862 research outputs found
Stability properties of black holes in self-gravitating nonlinear electrodynamics
We analyze the dynamical stability of black hole solutions in
self-gravitating nonlinear electrodynamics with respect to arbitrary linear
fluctuations of the metric and the electromagnetic field. In particular, we
derive simple conditions on the electromagnetic Lagrangian which imply linear
stability in the domain of outer communication. We show that these conditions
hold for several of the regular black hole solutions found by Ayon-Beato and
Garcia.Comment: 15 pages, no figure
Tricritical gravity waves in the four-dimensional generalized massive gravity
We construct a generalized massive gravity by combining quadratic curvature
gravity with the Chern-Simons term in four dimensions. This may be a candidate
for the parity-odd tricritical gravity theory. Considering the AdS vacuum
solution, we derive the linearized Einstein equation, which is not similar to
that of the three dimensional (3D) generalized massive gravity. When a
perturbed metric tensor is chosen to be the Kerr-Schild form, the linearized
equation reduces to a single massive scalar equation. At the tricritical points
where two masses are equal to -1 and 2, we obtain a log-square wave solution to
the massive scalar equation. This is compared to the 3D tricritical generalized
massive gravity whose dual is a rank-3 logarithmic conformal field theory.Comment: 17 pages, 1 figure, version to appear in EPJ
On the new massive gravity and AdS/CFT
Demanding the existence of a simple holographic -theorem, it is shown that
a general (parity preserving) theory of gravity in 2+1 dimensions involving
upto four derivative curvature invariants reduces to the new massive gravity
theory. We consider extending the theory including upto six derivative
curvature invariants. Black hole solutions are presented and consistency with
1+1 CFTs is checked. We present evidence that bulk unitarity is still in
conflict with a positive CFT central charge for generic choice of parameters.
However, for a special choice of parameters appearing in the four and six
derivative terms reduces the linearized equations to be two derivative, thereby
ameliorating the unitarity problem.Comment: 16 pages, 2 figures. v4: typo correcte
The Ebola crisis and people with disabilities' access to healthcare and government services in Liberia
BACKGROUND: There has been little research on the impact of the 2014-2015 West African Ebola crisis on people with disabilities. This paper outlines the way in which the Ebola Virus Disease (EVD) outbreak in Liberia in 2015 highlighted existing inequalities and exclusion of people with disabilities and their households. METHODS: The results presented here are part of a larger ESRC/DFID-funded mixed methods research project in Liberia (2014-2017) which included a quantitative household survey undertaken in five counties, complemented by qualitative focus group discussions and interviews with people with disabilities and other key stakeholders. Uniquely, this research gathered information about people with disabilities' experience of the EVD outbreak, as well as additional socioeconomic and inclusion data, that compared their experience with non-disabled community members. RESULTS: Reflections by people with disabilities themselves show knowledge, preparation, and responses to the EVD epidemic was often markedly different among people with disabilities due to limited resources, lack of inclusion by many mainstream public health and medical interventions and pre-existing discrimination, marginalisation and exclusion. Interviews with other key stakeholder revealed a lack of awareness of disability issues or sufficient training to include this population systematically in both Ebola response activities and general health services. Key findings include the need to understand and mitigate direct and indirect health consequences of unequal responses to the epidemic, as well as the limited capacity of healthcare and social services to respond to people with disabilities. CONCLUSION: There are lessons to be learned from Ebola outbreak around inclusion of people with disabilities, relevant to the current COVID-19 pandemic. Now is the time to undertake measures to ensure that people with disabilities do not continue to be marginalised and excluded during global public health emergencies
Spatially homogeneous Lifshitz black holes in five dimensional higher derivative gravity
We consider spatially homogeneous Lifshitz black hole solutions in five
dimensional higher derivative gravity theories, which can be possible near
horizon geometries of some systems that are interesting in the framework of
gauge/gravity duality. We show the solutions belonging to the nine Bianchi
classes in the pure R^2 gravity. We find that these black holes have zero
entropy at non-zero temperatures and this property is the same as the case of
BTZ black holes in new massive gravity at the critical point. In the most
general quadratic curvature gravity theories, we find new solutions in Bianchi
