748,289 research outputs found
A Concise Introduction to Perturbation Theory in Cosmology
We give a concise, self-contained introduction to perturbation theory in
cosmology at linear and second order, striking a balance between mathematical
rigour and usability. In particular we discuss gauge issues and the active and
passive approach to calculating gauge transformations. We also construct
gauge-invariant variables, including the second order tensor perturbation on
uniform curvature hypersurfaces.Comment: revtex4, 16 pages, 3 figures; v2: minor changes, typos corrected,
reference added, version accepted by CQ
Using mean field theory to determine the structure of uniform fluids
The structure of a uniform simple liquid is related to that of a reference
fluid with purely repulsive intermolecular forces in a self-consistently
determined external reference field (ERF) phi_ R. The ERF can be separated into
a harshly repulsive part phi_ R0 generated by the repulsive core of a reference
particle fixed at the origin and a more slowly varying part phi_ R1 arising
from a mean field treatment of the attractive forces. We use a generalized
linear response method to calculate the reference fluid structure, first
determining the response to the smoother part phi_ R1 of the ERF alone,
followed by the response to the harshly repulsive part. Both steps can be
carried out very accurately, as confirmed by MD simulations, and good agreement
with the structure of the full LJ fluid is found.Comment: 11 pages, 7 figure
Self-stabilizing TDMA Algorithms for Wireless Ad-hoc Networks without External Reference
Time division multiple access (TDMA) is a method for sharing communication
media. In wireless communications, TDMA algorithms often divide the radio time
into timeslots of uniform size, , and then combine them into frames of
uniform size, . We consider TDMA algorithms that allocate at least one
timeslot in every frame to every node. Given a maximal node degree, ,
and no access to external references for collision detection, time or position,
we consider the problem of collision-free self-stabilizing TDMA algorithms that
use constant frame size.
We demonstrate that this problem has no solution when the frame size is , where is the chromatic number for
distance- vertex coloring. As a complement to this lower bound, we focus on
proving the existence of collision-free self-stabilizing TDMA algorithms that
use constant frame size of . We consider basic settings (no hardware
support for collision detection and no prior clock synchronization), and the
collision of concurrent transmissions from transmitters that are at most two
hops apart. In the context of self-stabilizing systems that have no external
reference, we are the first to study this problem (to the best of our
knowledge), and use simulations to show convergence even with computation time
uncertainties
Fully permanent magnet atom chip for Bose-Einstein condensation
We describe a self-biased, fully permanent magnet atom chip used to study
ultracold atoms and to produce a Bose-Einstein condensate (BEC). The magnetic
trap is loaded efficiently by adiabatic transport of a magnetic trap via the
application of uniform external fields. Radio frequency spectroscopy is used
for in-trap analysis and to determine the temperature of the atomic cloud. The
formation of a Bose-Einstein condensate is observed in time of flight images
and as a narrow peak appearing in the radio frequency spectrum.Comment: changed title, substantial text modifications, journal reference
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Inference of plasmid copy number mean and noise from single cell gene expression data
Plasmids are extra-chromosomal DNA molecules which code for their own
replication. We previously reported a setup using genes coding for fluorescent
proteins of two colors that allowed us, using a simple model, to extract the
plasmid copy number noise in a monoclonal population of bacteria [J. Wong Ng et
al., Phys. Rev. E, 81, 011909 (2010)]. Here we present a detailed calculation
relating this noise to the measured levels of fluorescence, taking into account
all sources of fluorescence fluctuations: the fluctuation of gene expression as
in the simple model, but also the growth and division of bacteria, the
non-uniform distribution of their ages, the random partition of proteins at
divisions and the replication and partition of plasmids and chromosome. We show
how using the chromosome as a reference helps extracting the plasmid copy
number noise in a self-consistent manner.Comment: 9 pages, 3 figures, 2 table
Combination of geodetic observations and models for glacial isostatic adjustment fields in Fennoscandia
We demonstrate a new technique for using geodetic data to update a priori predictions for Glacial Isostatic Adjustment (GIA) in the Fennoscandia region. Global Positioning System (GPS), tide gauge, and Gravity Recovery and Climate Experiment (GRACE) gravity rates are assimilated into our model. The technique allows us to investigate the individual contributions from these data sets to the output GIA model in a self-consistent manner. Another benefit of the technique is that we are able to estimate uncertainties for the output model. These are reduced with each data set assimilated. Any uncertainties in the GPS reference frame are absorbed by reference frame adjustments that are estimated as part of the assimilation. Our updated model shows a spatial pattern and magnitude of peak uplift that is consistent with previous models, but our location of peak uplift is slightly to the east of many of these. We also simultaneously estimate a spatially averaged rate of local sea level rise. This regional rate (similar to 1.5 mm/yr) is consistent for all solutions, regardless of which data sets are assimilated or the magnitude of a priori GPS reference frame constraints. However, this is only the case if a uniform regional gravity rate, probably representing errors in, or unmodeled contributions to, the low-degree harmonic terms from GRACE, is also estimated for the assimilated GRACE data. Our estimated sea level rate is consistent with estimates obtained using a more traditional approach of direct "correction" using collocated GPS and tide gauge site
A novel technique for sterilization using a power self-regulated single-mode microwave cavity
In this paper, a novel technique to achieve precise temperatures in food sterilization has been proposed. An accurate temperature profile is needed in order to reach a commitment between the total removal of pathogens inside the product and the preservation of nutritional and organoleptic characteristics. The minimal variation of the target temperature in the sample by means of a monitoring and control software platform, allowing temperature stabilization over 100 °C, is the main goal of this work. A cylindrical microwave oven, under pressure conditions and continuous control of the microwave supply power as function of the final temperature inside the sample, has been designed and developed with conditions of single-mode resonance. The uniform heating in the product is achieved by means of sample movement and the self-regulated power control using the measured temperature. Finally, for testing the sterilization of food with this technology, specific biological validation based on Bacillus cereus as a biosensor of heat inactivation has been incorporated as a distribution along the sample in the experimental process to measure the colony-forming units (CFUs) for different food samples (laboratory medium, soup, or fish-based animal by-products). The obtained results allow the validation of this new technology for food sterilization with precise control of the microwave system to ensure the uniform elimination of pathogens using high temperatures.This work was supported by the Ministerio de Economía y Competitividad under the project with reference DPI2014-61857-EXP
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