125,413 research outputs found
Microwave Imaging from Limited-Angle Scattered Data using the Iterative Multi-Scaling Approach
In this paper, with reference to limited-angle data configurations, the performance of the nonlinear multi-scaling inversion approach (IMSA) is analyzed. Such an assessment is carried out by considering synthetically-generated as well as laboratory-controlled experimental data ('Marseille data') concerning two-dimensional dielectric scatterers. The obtained results demonstrate a satisfactory robustness and the reliability of the approach
Magnetized birefringence and CMB polarization
The polarization plane of the cosmic microwave background radiation can be
rotated either in a magnetized plasma or in the presence of a quintessential
background with pseudoscalar coupling to electromagnetism. A unified treatment
of these two phenomena is presented for cold and warm electron-ion plasmas at
the pre-recombination epoch. The electron temperature is only relevant to the
relativistic correction of the cold plasma results. The spectrum of plasma
excitations is obtained from a generalized Appleton--Hartree equation,
describing simultaneously the high-frequency propagation of electromagnetic
waves in a magnetized plasma with a dynamical quintessence field. It is shown
that these two effects are comparable for the plausible range of parameters
allowed by present constraints. It is then argued that the generalized
expressions derived in the present study may be relevant for direct searches of
a possible rotation of the cosmic microwave background polarization.Comment: 9 pages; corrected typos, references adde
Semantic Ambiguity and Perceived Ambiguity
I explore some of the issues that arise when trying to establish a connection
between the underspecification hypothesis pursued in the NLP literature and
work on ambiguity in semantics and in the psychological literature. A theory of
underspecification is developed `from the first principles', i.e., starting
from a definition of what it means for a sentence to be semantically ambiguous
and from what we know about the way humans deal with ambiguity. An
underspecified language is specified as the translation language of a grammar
covering sentences that display three classes of semantic ambiguity: lexical
ambiguity, scopal ambiguity, and referential ambiguity. The expressions of this
language denote sets of senses. A formalization of defeasible reasoning with
underspecified representations is presented, based on Default Logic. Some
issues to be confronted by such a formalization are discussed.Comment: Latex, 47 pages. Uses tree-dvips.sty, lingmacros.sty, fullname.st
Boosted Statistical Mechanics
Based on the fundamental principles of Relativistic Quantum Mechanics, we
give a rigorous, but completely elementary, proof of the relation between
fundamental observables of a statistical system when measured relatively to two
inertial reference frames, connected by a Lorentz transformation.Comment: 8 page
Stringy bounces and gradient instabilities
Bouncing solutions are obtained from a generally covariant action
characterized by a potential which is a nonlocal functional of the dilaton
field at two separated space-time points. Gradient instabilities are shown to
arise in this context but they are argued to be nongeneric. After performing a
gauge-invariant and frame-invariant derivation of the evolution equations of
the fluctuations, a heuristic criterion for the avoidance of pathological
instabilities is proposed and corroborated by a number of explicit examples
that turn out to be compatible with a quasi-flat spectrum of curvature
inhomogeneities for typical wavelengths larger than the Hubble radius.Comment: 25 pages; comments added and corrected typos; to appear in Phys. Rev.
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