448 research outputs found
Electromagnetic Field in Higher-Dimensional Black-Hole Spacetimes
A special test electromagnetic field in the spacetime of the
higher-dimensional generally rotating NUT-(A)dS black hole is found. It is
adjusted to the hidden symmetries of the background represented by the
principal Killing-Yano tensor. Such electromagnetic field generalizes the field
of charged black hole in four dimensions. In higher dimensions, however, the
gravitational back reaction of such a field cannot be consistently solved.Comment: 8 pages, no figures; presented at the Black hole VI conference in
White Point, Canada, May 12-16 2007, and at the GRG18 conference in Sydney,
Australia, July 8-13 200
Linear stability of Einstein-Gauss-Bonnet static spacetimes. Part II: vector and scalar perturbations
We study the stability under linear perturbations of a class of static
solutions of Einstein-Gauss-Bonnet gravity in dimensions with spatial
slices of the form \Sigma_{\k}^n \times {\mathbb R}^+, \Sigma_{\k}^n an
manifold of constant curvature \k. Linear perturbations for this class of
space-times can be generally classified into tensor, vector and scalar types.
In a previous paper, tensor perturbations were analyzed. In this paper we study
vector and scalar perturbations. We show that vector perturbations can be
analyzed in general using an S-deformation approach and do not introduce
instabilities. On the other hand, we show by analyzing an explicit example
that, contrary to what happens in Einstein gravity, scalar perturbations may
lead to instabilities in black holes with spherical horizons when the
Gauss-Bonnet string corrections are taken into account.Comment: 16 pages, 6 figure
Oral tradition in a technologically advanced world
In the field of folklore, the study of "oral tradition" cannot be an either/or proposition. Rather, the responsible study of oral tradition recognizes the interdependence of both of these concepts: while "oral" clearly modifies "tradition," there is an equally important coloring of "oral" by "tradition."//Note: Quotation marks removed to ensure alphabetical order. Difference as follows; "Oral Tradition" in a Technologically Advanced World
Critical escape velocity of black holes from branes
In recent work we have shown that a black hole stacked on a brane escapes
once it acquires a recoil velocity. This result was obtained in the {\it
probe-brane} approximation, {\it i.e.}, when the tension of the brane is
negligibly small. Therefore, it is not clear whether the effect of the brane
tension may prevent the black hole from escaping for small recoil velocities.
The question is whether a critical escape velocity exists. Here, we analyze
this problem by studying the interaction between a Dirac-Nambu-Goto brane and a
black hole assuming adiabatic (quasi-static) evolution. By describing the brane
in a fixed black hole spacetime, which restricts our conclusions to lowest
order effects in the tension, we find that the critical escape velocity does
not exist for co-dimension one branes, while it does for higher co-dimension
branes.Comment: 10 pages, revte
Merger Transitions in Brane--Black-Hole Systems: Criticality, Scaling, and Self-Similarity
We propose a toy model for study merger transitions in a curved spaceime with
an arbitrary number of dimensions. This model includes a bulk N-dimensional
static spherically symmetric black hole and a test D-dimensional brane
interacting with the black hole. The brane is asymptotically flat and allows
O(D-1) group of symmetry. Such a brane--black-hole (BBH) system has two
different phases. The first one is formed by solutions describing a brane
crossing the horizon of the bulk black hole. In this case the internal induced
geometry of the brane describes D-dimensional black hole. The other phase
consists of solutions for branes which do not intersect the horizon and the
induced geometry does not have a horizon. We study a critical solution at the
threshold of the brane-black-hole formation, and the solutions which are close
to it. In particular, we demonstrate, that there exists a striking similarity
of the merger transition, during which the phase of the BBH-system is changed,
both with the Choptuik critical collapse and with the merger transitions in the
higher dimensional caged black-hole--black-string system.Comment: 9 pages 2 figures; additional remarks and references are added at
Section IX "Discussion
D=5 Einstein-Maxwell-Chern-Simons Black Holes
5-dimensional Einstein-Maxwell-Chern-Simons theory with Chern-Simons
coefficient has supersymmetric black holes with vanishing horizon
angular velocity, but finite angular momentum. Here supersymmetry is associated
with a borderline between stability and instability, since for a
rotational instability arises, where counterrotating black holes appear, whose
horizon rotates in the opposite sense to the angular momentum. For
black holes are no longer uniquely characterized by their global charges, and
rotating black holes with vanishing angular momentum appear.Comment: 4 pages, 4 figures, RevTeX styl
Remark on the effective potential of the gravitational perturbation in the black hole background projected on the brane
The polar perturbation is examined when the spacetime is expressed by a 4d
metric induced from higher-dimensional Schwarzschild geometry. Since the
spacetime background is not a vacuum solution of 4d Einstein equation, the
various general principles are used to understand the behavior of the
energy-momentum tensor under the perturbation. It is found that although the
general principles fix many components, they cannot fix two components of the
energy-momentum tensor. Choosing two components suitably, we derive the
effective potential which has a correct 4d limit.Comment: 12 pages, no figure, CQG accepte
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Conspiracy in the Time of Corona: Automatic detection of Emerging Covid-19 Conspiracy Theories in Social Media and the News
Abstract
Rumors and conspiracy theories thrive in environments of low confi- dence and low trust. Consequently, it is not surprising that ones related to the Covid-19 pandemic are proliferating given the lack of scientific consensus on the virus’s spread and containment, or on the long term social and economic ramifications of the pandemic. Among the stories currently circulating are ones suggesting that the 5G telecommunication network activates the virus, that the pandemic is a hoax perpetrated by a global cabal, that the virus is a bio-weapon released deliberately by the Chinese, or that Bill Gates is using it as cover to launch a broad vaccination program to facilitate a global surveillance regime. While some may be quick to dismiss these stories as having little impact on real-world behavior, recent events including the destruction of cell phone towers, racially fueled attacks against Asian Americans, demonstrations espousing resistance to public health orders, and wide-scale defiance of scientifically sound public mandates such as those to wear masks and practice social distancing, countermand such conclusions. Inspired by narrative theory, we crawl social media sites and news reports and, through the application of automated machine-learning methods, discover the underlying narrative frame- works supporting the generation of rumors and conspiracy theories. We show how the various narrative frameworks fueling these stories rely on the alignment of otherwise disparate domains of knowledge, and consider how they attach to the broader reporting on the pandemic. These alignments and attachments, which can be monitored in near real-time, may be useful for identifying areas in the news that are particularly vulnerable to reinterpretation by conspiracy theorists. Understanding the dynamics of storytelling on social media and the narrative frameworks that provide the generative basis for these stories may also be helpful for devising methods to disrupt their spread
Black strings in (4+1)-dimensional Einstein-Yang-Mills theory
We study two classes of static uniform black string solutions in a
(4+1)-dimensional SU(2) Einstein-Yang-Mills model. These configurations possess
a regular event horizon and corresponds in a 4-dimensional picture to axially
symmetric black hole solutions in an Einstein-Yang-Mills-Higgs-U(1)-dilaton
theory. In this approach, one set of solutions possesses a nonzero magnetic
charge, while the other solutions represent black holes located in between a
monopole-antimonopole pair. A detailed analysis of the solutions' properties is
presented, the domain of existence of the black strings being determined. New
four dimensional solutions are found by boosting the five dimensional
configurations. We also present an argument for the non-existence of finite
mass hyperspherically symmetric black holes in SU(2) Einstein-Yang-Mills
theory.Comment: 19 Revtex pages, 27 eps-figures; discussion on rotating black holes
modifie
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