7,599 research outputs found
A Bestiary of Higher Dimensional Taub-NUT-AdS Spacetimes
We present a menagerie of solutions to the vacuum Einstein equations in six,
eight and ten dimensions. These solutions describe spacetimes which are either
locally asymptotically adS or locally asymptotically flat, and which have
non-trivial topology. We discuss the global structure of these solutions, and
their relevance within the context of M-theory.Comment: 11 pages, LaTex(v4: Comments and references added
Nuttier (A)dS Black Holes in Higher Dimensions
We construct new solutions of the vacuum Einstein field equations with
cosmological constant. These solutions describe spacetimes with non-trivial
topology that are asymptotically dS, AdS or flat. For a negative cosmological
constant these solutions are NUT charged generalizations of the topological
black hole solutions in higher dimensions. We also point out the existence of
such NUT charged spacetimes in odd dimensions and we explicitly construct such
spaces in 5 and 7 dimensions. The existence of such spacetimes with non-trivial
topology is closely related to the existence of the cosmological constant.
Finally, we discuss the global structure of such solutions and possible
applications in string theory.Comment: latex, 30 pages, added reference
On zero-point energy, stability and Hagedorn behavior of Type IIB strings on pp-waves
Type IIB strings on many pp-wave backgrounds, supported either by 5-form or
3-form fluxes, have negative light-cone zero-point energy. This raises the
question of their stability and poses possible problems in the definition of
their thermodynamic properties. After having pointed out the correct way of
calculating the zero-point energy, an issue not fully discussed in literature,
we show that these Type IIB strings are classically stable and have well
defined thermal properties, exhibiting a Hagedorn behavior.Comment: Latex, 13 pages. v2: regularization/renormalization prescription
clarified, refs. adde
A deformation of AdS_5 x S^5
We analyse a one parameter family of supersymmetric solutions of type IIB
supergravity that includes AdS_5 x S^5. For small values of the parameter the
solutions are causally well-behaved, but beyond a critical value closed
timelike curves (CTC's) appear. The solutions are holographically dual to N=4
supersymmetric Yang-Mills theory on a non-conformally flat background with
non-vanishing R-currents. We compute the holographic energy-momentum tensor for
the spacetime and show that it remains finite even when the CTC's appear. The
solutions, as well as the uplift of some recently discovered AdS_5 black hole
solutions, are shown to preserve precisely two supersymmetries.Comment: 16 pages, v2: typos corrected and references adde
Ricci-flat Metrics with U(1) Action and the Dirichlet Boundary-value Problem in Riemannian Quantum Gravity and Isoperimetric Inequalities
The Dirichlet boundary-value problem and isoperimetric inequalities for
positive definite regular solutions of the vacuum Einstein equations are
studied in arbitrary dimensions for the class of metrics with boundaries
admitting a U(1) action. We show that in the case of non-trivial bundles
Taub-Bolt infillings are double-valued whereas Taub-Nut and Eguchi-Hanson
infillings are unique. In the case of trivial bundles, there are two
Schwarzschild infillings in arbitrary dimensions. The condition of whether a
particular type of filling in is possible can be expressed as a limitation on
squashing through a functional dependence on dimension in each case. The case
of the Eguchi-Hanson metric is solved in arbitrary dimension. The Taub-Nut and
the Taub-Bolt are solved in four dimensions and methods for arbitrary dimension
are delineated. For the case of Schwarzschild, analytic formulae for the two
infilling black hole masses in arbitrary dimension have been obtained. This
should facilitate the study of black hole dynamics/thermodynamics in higher
dimensions. We found that all infilling solutions are convex. Thus convexity of
the boundary does not guarantee uniqueness of the infilling. Isoperimetric
inequalities involving the volume of the boundary and the volume of the
infilling solutions are then investigated. In particular, the analogues of
Minkowski's celebrated inequality in flat space are found and discussed
providing insight into the geometric nature of these Ricci-flat spaces.Comment: 40 pages, 3 figure
The morphology of the emission line region of Compact Steep Spectrum radio sources
We present the results of HST narrow band imaging of eleven Compact Steep
Spectrum (CSS) radio sources. Five of them (3C 48, 3C 147, 3C303.1, 3C 277.1
and 4C 12.50) were part of a dedicated ``pointed'' program of deep line imaging
([OIII]). For six additional sources (3C 49, 3C 93.1, 3C 138, 3C 268.3, 3C305.1
and 3C343.1) ``snapshot'' images ([OIII] or [OII]) were taken from the HST
archive. In all but one of the targets (3C 49) line emission has been detected
and only in one case (3C 138) is unresolved. In four out five of the sources
with deep observations, the line emission extends well beyond the size of the
radio source but along the radio axis. Structures of similar surface brightness
would have not been seen in the snapshot images. These emission line structures
extend to scales of 10 to 30 kpc and cover a projected angle, when seen from
the nucleus, of 30 to 110 degrees, indicating that the nuclear illumination is
anisotropic. Photon counting arguments also support this interpretation. In six
objects the radio emission extends over more than 1 arcsec. In these cases the
line emission has an elongated structure, linking the nucleus to the
radio-lobes, possibly tracing the path of the invisible radio jets.
