9,223 research outputs found
From Hubble diagrams to scale factors
We present a lower bound on the radius of the universe today and a
monotonicity constraint on the Hubble diagram. Our theoretical input is
Einstein's kinematics and maximally symmetric universes. Present supernova data
yield m. A first attempt to quantify the monotonicity
constraint is described. We do not see any indication of non-monotonicity.Comment: 13 pages, 6 figure
Maximal symmetry at the speed of light
We propose a relativistic version of the cosmological principle and confront
it to the Hubble diagram of supernovae and other probes.Comment: 11 pages, 1 figur
Institutions and Economic Growth: The Successful Experience of Switzerland (1870-1950)
economic growth, institutions, Switzerland, small countries
Einstein-Cartan, Bianchi I and the Hubble Diagram
We try to solve the dark matter problem in the fit between theory and the
Hubble diagram of supernovae by allowing for torsion via Einstein-Cartan's
gravity and for anisotropy via the axial Bianchi I metric. Otherwise we are
conservative and admit only the cosmological constant and dust. The failure of
our model is quantified by the relative amount of dust in our best fit:
Omega_{m0}= 27 % +/- 5 % at 1 sigma level.Comment: 11 pages, 1 figur
Well-Founded Semantics for Extended Datalog and Ontological Reasoning
The Datalog± family of expressive extensions of Datalog has recently been introduced as a new paradigm for query answering over ontologies, which captures and extends several common description logics. It extends plain Datalog by features such as existentially quantified rule heads and, at the same time, restricts the rule syntax so as to achieve decidability and tractability. In this paper, we continue the research on Datalog±. More precisely, we generalize the well-founded semantics (WFS), as the standard semantics for nonmonotonic normal programs in the database context, to Datalog± programs with negation under the unique name assumption (UNA). We prove that for guarded Datalog± with negation under the standard WFS, answering normal Boolean conjunctive queries is decidable, and we provide precise complexity results for this problem, namely, in particular, completeness for PTIME (resp., 2-EXPTIME) in the data (resp., combined) complexity
Forward and inverse problems in fundamental and applied magnetohydrodynamics
This Minireview summarizes the recent efforts to solve forward and inverse
problems as they occur in different branches of fundamental and applied
magnetohydrodynamics. As for the forward problem, the main focus is on the
numerical treatment of induction processes, including self-excitation of
magnetic fields in non-spherical domains and/or under the influence of
non-homogeneous material parameters. As an important application of the
developed numerical schemes, the functioning of the von-K\'{a}rm\'{a}n-sodium
(VKS) dynamo experiment is shown to depend crucially on the presence of
soft-iron impellers. As for the inverse problem, the main focus is on the
mathematical background and some first practical applications of the
Contactless Inductive Flow Tomography (CIFT), in which flow induced magnetic
field perturbations are utilized for the reconstruction of the velocity field.
The promises of CIFT for flow field monitoring in the continuous casting of
steel are substantiated by results obtained at a test rig with a low melting
liquid metal. While CIFT is presently restricted to flows with low magnetic
Reynolds numbers, some selected problems of non-linear inverse dynamo theory,
with possible application to geo- and astrophysics, are also discussed.Comment: 15 pages, 4 figures, accepted for publication in European Physical
Journal Special Topic
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