Type I and Type IX cases.Comment: 15 pages, no figure; v2, refs added, version to appear in JHE
Chiral Topologically Massive Gravity and Extremal B-F Scalars
At a critical ``chiral'' coupling, topologically massive gravity with a
negative cosmological constant exhibits several unusual features, including the
emergence of a new logarithmic branch of solutions and a linearization
instability for certain boundary conditions. I show that at this coupling, the
linearized theory may be parametrized by a free scalar field at the
Breitenlohner-Freedman bound, and use this description to investigate these
features.Comment: 16 pages; v2: added references, typos correcte
Analytic Lifshitz black holes in higher dimensions
We generalize the four-dimensional R^2-corrected z=3/2 Lifshitz black hole to
a two-parameter family of black hole solutions for any dynamical exponent z and
for any dimension D. For a particular relation between the parameters, we find
the first example of an extremal Lifshitz black hole. An asymptotically
Lifshitz black hole with a logarithmic decay is also exhibited for a specific
critical exponent depending on the dimension. We extend this analysis to the
more general quadratic curvature corrections for which we present three new
families of higher-dimensional D>=5 analytic Lifshitz black holes for generic
z. One of these higher-dimensional families contains as critical limits the z=3
three-dimensional Lifshitz black hole and a new z=6 four-dimensional black
hole. The variety of analytic solutions presented here encourages to explore
these gravity models within the context of non-relativistic holographic
correspondence.Comment: 14 page
A Raman study of the Charge-Density-Wave State in AMoO (A = K,Rb)
We report a comparative Raman spectroscopic study of the
quasi-one-dimensional charge-density-wave systems \ab (A = K, Rb). The
temperature and polarization dependent experiments reveal charge-coupled
vibrational Raman features. The strongly temperature-dependent collective
amplitudon mode in both materials differ by about 3 cm, thus revealing the role
of alkali atom. We discus the observed vibrational features in terms of
charge-density-wave ground state accompanied by change in the crystal symmetry.
A frequency-kink in some modes seen in \bb between T = 80 K and 100 K supports
the first-order lock-in transition, unlike \rb. The unusually sharp Raman
lines(limited by the instrumental response) at very low temperatures and their
temperature evolution suggests that the decay of the low energy phonons is
strongly influenced by the presence of the temperature dependent charge density
wave gap.Comment: 13 pages, 7 figure
The family of regular interiors for non-rotating black holes with
We find the general solution for the spacetimes describing the interior of
static black holes with an equation of state of the type (
being the stress-energy tensor). This form is the one expected from taking into
account different quantum effects associated with strong gravitational fields.
We recover all the particular examples found in the literature. We remark that
all the solutions found follow the natural scheme of an interior core linked
smoothly with the exterior solution by a transient region. We also discuss
their local energy properties and give the main ideas involved in a possible
generalization of the scheme, in order to include other realistic types of
sources.Comment: 31 pages, 1 figure, version to appear in Physical Review
Universal thermal and electrical conductivity from holography
It is known from earlier work of Iqbal, Liu (arXiv:0809.3808) that the
boundary transport coefficients such as electrical conductivity (at vanishing
chemical potential), shear viscosity etc. at low frequency and finite
temperature can be expressed in terms of geometrical quantities evaluated at
the horizon. In the case of electrical conductivity, at zero chemical potential
gauge field fluctuation and metric fluctuation decouples, resulting in a
trivial flow from horizon to boundary. In the presence of chemical potential,
the story becomes complicated due to the fact that gauge field and metric
fluctuation can no longer be decoupled. This results in a nontrivial flow from
horizon to boundary. Though horizon conductivity can be expressed in terms of
geometrical quantities evaluated at the horizon, there exist no such neat
result for electrical conductivity at the boundary. In this paper we propose an
expression for boundary conductivity expressed in terms of geometrical
quantities evaluated at the horizon and thermodynamical quantities. We also
consider the theory at finite cutoff outside the horizon (arXiv:1006.1902) and
give an expression for cutoff dependent electrical conductivity, which
interpolates smoothly between horizon conductivity and boundary conductivity .
Using the results about the electrical conductivity we gain much insight into
the universality of thermal conductivity to viscosity ratio proposed in
arXiv:0912.2719.Comment: An appendix added discussing relation between boundary conductivity
and universal conductivity of stretched horizon, version to be published in
JHE
- …