67,184 research outputs found
Instability of three dimensional conformally dressed black hole
The three dimensional black hole solution of Einstein equations with negative
cosmological constant coupled to a conformal scalar field is proved to be
unstable against linear circularly symmetric perturbations.Comment: 5 pages, REVTe
The signature of the magnetorotational instability in the Reynolds and Maxwell stress tensors in accretion discs
The magnetorotational instability is thought to be responsible for the
generation of magnetohydrodynamic turbulence that leads to enhanced outward
angular momentum transport in accretion discs. Here, we present the first
formal analytical proof showing that, during the exponential growth of the
instability, the mean (averaged over the disc scale-height) Reynolds stress is
always positive, the mean Maxwell stress is always negative, and hence the mean
total stress is positive and leads to a net outward flux of angular momentum.
More importantly, we show that the ratio of the Maxwell to the Reynolds
stresses during the late times of the exponential growth of the instability is
determined only by the local shear and does not depend on the initial spectrum
of perturbations or the strength of the seed magnetic. Even though we derived
these properties of the stress tensors for the exponential growth of the
instability in incompressible flows, numerical simulations of shearing boxes
show that this characteristic is qualitatively preserved under more general
conditions, even during the saturated turbulent state generated by the
instability.Comment: 9 pages, 4 figures. Minor revisions. Accepted for publication in
MNRA
Adsorbate Electric Fields on a Cryogenic Atom Chip
We investigate the behaviour of electric fields originating from adsorbates
deposited on a cryogenic atom chip as it is cooled from room temperature to
cryogenic temperature. Using Rydberg electromagnetically induced transparency
we measure the field strength versus distance from a 1 mm square of YBCO
patterned onto a YSZ chip substrate. We find a localized and stable dipole
field at room temperature and attribute it to a saturated layer of chemically
adsorbed rubidium atoms on the YBCO. As the chip is cooled towards 83 K we
observe a change in sign of the electric field as well as a transition from a
localized to a delocalized dipole density. We relate these changes to the onset
of physisorption on the chip surface when the van der Waals attraction
overcomes the thermal desorption mechanisms. Our findings suggest that, through
careful selection of substrate materials, it may be possible to reduce the
electric fields caused by atomic adsorption on chips, opening up experiments to
controlled Rydberg-surface coupling schemes.Comment: 5 pages, 4 figure
Jaynes' MaxEnt, Steady State Flow Systems and the Maximum Entropy Production Principle
Jaynes' maximum entropy (MaxEnt) principle was recently used to give a
conditional, local derivation of the ``maximum entropy production'' (MEP)
principle, which states that a flow system with fixed flow(s) or gradient(s)
will converge to a steady state of maximum production of thermodynamic entropy
(R.K. Niven, Phys. Rev. E, in press). The analysis provides a steady state
analog of the MaxEnt formulation of equilibrium thermodynamics, applicable to
many complex flow systems at steady state. The present study examines the
classification of physical systems, with emphasis on the choice of constraints
in MaxEnt. The discussion clarifies the distinction between equilibrium, fluid
flow, source/sink, flow/reactive and other systems, leading into an appraisal
of the application of MaxEnt to steady state flow and reactive systems.Comment: 6 pages; paper for MaxEnt0
Habitat conversion and global avian biodiversity loss
The magnitude of the impacts of human activities on global biodiversity has been documented at several organizational levels. However, although there have been numerous studies of the effects of local-scale changes in land use (e.g. logging) on the abundance of groups of organisms, broader continental or global-scale analyses addressing the same basic issues remain largely wanting. None the less, changing patterns of land use, associated with the appropriation of increasing proportions of net primary productivity by the human population, seem likely not simply to have reduced the diversity of life, but also to have reduced the carrying capacity of the environment in terms of the numbers of other organisms that it can sustain.
Here, we estimate the size of the existing global breeding bird population, and then make a first approximation as to how much this has been modified as a consequence of land-use changes wrought by human activities. Summing numbers across different land-use classes gives a best current estimate of a global population of less than 100 billion breeding bird individuals. Applying the same methodology to estimates of original land-use distributions suggests that conservatively this may represent a loss of between a fifth and a quarter of pre-agricultural bird numbers. This loss is shared across a range of temperate and tropical land-use types
The 2D analogue of the Reissner-Nordstrom solution
A two-dimensional (2D) dilaton gravity model, whose static solutions have the
same features of the Reissner-Nordstrom solutions, is obtained from the
dimensional reduction of a four-dimensional (4D) string effective action
invariant under S-duality transformations. The black hole solutions of the 2D
model and their relationship with those of the 4D theory are discussed.Comment: 5 pages, Plain-Tex, no figure
Reconfigurable self-sufficient traps for ultracold atoms based on a superconducting square
We report on the trapping of ultracold atoms in the magnetic field formed
entirely by persistent supercurrents induced in a thin film type-II
superconducting square. The supercurrents are carried by vortices induced in
the 2D structure by applying two magnetic field pulses of varying amplitude
perpendicular to its surface. This results in a self-sufficient quadrupole trap
that does not require any externally applied fields. We investigate the
trapping parameters for different supercurrent distributions. Furthermore, to
demonstrate possible applications of these types of supercurrent traps we show
how a central quadrupole trap can be split into four traps by the use of a bias
field.Comment: 5 pages, 7 figure
Gravastars and Black Holes of Anisotropic Dark Energy
Dynamical models of prototype gravastars made of anisotropic dark energy are
constructed, in which an infinitely thin spherical shell of a perfect fluid
with the equation of state divides the whole spacetime
into two regions, the internal region filled with a dark energy fluid, and the
external Schwarzschild region. The models represent "bounded excursion" stable
gravastars, where the thin shell is oscillating between two finite radii, while
in other cases they collapse until the formation of black holes. Here we show,
for the first time in the literature, a model of gravastar and formation of
black hole with both interior and thin shell constituted exclusively of dark
energy. Besides, the sign of the parameter of anisotropy () seems to
be relevant to the gravastar formation. The formation is favored when the
tangential pressure is greater than the radial pressure, at least in the
neighborhood of the isotropic case ().Comment: 16 pages, 8 figures. Accepted for publication in Gen. Rel. Gra
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