21,013 research outputs found
Bose-enhanced chemistry: Amplification of selectivity in the dissociation of molecular Bose-Einstein condensates
We study the photodissociation chemistry of a quantum degenerate gas of
bosonic triatomic molecules, assuming two open rearrangement channels
( or ). The equations of motion are equivalent to those of a
parametric multimode laser, resulting in an exponential buildup of macroscopic
mode populations. By exponentially amplifying a small differential in the
single-particle rate-coefficients, Bose stimulation leads to a nearly complete
selectivity of the collective -body process, indicating a novel type of
ultra-selective quantum degenerate chemistry.Comment: 5 pages, 3 figure
Cosmic Acceleration Data and Bulk-Brane Energy Exchange
We consider a braneworld model with bulk-brane energy exchange. This allows
for crossing of the w=-1 phantom divide line without introducing phantom energy
with quantum instabilities. We use the latest SnIa data included in the Gold06
dataset to provide an estimate of the preferred parameter values of this
braneworld model. We use three fitting approaches which provide best fit
parameter values and hint towards a bulk energy component that behaves like
relativistic matter which is propagating in the bulk and is moving at a speed v
along the fifth dimension, while the bulk-brane energy exchange component
corresponds to negative pressure and signifies energy flowing from the bulk
into the brane. We find that the best fit effective equation of state parameter
marginally crosses the phantom divide line w=-1. Thus, we have
demonstrated both the ability of this class of braneworld models to provide
crossing of the phantom divide and also that cosmological data hint towards
natural values for the model parameters.Comment: 12 pages, 2 figures, added comments, references update
Light Deflection, Lensing, and Time Delays from Gravitational Potentials and Fermat's Principle in the Presence of a Cosmological Constant
The contribution of the cosmological constant to the deflection angle and the
time delays are derived from the integration of the gravitational potential as
well as from Fermat's Principle. The findings are in agreement with recent
results using exact solutions to Einstein's equations and reproduce precisely
the new -term in the bending angle and the lens equation. The
consequences on time delay expressions are explored. While it is known that
contributes to the gravitational time delay, it is shown here that a
new -term appears in the geometrical time delay as well. Although
these newly derived terms are perhaps small for current observations, they do
not cancel out as previously claimed. Moreover, as shown before, at galaxy
cluster scale, the contribution can be larger than the second-order
term in the Einstein deflection angle for several cluster lens systems.Comment: 6 pages, 1 figure, matches version published in PR
Anisotropic Cosmological Constant and the CMB Quadrupole Anomaly
There are evidences that the cosmic microwave background (CMB) large-angle
anomalies imply a departure from statistical isotropy and hence from the
standard cosmological model. We propose a LCDM model extension whose dark
energy component preserves its nondynamical character but wield anisotropic
vacuum pressure. Exact solutions for the cosmological scale factors are
presented, upper bounds for the deformation parameter are evaluated and its
value is estimated considering the elliptical universe proposal to solve the
quadrupole anomaly. This model can be constructed from a Bianchi I cosmology
with cosmological constant from two different ways: i) a straightforward
anisotropic modification of the vacuum pressure consistently with
energy-momentum conservation; ii) a Poisson structure deformation between
canonical momenta such that the dynamics remain invariant under scale factors
rescalings.Comment: 8 pages, 2 columns, 1 figure. v2: figure improved, added comments on
higher eccentricity powers and references. v3: typos corrected, version to
appear in PR
Nonlocal Cosmology
We explore nonlocally modified models of gravity, inspired by quantum loop
corrections, as a mechanism for explaining current cosmic acceleration. These
theories enjoy two major advantages: they allow a delayed response to cosmic
events, here the transition from radiation to matter dominance, and they avoid
the usual level of fine tuning; instead, emulating Dirac's dictum, the required
large numbers come from the large time scales involved. Their solar system
effects are safely negligible, and they may even prove useful to the black hole
information problem.Comment: Expanded(!) version, to appear in Phys. Rev. Letter
Chaotic Free-Space Laser Communication over Turbulent Channel
The dynamics of errors caused by atmospheric turbulence in a
self-synchronizing chaos based communication system that stably transmits
information over a 5 km free-space laser link is studied experimentally.
Binary information is transmitted using a chaotic sequence of short-term pulses
as carrier. The information signal slightly shifts the chaotic time position of
each pulse depending on the information bit. We report the results of an
experimental analysis of the atmospheric turbulence in the channel and the
impact of turbulence on the Bit-Error-Rate (BER) performance of this chaos
based communication system.Comment: 4 pages, 5 figure
On the Miura map between the dispersionless KP and dispersionless modified KP hierarchies
We investigate the Miura map between the dispersionless KP and dispersionless
modified KP hierarchies. We show that the Miura map is canonical with respect
to their bi-Hamiltonian structures. Moreover, inspired by the works of Takasaki
and Takebe, the twistor construction of solution structure for the
dispersionless modified KP hierarchy is given.Comment: 19 pages, Latex, no figure
Towards A Framework for Effective User Participation in Nonprofit Community Contexts: Beyond User Involvement
In recent years, IS researchers have begun to examine the broader societal impacts of information systems (IS) and technologies. As such, researchers have sought out various approaches to develop the technological capacity of nonprofit and community-based organizations (NCOs). The concept of user participation is gaining increasing attention as a malleable approach to achieve this goal. This paper develops a strategic framework that theorizes user participation in nonprofit and community-based contexts. We conclude with implications for research and practice
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