42,214 research outputs found
Reionization Through the Lens of Percolation Theory
The reionization of intergalactic hydrogen has received intense theoretical
scrutiny over the past two decades. Here, we approach the process formally as a
percolation process and phase transition. Using semi-numeric simulations, we
demonstrate that an infinitely-large ionized region abruptly appears at an
ionized fraction of ~0.1 and quickly grows to encompass most of the ionized
gas: by an ionized fraction of 0.3, nearly ninety percent of the ionized
material is part of this region. Throughout most of reionization, nearly all of
the intergalactic medium is divided into just two regions, one ionized and one
neutral, and both infinite in extent. We also show that the discrete ionized
regions that exist before and near this transition point follow a near-power
law distribution in volume, with equal contributions to the total filling
factor per logarithmic interval in size up to a sharp cutoff in volume. These
qualities are generic to percolation processes, with the detailed behavior a
result of long-range correlations in the underlying density field. These
insights will be crucial to understanding the distribution of ionized and
neutral gas during reionization and provide precise meaning to the intuitive
description of reionization as an "overlap" process.Comment: 16 pages, version accepted by MNRAS (conclusions unchanged from
original
When and how much to talk: credibility and flexibility in monetary policy with private information
This paper analyzes how noisy or imprecise announcements might partially remove the inefficiencies resulting from the credibility problem in monetary policy when the presence of non-verifiable private information adds another dimension to that problem. The analysis finds that imprecise or noisy announcements can be a meaningful form of communication only if it is possible to "tie" the hands of the monetary authority somehow. To the extent that it is otherwise efficient for policy to react to the monetary authority?s private information, such announcements can be extremely costly in terms of the sacrifice in flexibility required to make them relevant. Suprisingly, the conditions under which the monetary authority can make more precise announcements are identical to those under which the monetary authority is less likely to prefer the noisy announcement equilibrium.Monetary policy
Stability and asymptotic behavior of periodic traveling wave solutions of viscous conservation laws in several dimensions
Under natural spectral stability assumptions motivated by previous
investigations of the associated spectral stability problem, we determine sharp
estimates on the linearized solution operator about a multidimensional
planar periodic wave of a system of conservation laws with viscosity, yielding
linearized stability for all and dimensions and nonlinear stability and
-asymptotic behavior for and . The behavior can in
general be rather complicated, involving both convective (i.e., wave-like) and
diffusive effects
Kinetics and thermodynamics of ceramic/metal interface reactions related to high T(sub c) superconducting applications
Superconducting ceramic materials, no matter what their form, size or shape, must eventually make contact with non-superconducting materials in order to accomplish current transfer to other parts of a real operating system, or for testing and measurement of properties. Thus, whether the configuration is a clad wire, a bulk superconducting disc, tape, or a thick or thin superconducting film on a substrate, the physical and mechanical behavior of interface (interconnections, joints, etc.) between superconductors and normal conductor materials of all kinds is of extreme importance to the technological development of these systems. Fabrication heat treatments associated with the particular joining process allow possible reactions between the superconducting ceramic and the contact to occur, and consequently influence properties at the interface region. The nature of these reactions is therefore of great broad interest, as these may be a primary determinant for the real capability of these materials. Research related both to fabrication of composite sheathed wire products, and the joining contacts for physical property measurements, as well as, a review of other related literature in the field are described. Comparison are made between 1-2-3, Bi-, and Tl-based ceramic superconductors joined to a variety of metals including Cu, Ni, Fe, Cr, Ag, Ag-Pd, Au, In, and Ga. The morphology of reaction products and the nature of interface degradation as a function of time will be highlighted
Maneuvering strategies using CMGs
This paper considers control strategies for maneuvering spacecraft using Single-Gimbal Control Momentum Gyros (CMGs). A pyramid configuration using four gyros is utilized. Preferred initial gimbal angles for maximum utilization of CMG momentum are obtained for some known torque commands. Feedback control laws are derived from the stability point of view by using the Liapunov's Second Theorem. The gyro rates are obtained by the pseudo-inverse technique. The effect of gimbal rate bounds on controllability are studied for an example maneuver. Singularity avoidance is based on limiting the gyro rates depending on a singularity index
SQCD Vacua and Geometrical Engineering
We consider the geometrical engineering constructions for the N = 1 SQCD
vacua recently proposed by Giveon and Kutasov. After one T-duality, the
geometries with wrapped D5 branes become N = 1 brane configurations with NS
branes and D4 branes. The field theories encoded by the geometries contain
extra massive adjoint fields for the flavor group. After performing a flop, the
geometries contain branes, antibranes and branes wrapped on non-holomorphic
cycles. The various tachyon condensations between pairs of wrapped D5 branes
and anti D5 branes together with deformations of the cycles give rise to a
variety of supersymmetric and metastable non-supersymmetric vacua.Comment: 21 Pages, Latex, 8 Figure
Photo-heating and the fate of hard photons during the reionisation of HeII by quasars
We use a combination of analytic and numerical arguments to consider the
impact of quasar photo-heating during HeII reionisation on the thermal
evolution of the intergalactic medium (IGM). We demonstrate that rapid (\Delta
z 10^4 K) photo-heating is difficult to achieve
across the entire IGM unless quasar spectra are significantly harder than
implied by current observational constraints. Although filtering of intrinsic
quasar radiation through dense regions in the IGM does increase the mean excess
energy per HeII photo-ionisation, it also weakens the radiation intensity and
lowers the photo-ionisation rate, preventing rapid heating over time intervals
shorter than the local photo-ionisation timescale. Moreover, the hard photons
responsible for the strongest heating are more likely to deposit their energy
inside dense clumps. The abundance of such clumps is, however, uncertain and
model-dependent, leading to a fairly large uncertainty in the photo-heating
rates. Nevertheless, although some of the IGM may be exposed to a hardened and
weakened ionising background for long periods, most of the IGM must instead be
reionised by the more abundant, softer photons and with accordingly modest
heating rates (\Delta T < 10^4 K). The repeated ionisation of fossil quasar
HeIII regions does not increase the net heating because the recombination times
in these regions typically exceed the IGM cooling times and the average time
lag between successive rounds of quasar activity. Detailed line-of-sight
radiative transfer simulations confirm these expectations and predict a rich
thermal structure in the IGM during HeII reionisation. [Abridged]Comment: 20 pages, 6 figures, accepted by MNRA
-kaon cross section in meson exchange model
We calculate the cross section for the dissociation of by kaons
within the framework of a meson exchange model including anomalous parity
interactions. Off-shell effects at the vertices were handled with QCD sum rule
estimates for the running coupling constants. The total -kaon cross
section was found to be mb for 4.1\leq\sqrt{s}\leq5 \GeV.Comment: 13 pages, 4 eps figure
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