6,209 research outputs found
The Thomson scattering cross section in a magnetized, high density plasma
We calculate the Thomson scattering cross section in a non-relativistic,
magnetized, high density plasma -- in a regime where collective excitations can
be described by magnetohydrodynamics. We show that, in addition to cyclotron
resonances and an elastic peak, the cross section exhibits two pairs of peaks
associated with slow and fast magnetosonic waves; by contrast, the cross
section arising in pure hydrodynamics possesses just a single pair of Brillouin
peaks. Both the position and the width of these magnetosonic-wave peaks depend
on the ambient magnetic field and temperature, as well as transport and
thermodynamic coefficients, and so can therefore serve as a diagnostic tool for
plasma properties that are otherwise challenging to measure.Comment: Main paper: pp 1-8. Appendix: pp 8-10. 2 figure
Quantum cosmological consistency condition for inflation
We investigate the quantum cosmological tunneling scenario for inflationary
models. Within a path-integral approach, we derive the corresponding tunneling
probability distribution. A sharp peak in this distribution can be interpreted
as the initial condition for inflation and therefore as a quantum cosmological
prediction for its energy scale. This energy scale is also a genuine prediction
of any inflationary model by itself, as the primordial gravitons generated
during inflation leave their imprint in the B-polarization of the cosmic
microwave background. In this way, one can derive a consistency condition for
inflationary models that guarantees compatibility with a tunneling origin and
can lead to a testable quantum cosmological prediction. The general method is
demonstrated explicitly for the model of natural inflation.Comment: 1+16 pages, 3 figures. v2: typos corrected, minor improvement of the
discussio
What can quantum cosmology say about the inflationary universe?
We propose a method to extract predictions from quantum cosmology for
inflation that can be confronted with observations. Employing the tunneling
boundary condition in quantum geometrodynamics, we derive a probability
distribution for the inflaton field. A sharp peak in this distribution can be
interpreted as setting the initial conditions for the subsequent phase of
inflation. In this way, the peak sets the energy scale at which the
inflationary phase has started. This energy scale must be consistent with the
energy scale found from the inflationary potential and with the scale found
from a potential observation of primordial gravitational waves. Demanding a
consistent history of the universe from its quantum origin to its present
state, which includes decoherence, we derive a condition that allows one to
constrain the parameter space of the underlying model of inflation. We
demonstrate our method by applying it to two models: Higgs inflation and
natural inflation.Comment: 13 pages, 2 figures. Contribution to the Proceedings of the DICE14
meeting, Castiglioncello, September 201
Factorizing the Stochastic Galerkin System
Recent work has explored solver strategies for the linear system of equations
arising from a spectral Galerkin approximation of the solution of PDEs with
parameterized (or stochastic) inputs. We consider the related problem of a
matrix equation whose matrix and right hand side depend on a set of parameters
(e.g. a PDE with stochastic inputs semidiscretized in space) and examine the
linear system arising from a similar Galerkin approximation of the solution. We
derive a useful factorization of this system of equations, which yields bounds
on the eigenvalues, clues to preconditioning, and a flexible implementation
method for a wide array of problems. We complement this analysis with (i) a
numerical study of preconditioners on a standard elliptic PDE test problem and
(ii) a fluids application using existing CFD codes; the MATLAB codes used in
the numerical studies are available online.Comment: 13 pages, 4 figures, 2 table
Primitive abundant and weird numbers with many prime factors
We give an algorithm to enumerate all primitive abundant numbers (briefly,
PANs) with a fixed (the number of prime factors counted with their
multiplicity), and explicitly find all PANs up to , count all PANs
and square-free PANs up to and count all odd PANs and odd
square-free PANs up to . We find primitive weird numbers (briefly,
PWNs) with up to 16 prime factors, improving the previous results of
[Amato-Hasler-Melfi-Parton] where PWNs with up to 6 prime factors have been
given. The largest PWN we find has 14712 digits: as far as we know, this is the
largest example existing, the previous one being 5328 digits long [Melfi]. We
find hundreds of PWNs with exactly one square odd prime factor: as far as we
know, only five were known before. We find all PWNs with at least one odd prime
factor with multiplicity greater than one and and prove that there
are none with . Regarding PWNs with a cubic (or higher) odd prime
factor, we prove that there are none with , and we did not find
any with larger . Finally, we find several PWNs with 2 square odd prime
factors, and one with 3 square odd prime factors. These are the first such
examples.Comment: New section on open problems. A mistake in table 2 corrected (# odd
PAN with Omega=8). New PWN in table 5, last line, 2 squared prime factors,
Omega=15. Updated bibliograph
Fractal universe and quantum gravity
We propose a field theory which lives in fractal spacetime and is argued to
be Lorentz invariant, power-counting renormalizable, ultraviolet finite, and
causal. The system flows from an ultraviolet fixed point, where spacetime has
Hausdorff dimension 2, to an infrared limit coinciding with a standard
four-dimensional field theory. Classically, the fractal world where fields live
exchanges energy momentum with the bulk with integer topological dimension.
However, the total energy momentum is conserved. We consider the dynamics and
the propagator of a scalar field. Implications for quantum gravity, cosmology,
and the cosmological constant are discussed.Comment: 4 pages. v2: typos corrected; v3: discussion improved, intuitive
introduction added, matches the published versio
A New Fast Motion Estimation and Mode Decision algorithm for H.264 Depth Maps encoding in Free Viewpoint TV
In this paper, we consider a scenario where 3D scenes are modeled through a View+Depth representation. This representation is to be used at the rendering side to generate synthetic views for free viewpoint video. The encoding of both type of data (view and depth) is carried out using two H.264/AVC encoders. In this scenario we address the reduction of the encoding complexity of depth data. Firstly, an analysis of the Mode Decision and Motion Estimation processes has been conducted for both view and depth sequences, in order to capture the correlation between them. Taking advantage of this correlation, we propose a fast mode decision and motion estimation algorithm for the depth encoding. Results show that the proposed algorithm reduces the computational burden with a negligible loss in terms of quality of the rendered synthetic views. Quality measurements have been conducted using the Video Quality Metric
Effects produced by breach morphology on the outflow discharge due to the overtopping of earthfill dams
River hydrodynamicsUnsteady open channel flow and dam brea
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