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Fighting HIV/AIDS: Reconfiguring the State?
The author wishes to thank the anonymous reviewers of the article and the ESRC for funding part of this research
First Order Description of Black Holes in Moduli Space
We show that the second order field equations characterizing extremal
solutions for spherically symmetric, stationary black holes are in fact implied
by a system of first order equations given in terms of a prepotential W. This
confirms and generalizes the results in [14]. Moreover we prove that the
squared prepotential function shares the same properties of a c-function and
that it interpolates between M^2_{ADM} and M^2_{BR}, the parameter of the
near-horizon Bertotti-Robinson geometry. When the black holes are solutions of
extended supergravities we are able to find an explicit expression for the
prepotentials, valid at any radial distance from the horizon, which reproduces
all the attractors of the four dimensional N>2 theories. Far from the horizon,
however, for N-even our ansatz poses a constraint on one of the U-duality
invariants for the non-BPS solutions with Z \neq 0. We discuss a possible
extension of our considerations to the non extremal case.Comment: Some points clarified, a comment on the interpretation of the
prepotential W in terms of c-function added, typos corrected. Version to
appear on JHE
A class of exact solutions of Einstein's field equations in higher dimensional spacetimes, d: Majumdar-Papapetrou solutions
The Newtonian theory of gravitation and electrostatics admit equilibrium
configurations of charged fluids where the charge density can be equal to the
mass density, in appropriate units. The general relativistic analog for charged
dust stars was discovered by Majumdar and by Papapetrou. In the present work we
consider Einstein-Maxwell solutions in d-dimensional spacetimes and show that
there are Majumdar-Papapetrou type solutions for all . It is
verified that the equilibrium is independent of the shape of the distribution
of the charged matter. It is also showed that for perfect fluid solutions
satisfying the Majumdar-Papapetrou condition with a boundary where the pressure
is zero, the pressure vanishes everywhere, and that the -dimensional spatial section of the spacetime is conformal to a
Ricci-flat space. The Weyl d-dimensional axisymmetric solutions are generalized
to include electric field and charged matter.Comment: 26 pages, no figure
Generic isolated horizons in loop quantum gravity
Isolated horizons model equilibrium states of classical black holes. A
detailed quantization, starting from a classical phase space restricted to
spherically symmetric horizons, exists in the literature and has since been
extended to axisymmetry. This paper extends the quantum theory to horizons of
arbitrary shape. Surprisingly, the Hilbert space obtained by quantizing the
full phase space of \textit{all} generic horizons with a fixed area is
identical to that originally found in spherical symmetry. The entropy of a
large horizon remains one quarter its area, with the Barbero-Immirzi parameter
retaining its value from symmetric analyses. These results suggest a
reinterpretation of the intrinsic quantum geometry of the horizon surface.Comment: 13 page
Intercontinental comparison of habitat levels of invasion between temperate North America and Europe
Several studies have demonstrated that floras of the New World contain larger proportions of alien species than those of the Old World; however, the differences in fine-scale invasion patterns are poorly known. We compared the levels of invasion in analogous habitats of two environmentally similar regions in temperate North America and Europe (the Carolinas and the Czech Republic), using comprehensive vegetation-plot databases. Native and alien vascular plant species were identified within 4165 vegetation plots assigned to 12 habitats occurring in both areas. The level of invasion was calculated for each habitat (1) as the proportion of aliens recorded cumulatively across multiple plots (habitat scale) and (2) as the mean proportion of aliens per plot (plot scale), both separately for all alien species and for the subgroup of aliens originating in one region and invading the other. The proportions of species native on one continent and invading the other were also calculated for each habitat to compare the alien species exchange between continents. Habitat levels of invasion showed remarkably similar patterns on the two continents. There were significant positive relationships for the levels of invasion, both for all alien species (habitat-scale R2 = 0.907; plot-scale R2 = 0.676) and for those that originated on the opposite continent (habitat-scale R2 = 0.624; plot-scale R2 = 0.708). In both regions, the most and the least invaded habitats were the same, but on average, North American habitats showed higher habitat-scale levels of invasion than their European counterparts. At the same time, a larger proportion of alien species was provided by European habitats for invasion to North America than vice versa. The consistent intercontinental pattern of habitat levels of invasion suggests that these levels are driven by similar mechanisms in distant regions. Habitat conditions are likely to have stronger effect on the level of invasion than the identity of alien species, as shown by similar levels of invasion in analogous habitats despite different geographical origins of alien species. The higher flux of alien species from Europe to North America is consistent with a generally higher level of invasion of North American habitats
Non-supersymmetric black rings as thermally excited supertubes
We construct a seven-parameter family of supergravity solutions that describe
non-supersymmetric black rings and black tubes with three charges, three
dipoles and two angular momenta. The black rings have regular horizons and
non-zero temperature. They are naturally interpreted as the supergravity
descriptions of thermally excited configurations of supertubes, specifically of
supertubes with two charges and one dipole, and of supertubes with three
charges and two dipoles. In order to fully describe thermal excitations near
supersymmetry of the black supertubes with three charges and three dipoles a
