2,716 research outputs found
An Ontology for Submarine Feature Representation on Charts
A landform is a subjective individuation of a part of a terrain. Landform recognition is a difficult task because its definition usually relies on a qualitative and fuzzy description. Achieving automatic recognition of landforms requires a formal definition of the landforms properties and their modelling. In the maritime domain, the International Hydrographic Organisation published a standard terminology of undersea feature names which formalises a set of definition mainly for naming and communication purpose. This terminology is here used as a starting point for the definition of an ontology of undersea features and their automatic classification from a terrain model. First, an ontology of undersea features is built. The ontology is composed of an application domain ontology describing the main properties and relationships between features and a representation ontology deals with representation on a chart where features are portrayed by soundings and isobaths. A database model was generated from the ontology. Geometrical properties describing the feature shape are computed from soundings and isobaths and are used for feature classification. An example of automatic classification on a nautical chart is presented and results and on-going research are discussed
Instability Proof for Einstein-Yang-Mills Solitons and Black Holes with Arbitrary Gauge Groups
We prove that static, spherically symmetric, asymptotically flat soliton and
black hole solutions of the Einstein-Yang-Mills equations are unstable for
arbitrary gauge groups, at least for the ``generic" case. This conclusion is
derived without explicit knowledge of the possible equilibrium solutions.Comment: 26 pages, LATEX, no figure
Effective action approach to higher-order relativistic tidal interactions in binary systems and their effective one body description
The gravitational-wave signal from inspiralling neutron-star--neutron-star
(or black-hole--neutron-star) binaries will be influenced by tidal coupling in
the system. An important science goal in the gravitational-wave detection of
these systems is to obtain information about the equation of state of neutron
star matter via the measurement of the tidal polarizability parameters of
neutron stars. To extract this piece of information will require to have
accurate analytical descriptions of both the motion and the radiation of
tidally interacting binaries. We improve the analytical description of the late
inspiral dynamics by computing the next-to-next-to-leading order relativistic
correction to the tidal interaction energy. Our calculation is based on an
effective-action approach to tidal interactions, and on its transcription
within the effective-one-body formalism. We find that second-order relativistic
effects (quadratic in the relativistic gravitational potential ) significantly increase the effective tidal polarizability of
neutron stars by a distance-dependent amplification factor of the form where, say for an equal-mass binary,
(as previously known) and (as
determined here for the first time). We argue that higher-order relativistic
effects will lead to further amplification, and we suggest a Pad\'e-type way of
resumming them. We recommend to test our results by comparing
resolution-extrapolated numerical simulations of inspiralling-binary neutron
stars to their effective one body description.Comment: 29 pages, Physical Review D, to appea
On the Stability of Gravitating Nonabelian Monopoles
The behaviour of magnetic monopole solutions of the Einstein-Yang-Mills-Higgs
equations subject to linear spherically symmetric perturbations is studied.
Using Jacobi's criterion some of the monopoles are shown to be unstable.
Furthermore the numerical results and analytical considerations indicate the
existence of a set of stable solutions.Comment: 9 pages, 7 figures, minor change
On Gravitational Waves in Spacetimes with a Nonvanishing Cosmological Constant
We study the effect of a cosmological constant on the propagation
and detection of gravitational waves. To this purpose we investigate the
linearised Einstein's equations with terms up to linear order in in a
de Sitter and an anti-de Sitter background spacetime. In this framework the
cosmological term does not induce changes in the polarization states of the
waves, whereas the amplitude gets modified with terms depending on .
Moreover, if a source emits a periodic waveform, its periodicity as measured by
a distant observer gets modified. These effects are, however, extremely tiny
and thus well below the detectability by some twenty orders of magnitude within
present gravitational wave detectors such as LIGO or future planned ones such
as LISA.Comment: 8 pages, 4 figures, accepted for publication in Physical Review
Aspects of hairy black holes in spontaneously-broken Einstein-Yang-Mills systems: Stability analysis and Entropy considerations
We analyze (3+1)-dimensional black-hole space-times in spontaneously broken
Yang-Mills gauge theories that have been recently presented as candidates for
an evasion of the scalar-no-hair theorem. Although we show that in principle
the conditions for the no-hair theorem do not apply to this case, however we
prove that the `spirit' of the theorem is not violated, in the sense that there
exist instabilities, in both the sphaleron and gravitational sectors. The
instability analysis of the sphaleron sector, which was expected to be unstable
for topological reasons, is performed by means of a variational method. As
shown, there exist modes in this sector that are unstable against linear
perturbations. Instabilities exist also in the gravitational sector. A method
for counting the gravitational unstable modes, which utilizes a
catastrophe-theoretic approach is presented. The r\^ole of the catastrophe
functional is played by the mass functional of the black hole. The Higgs vacuum
expectation value (v.e.v.) is used as a control parameter, having a critical
value beyond which instabilities are turned on. The (stable) Schwarzschild
solution is then understood from this point of view. The catastrophe-theory
appproach facilitates enormously a universal stability study of non-Abelian
black holes, which goes beyond linearized perturbations. Some elementary
entropy considerations are also presented...Comment: Latex file, 50 pages, 2 figures (included as PS files at the end:
plot1.ps, plot2.ps
Rotating solitons and non-rotating, non-static black holes
It is shown that the non-Abelian black hole solutions have stationary
generalizations which are parameterized by their angular momentum and electric
Yang-Mills charge. In particular, there exists a non-static class of stationary
black holes with vanishing angular momentum. It is also argued that the
particle-like Bartnik-McKinnon solutions admit slowly rotating, globally
regular excitations. In agreement with the non-Abelian version of the staticity
theorem, these non-static soliton excitations carry electric charge, although
their non-rotating limit is neutral.Comment: 5 pages, REVTe
O(4) texture with a cosmological constant
We investigate O(4) textures in a background with a positive cosmological
constant. We find static solutions which co-move with the expanding background.
There exists a solution in which the scalar field is regular at the horizon.
This solution has a noninteger winding number smaller than one. There also
exist solutions in which scalar-field derivatives are singular at the horizon.
Such solutions can complete one winding within the horizon. If the winding
number is larger than some critical value, static solutions including the
regular one are unstable under perturbations.Comment: 25 pages, revtex, 6 eps figure
What measurable zero point fluctuations can(not) tell us about dark energy
We show that laboratory experiments cannot measure the absolute value of dark
energy. All known experiments rely on electromagnetic interactions. They are
thus insensitive to particles and fields that interact only weakly with
ordinary matter. In addition, Josephson junction experiments only measure
differences in vacuum energy similar to Casimir force measurements. Gravity,
however, couples to the absolute value. Finally we note that Casimir force
measurements have tested zero point fluctuations up to energies of ~10 eV, well
above the dark energy scale of ~0.01 eV. Hence, the proposed cut-off in the
fluctuation spectrum is ruled out experimentally.Comment: 4 page
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