Nevertheless the emission line morphologies do not show the bow shocks at the
extremities of the radio lobes expected if they are sources whose expansion is
frustrated by a dense external medium. Our data favour the alternative model in
which CSSs are the young phase of the large size radio sources. When
``pointed'' pure continuum images are available, there appears to be no
alignment between radio and continuum emission which contradicts previous
suggestions based on broad-band HST imaging.Comment: To appear in AJ; LateX, 32 pages, 12 figures (in gif format
Transgressing the moral economy: Wheelerism and management of the nationalised coal industry in Scotland
This article illuminates the links between managerial style and political economy in post-1945 Britain, and explores the origins of the 1984â1985 miners' strike, by examining in longer historical context the abrasive attitudes and policies of Albert Wheeler, Scottish Area Director of the National Coal Board (NCB). Wheeler built on an earlier emphasis on production and economic criteria, and his micro-management reflected pre-existing centralising tendencies in the industries. But he was innovative in one crucial aspect, transgressing the moral economy of the Scottish coalfield, which emphasised the value of economic security and changes by joint industrial agreement
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Optimal management of an insurer's exposure in a competitive general insurance market
The qualitative behavior of the optimal premium strategy is determined for an insurer in a finite and an infinite market using a deterministic general insurance model. The optimization problem leads to a system of forward-backward differential equations obtained from Pontryaginâs Maximum Principle. The focus of the modelling is on how this optimization problem can be simplified by the choice of demand function and the insurerâs objective. Phase diagrams are used to characterize the optimal control. When the demand is linear in the relative premium, the structure of the phase diagram can be determined analytically. Two types of premium strategy are identified for an insurer in an infinite market, and which is optimal depends on the existence of equilibrium points in the phase diagram. In a finite market there are four more types of premium strategy, and optimality depends on the initial exposure of the insurer and the position of a saddle point in the phase diagram. The effect of a nonlinear demand function is examined by perturbing the linear price function. An analytical optimal premium strategy is also found using inverse methods when the price function is nonlinear
Planning ahead in public health? A qualitative study of the time horizons used in public health decision-making
<p>Abstract</p> <p>Background</p> <p>In order to better understand factors that influence decisions for public health, we undertook a qualitative study to explore issues relating to the time horizons used in decision-making.</p> <p>Methods</p> <p>Qualitative study using semi-structured interviews. 33 individuals involved in the decision making process around coronary heart disease were purposively sampled from the UK National Health Service (national, regional and local levels), academia and voluntary organizations. Analysis was based on the framework method using N-VIVO software. Interviews were transcribed, coded and emergent themes identified.</p> <p>Results</p> <p>Many participants suggested that the timescales for public health decision-making are too short. Commissioners and some practitioners working at the national level particularly felt constrained in terms of planning for the long-term. Furthermore respondents felt that longer term planning was needed to address the wider determinants of health and to achieve societal level changes. Three prominent 'systems' issues were identified as important drivers of short term thinking: the need to demonstrate impact within the 4 year political cycle; the requirement to 'balance the books' within the annual commissioning cycle and the disruption caused by frequent re-organisations within the health service. In addition respondents suggested that the tools and evidence base for longer term planning were not well established.</p> <p>Conclusion</p> <p>Many public health decision and policy makers feel that the timescales for decision-making are too short. Substantial systemic barriers to longer-term planning exist. Policy makers need to look beyond short-term targets and budget cycles to secure investment for long-term improvement in public health.</p
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