more general family of black ring solutions is required.Comment: 35 pages. v2: ref added and minor typo correcte
Resilience and Alternative Stable States After Desert Wildfires
Improving models of community change is a fundamental goal in ecology and has renewed importance during global change and increasing human disturbance of the biosphere. Using the Mojave Desert (southwestern United States) as a model system, invaded by nonnative plants and subject to wildfire disturbances, we examined models of resilience, alternative stable states, and convergent-divergent trajectories for 36 yr of plant community change after 31 wildfires in communities dominated by the native shrubs Larrea tridentata or Coleogyne ramosissima. Perennial species richness on average was fully resilient within 23 yr after disturbance in both community types. Perennial cover was fully resilient within 25 yr in the Larrea community, but recovery was projected to require 52 yr in the Coleogyne community. Species composition shifts were persistent, and in the Coleogyne community, the projected compositional recovery time of 550 yr and increasing resembled a deflected trajectory toward potential alternative states. Disturbed sites contained a perennial species composition of predominately short-statured forbs, subshrubs, and grasses, contrasting with the larger-statured shrub and tree structure of undisturbed sites. Auxiliary data sets characterizing species recruitment, annual plants including nonnative grasses, biocrust communities, and soils showed persistent differences between disturbed and undisturbed sites consistent with positive feedbacks potentially contributing to alternative stable states. Resprouting produced limited resilience for the large shrubs L. tridentata and Yucca spp. important to population persistence but did not forestall long-term reduced abundance of the species. The nonnative annual grass Bromus rubens increased on disturbed sites over time, suggesting persistently abundant nonnative plant fuels and reburn potential. Biocrust cover on disturbed sites was half and species richness a third of amounts on undisturbed sites. Soil nitrogen was 30% greater on disturbed sites and no significant trend was evident for it to decline on even the oldest burns. Disturbed desert plant communities simultaneously supported all three models of resilience, alternative stable states, and convergent-divergent trajectories among community measures (e.g., species richness, composition), timeframes since disturbance, and spatial resolutions. Accommodating expression within ecosystems of multiple models, including those opposing each other, may help broaden theoretical models of ecosystem change
Black Rings, Supertubes, and a Stringy Resolution of Black Hole Non-Uniqueness
In order to address the issues raised by the recent discovery of
non-uniqueness of black holes in five dimensions, we construct a solution of
string theory at low energies describing a five-dimensional spinning black ring
with three charges that can be interpreted as D1-brane, D5-brane, and momentum
charges. The solution possesses closed timelike curves (CTCs) and other
pathologies, whose origin we clarify. These pathologies can be avoided by
setting any one of the charges, e.g. the momentum, to zero. We argue that the
D1-D5-charged black ring, lifted to six dimensions, describes the thermal
excitation of a supersymmetric D1-D5 supertube, which is in the same U-duality
class as the D0-F1 supertube. We explain how the stringy microscopic
description of the D1-D5 system distinguishes between a spherical black hole
and a black ring with the same asymptotic charges, and therefore provides a
(partial) resolution of the non-uniqueness of black holes in five dimensions.Comment: 33 pages, 1 figur
Brane Baldness vs. Superselection Sectors
The search for intersecting brane solutions in supergravity is a large and
profitable industry. Recently, attention has focused on finding localized forms
of known `delocalized' solutions. However, in some cases, a localized version
of the delocalized solution simply does not exist. Instead, localized separated
branes necessarily delocalize as the separation is removed. This phenomenon is
related to black hole no-hair theorems, i.e. `baldness.' We continue the
discussion of this effect and describe how it can be understood, in the case of
Dirichlet branes, in terms of the corresponding intersection field theory. When
it occurs, it is associated with the quantum mixing of phases and lack of
superselection sectors in low dimensional field theories. We find surprisingly
wide agreement between the field theory and supergravity both with respect to
which examples delocalize and with respect to the rate at which this occurs.Comment: 26 pages, ReVTeX, 2 figures, reference added, version to appear in
PR
CONNECTING FINE- AND BROAD-SCALE SPECIES–AREA RELATIONSHIPS OF SOUTHEASTERN U.S. FLORA
Although the rate that species accumulate with area has long been regarded as an important component of fine-scale community structure and several studies have examined this rate in meta-analyses, few if any studies have systematically examined fine-scale species-area relationships using a consistent survey protocol over a large region. We examined fine-scale species-area relationships using the extensive database of the Carolina Vegetation Survey (North Carolina, South Carolina, Georgia, and Tennessee, USA), including 1472 plots wherein vascular plant richness was recorded for each of six subplot sizes regularly spaced on a log10 scale, from 0.01 to 1000 m2. Contrary to prevailing theory, our data closely and consistently fit an Arrhenius (power law) species-area model, echoing broader-scale patterns. Species accumulation rate (Z) values fell within a narrow range (95% between 0.2 and 0.5) despite a 30-fold range in 1000-m2 richness. When we added regional- and global-scale richness estimates to our results, a Preston-type triphasic curve emerged. We suggest that (1) fine-scale species-area relationships are remarkably consistent and (2) full-scale species-area curves reveal scale dependencies in diversity data that are not accounted for by current species-area theory
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