2,985 research outputs found
Dimension-Dependence of the Critical Exponent in Spherically Symmetric Gravitational Collapse
We study the critical behaviour of spherically symmetric scalar field
collapse to black holes in spacetime dimensions other than four. We obtain
reliable values for the scaling exponent in the supercritical region for
dimensions in the range . The critical exponent increases
monotonically to an asymptotic value at large of . The
data is well fit by a simple exponential of the form: .Comment: 5 pages, including 7 figures New version contains more data points,
one extra graph and more accurate error bars. No changes to result
Spherically symmetric scalar field collapse in any dimension
We describe a formalism and numerical approach for studying spherically
symmetric scalar field collapse for arbitrary spacetime dimension d and
cosmological constant Lambda. The presciption uses a double null formalism, and
is based on field redefinitions first used to simplify the field equations in
generic two-dimensional dilaton gravity. The formalism is used to construct
code in which d and Lambda are input parameters. The code reproduces known
results in d = 4 and d = 6 with Lambda = 0. We present new results for d = 5
with zero and negative Lambda.Comment: 16 pages, 6 figures, typos corrected, presentational changes, PRD in
pres
Dynamic range of nanotube- and nanowire-based electromechanical systems
Nanomechanical resonators with high aspect ratio, such as nanotubes and nanowires are of interest due to their expected high sensitivity. However, a strongly nonlinear response combined with a high thermomechanical noise level limits the useful linear dynamic range of this type of device. We derive the equations governing this behavior and find a strong dependence [[proportional]dsqrt((d/L)[sup 5])] of the dynamic range on aspect ratio
Scalar field collapse in three-dimensional AdS spacetime
We describe results of a numerical calculation of circularly symmetric scalar
field collapse in three spacetime dimensions with negative cosmological
constant. The procedure uses a double null formulation of the Einstein-scalar
equations. We see evidence of black hole formation on first implosion of a
scalar pulse if the initial pulse amplitude is greater than a critical
value . Sufficiently near criticality the apparent horizon radius
grows with pulse amplitude according to the formula .Comment: 10 pages, 1 figure; references added, to appear in CQG(L
Two dimensional general covariance from three dimensions
A 3d generally covariant field theory having some unusual properties is
described. The theory has a degenerate 3-metric which effectively makes it a 2d
field theory in disguise. For 2-manifolds without boundary, it has an infinite
number of conserved charges that are associated with graphs in two dimensions
and the Poisson algebra of the charges is closed. For 2-manifolds with boundary
there are additional observables that have a Kac-Moody Poisson algebra. It is
further shown that the theory is classically integrable and the general
solution of the equations of motion is given. The quantum theory is described
using Dirac quantization, and it is shown that there are quantum states
associated with graphs in two dimensions.Comment: 10 pages (Latex), Alberta-Thy-19-9
Constants of motion for vacuum general relativity
The 3+1 Hamiltonian Einstein equations, reduced by imposing two commuting
spacelike Killing vector fields, may be written as the equations of the
principal chiral model with certain `source' terms. Using this
formulation, we give a procedure for generating an infinite number of non-local
constants of motion for this sector of the Einstein equations. The constants of
motion arise as explicit functionals on the phase space of Einstein gravity,
and are labelled by sl(2,R) indices.Comment: 10 pages, latex, version to appear in Phys. Rev. D
Basins of attraction of a nonlinear nanomechanical resonator
We present an experiment that systematically probes the basins of attraction
of two fixed points of a nonlinear nanomechanical resonator and maps them out
with high resolution. We observe a separatrix which progressively alters shape
for varying drive strength and changes the relative areas of the two basins of
attraction. The observed separatrix is blurred due to ambient fluctuations,
including residual noise in the drive system, which cause uncertainty in the
preparation of an initial state close to the separatrix. We find a good
agreement between the experimentally mapped and theoretically calculated basins
of attraction
Nanowire-based very-high-frequency electromechanical resonator
Fabrication and readout of devices with progressively smaller size, ultimately down to the molecular scale, is critical for the development of very-high-frequency nanoelectromechanical systems (NEMS). Nanomaterials, such as carbon nanotubes or nanowires, offer immense prospects as active elements for these applications. We report the fabrication and measurement of a platinum nanowire resonator, 43 nm in diameter and 1.3 ”m in length. This device, among the smallest NEMS reported, has a fundamental vibration frequency of 105.3 MHz, with a quality factor of 8500 at 4 K. Its resonant motion is transduced by a technique that is well suited to ultrasmall mechanical structures
Einstein's equations and the chiral model
The vacuum Einstein equations for spacetimes with two commuting spacelike
Killing field symmetries are studied using the Ashtekar variables. The case of
compact spacelike hypersurfaces which are three-tori is considered, and the
determinant of the Killing two-torus metric is chosen as the time gauge. The
Hamiltonian evolution equations in this gauge may be rewritten as those of a
modified SL(2) principal chiral model with a time dependent `coupling
constant', or equivalently, with time dependent SL(2) structure constants. The
evolution equations have a generalized zero-curvature formulation. Using this
form, the explicit time dependence of an infinite number of
spatial-diffeomorphism invariant phase space functionals is extracted, and it
is shown that these are observables in the sense that they Poisson commute with
the reduced Hamiltonian. An infinite set of observables that have SL(2) indices
are also found. This determination of the explicit time dependence of an
infinite set of spatial-diffeomorphism invariant observables amounts to the
solutions of the Hamiltonian Einstein equations for these observables.Comment: 22 pages, RevTeX, to appear in Phys. Rev.
Identification of genes required for the survival of B. fragilis using massive parallel sequencing of a saturated transposon mutant library
BACKGROUND: Bacteroides fragilis is a Gram-negative anaerobe that is normally a human gut commensal; it comprises a small percentage of the gut Bacteroides but is the most frequently isolated Bacteroides from human infections. Identification of the essential genes necessary for the survival of B. fragilis provides novel information which can be exploited for the treatment of bacterial infections. RESULTS: Massive parallel sequencing of saturated transposon mutant libraries (two mutant pools of approximately 50,000 mutants each) was used to determine the essential genes for the growth of B. fragilis 638R on nutrient rich medium. Among the 4326 protein coding genes, 550 genes (12.7%) were found to be essential for the survival of B. fragilis 638R. Of the 550 essential genes, only 367 genes were assigned to a Cluster of Orthologous Genes, and about 290 genes had Kyoto Encyclopedia of Genes and Genomes orthologous members. Interestingly, genes with hypothetical functions accounted for 41.3% of essential genes (227 genes), indicating that the functions of a significant percentage of the genes used by B. fragilis 638R are still unknown. Global transcriptome analysis using RNA-Seq indicated that most of the essential genes (92%) are, in fact, transcribed in B. fragilis 638R including most of those coding for hypothetical proteins. Three hundred fifty of the 550 essential genes of B. fragilis 638R are present in Database of Essential Genes. 10.02 and 31% of those are genes included as essential genes for nine species (including Gram-positive pathogenic bacteria). CONCLUSIONS: The essential gene data described in this investigation provides a valuable resource to study gene function and pathways involved in B. fragilis survival. Thorough examination of the B. fragilis-specific essential genes and genes that are shared between divergent organisms opens new research avenues that will lead to enhanced understanding of survival strategies used by bacteria in different microniches and under different stress situations. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi: 10.1186/1471-2164-15-429) contains supplementary material, which is available to authorized users
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