regions. If on the boundary the evolution vector
normal to the 2-surface is chosen to be proportional to the dual expansion
vector, we obtain a generalization of the Hawking energy associated with a
generalized Kodama vector. This vector plays the role for the stationary
untrapped boundary conditions which the stationary Killing vector plays for
stationary black holes. When the dual expansion vector is null, the boundary
conditions reduce to the ones given by the non-expanding horizons and the null
trapping horizons.Comment: 11 pages, improved discussion section, a reference added, accepted
for publication in Classical and Quantum Gravit
Knowledge mobilization in bridging patientâpractitionerâresearcher boundaries : a systematic integrative review
Aim
To review when, how, and in what context knowledge mobilization (KMb) has crossed patientâpractitionerâresearcher boundaries.
Background
KMb is essential in contemporary health care, yet little is known about how patients are engaged.
Design
Integrative review.
Data sources
Ten academic databases and grey literature.
Review methods
We followed integrative review methodology to identify publications from 2006â2019 which contributed to understanding of crossâboundary KMb. We extracted data using a bespoke spreadsheet and the Template for Intervention Description and Replication (TIDieR) framework. We used metaâsummary to organize key findings.
Results
Thirtyâthree papers collectively provide new insights into âwhenâ and âhowâ KMb has crossed patientâresearcherâpractitioner boundaries and the impact this has achieved. Knowledge is mobilized to improve care, promote health, or prevent ill health. Most studies focus on creating or reâshaping knowledge to make it more useful. Knowledge is mobilized in small community groups, in larger networks, and intervention studies. Finding the right people to engage in activities is crucial, as activities can be demanding and timeâconsuming. Devolving power to communities and using local people to move knowledge can be effective. Few studies report definitive outcomes of KMb.
Conclusion
Crossâboundary KMb can and does produce new and shared knowledge for health care. Positive outcomes can be achieved using diverse public engagement strategies. KMb process and theory is an emerging discipline, further research is needed on effective crossâboundary working and on measuring the impact of KMb.
Impact
This review provides new and nuanced understandings of how KMb theory has been used to bridge patientâresearcherâpractitioner boundaries. We have assessed âhowâ, âwhenâ, and in what context patients, practitioners and researchers have attempted to mobilize knowledge and identified impact. We have developed a knowledge base about good practice and what can and potentially should be avoided in crossâboundary KMb
The influence of phosphatidylserine localisation and lipid phase on membrane remodelling by the ESCRT-II/ESCRT-III complex
The endosomal sorting complex required for transport (ESCRT) organises in supramolecular structures on the surface of lipid bilayers to drive membrane invagination and scission of intraluminal vesicles (ILVs), a process also controlled by membrane mechanics. However, ESCRT association with the membrane is also mediated by electrostatic interactions with anionic phospholipids. Phospholipid distribution within natural biomembranes is inhomogeneous due to, for example, the formation of lipid rafts and curvature-driven lipid sorting. Here, we have used phase-separated giant unilamellar vesicles (GUVs) to investigate the link between phosphatidylserine (PS)-rich lipid domains and ESCRT activity. We employ GUVs composed of phase separating lipid mixtures, where unsaturated DOPS and saturated DPPS lipids are incorporated individually or simultaneously to enhance PS localisation in liquid disordered (Ld) and/or liquid ordered (Lo) domains, respectively. PS partitioning between the coexisting phases is confirmed by a fluorescent Annexin V probe. Ultimately, we find that ILV generation promoted by ESCRTs is significantly enhanced when PS lipids localise within Ld domains. However, the ILVs that form are rich in Lo lipids. We interpret this surprising observation as preferential recruitment of the Lo phase beneath the ESCRT complex due to its increased rigidity, where the Ld phase is favoured in the neck of the resultant buds to facilitate the high membrane curvature in these regions of the membrane during the ILV formation process. Ld domains offer lower resistance to membrane bending, demonstrating a mechanism by which the composition and mechanics of membranes can be coupled to regulate the location and efficiency of ESCRT activity
P and T Violation From Certain Dimension Eight Weinberg Operators
Dimension eight operators of the Weinberg type have been shown to give
important contributions to CP violating phenomena, such as the electric dipole
moment of the neutron. In this note we show how operators related to these (and
expected to occur on equal footing) can give rise to time-reversal violating
phenomena such as atomic electric dipole moments. We also estimate the induced
parity violating phenomena such as small ``wrong'' parity admixtures in atomic
states and find that they are negligible. Uses harvmac.tex and epsf.tex; one
figure submitted as a uuencoded, compressed EPS file.Comment: 6 pages, EFI-92-5
Hamiltonian, Energy and Entropy in General Relativity with Non-Orthogonal Boundaries
A general recipe to define, via Noether theorem, the Hamiltonian in any
natural field theory is suggested. It is based on a Regge-Teitelboim-like
approach applied to the variation of Noether conserved quantities. The
Hamiltonian for General Relativity in presence of non-orthogonal boundaries is
analysed and the energy is defined as the on-shell value of the Hamiltonian.
The role played by boundary conditions in the formalism is outlined and the
quasilocal internal energy is defined by imposing metric Dirichlet boundary
conditions. A (conditioned) agreement with previous definitions is proved. A
correspondence with Brown-York original formulation of the first principle of
black hole thermodynamics is finally established.Comment: 29 pages with 1 figur
Marginally trapped tubes and dynamical horizons
We investigate the generic behaviour of marginally trapped tubes (roughly
time-evolved apparent horizons) using simple, spherically symmetric examples of
dust and scalar field collapse/accretion onto pre-existing black holes. We find
that given appropriate physical conditions the evolution of the marginally
trapped tube may be either null, timelike, or spacelike and further that the
marginally trapped two-sphere cross-sections may either expand or contract in
area. Spacelike expansions occur when the matter falling into a black hole
satisfies , where is the area of the horizon while
and are respectively the density and pressure of the matter.
Timelike evolutions occur when is greater than this cut-off and so
would be expected to be more common for large black holes. Physically they
correspond to horizon "jumps" as extreme conditions force the formation of new
horizons outside of the old.Comment: 31 pages, many figures. Final Version to appear in CQG: improvements
include more complete references, a discussion of those references,
Penrose-Carter diagrams for several of the spacetimes, and improved numerics
for the scalar field
Evidence for improved urban flood resilience by sustainable drainage retrofit
The rapid growth of cities under modern development pressure has resulted in surface water flooding becoming an increasing hazard and future climate change uncertainties may exacerbate this threat still further: retrofitting sustainable drainage systems to attenuate stormwater runoff has been advocated as part of an integrated solution required to address this problem. Many of these adaptations not only enhance a communityâs resilience to flooding, but may also offer additional benefits in terms of improved environmental amenity and quality of life. The evidence base for sustainable drainage is critically evaluated in respect of the implications for urban planning, as applied to existing housing stocks and business properties in urban areas worldwide. It is concluded that this approach can make a substantial contribution towards urban resilience as part of an integrated approach to managing extreme storms. This will be of interest to urban planners and designers considering the implementation of integrated flood risk management
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