3,669 research outputs found
Reconstructing the linear power spectrum of cosmological mass fluctuations
We describe an attempt to reconstruct the initial conditions for the
formation of cosmological large-scale structure. The power spectrum of the
primordial fluctuations is affected by bias, nonlinear evolution and
redshift-space distortions, but we show how these effects can be corrected for
analytically. Using eight independent datasets, we obtain excellent agreement
in the estimated linear power spectra given the following conditions. First,
the relative bias factors for Abell clusters, radio galaxies, optical galaxies
and IRAS galaxies must be in the ratios 4.5:1.9:1.3:1. Second, the data require
redshift-space distortion: \Omega^{0.6}/b_{\ss I}
= 1.0 \pm 0.2. Third, low values of and bias are disfavoured. The
shape of the spectrum is extremely well described by a CDM transfer function
with an apparent value of the fitting parameter . Tilted models
predict too little power at 100 Mpc wavelengths.Comment: Edinburgh Astronomy Preprint 26/93. Accepted for publication in
Monthly Notices of the RAS. 13 pages of LaTeX, plus 10 PostScript figures.
You will need the mn.sty style file (from babbage: get mn.sty). The figure
.ps files are in the usual self-unpacking unix scrip
Identity and Search in Social Networks
Social networks have the surprising property of being "searchable": Ordinary
people are capable of directing messages through their network of acquaintances
to reach a specific but distant target person in only a few steps. We present a
model that offers an explanation of social network searchability in terms of
recognizable personal identities: sets of characteristics measured along a
number of social dimensions. Our model defines a class of searchable networks
and a method for searching them that may be applicable to many network search
problems, including the location of data files in peer-to-peer networks, pages
on the World Wide Web, and information in distributed databases.Comment: 4 page, 3 figures, revte
Nonlinear evolution of cosmological power spectra
Hamilton et al. have suggested an invaluable scaling formula which describes
how the power spectra of density fluctuations evolve into the nonlinear regime
of hierarchical clustering. This paper presents an extension of their method to
low-density universes and universes with nonzero cosmological constant. We pay
particular attention to models with large negative spectral indices, and give a
spectrum-dependent fitting formula which is of significantly improved accuracy
by comparison with an earlier version of this work. The tendency of nonlinear
effects to increase power on small scales is stronger for spectra with more
negative spectral indices, and for lower densities. However, for low-density
models with a cosmological constant, the nonlinear effects are less strong than
for an open universe of the same .Comment: MNRAS in press. 8 pages TeX including Postscript figures. Uses mn.tex
and epsf.te
Recommended from our members
Microwave Heating of Lunar Simulants JSC-1A and NU-LHT-3M: Experimental And Theoretical Analysis
Recommended from our members
mHealth: providing a mindfulness app for women with chronic pelvic pain in gynaecology outpatient clinics: qualitative data analysis of user experience and lessons learnt
OBJECTIVES: To determine whether a pre-existing smartphone app to teach mindfulness meditation is acceptable to women with chronic pelvic pain (CPP) and can be integrated into clinical practice within the National Health Service (NHS) CPP pathways, and to inform the design of a potential randomised clinical trial.
DESIGN: A prestudy patient and public involvement (PPI) group to collect feedback on the acceptability of the existing app and study design was followed by a three-arm randomised feasibility trial. In addition, we undertook interviews and focus groups with patients and staff to explore app usability and acceptability. We also obtained participant comments on the research process, such as acceptability of the study questionnaires.
SETTING: Two gynaecology clinics within Barts Health NHS, London, UK.
PARTICIPANTS: Patients with CPP lasting ≥6 months with access to smartphone or personal computer and understanding of basic English.
INTERVENTION: The intervention was mindfulness meditation content plus additional pain module delivered by a smartphone app. Active controls received muscle relaxation content from the same app. Passive (waiting list) controls received usual care.
MAIN OUTCOME MEASURES: Themes on user feedback, app usability and integration, and reasons for using/not using the app.
RESULTS: The use of the app was low in both active groups. Patients in the prestudy PPI group, all volunteers, were enthusiastic about the app (convenience, content, portability, flexibility, ease of use). Women contributing to the interview or focus group data (n=14), from a 'real world' clinic (some not regular app users), were less positive, citing as barriers lack of opportunities/motivation to use the app and lack of familiarity and capabilities with technology. Staff (n=7) were concerned about the potential need for extra support for them and for the patients, and considered the app needed organisational backing and peer acceptance.
CONCLUSION: The opinions of prestudy PPI volunteers meeting in their private time may not represent those of patients recruited at a routine clinic appointment. It may be more successful to codesign/codevelop an app with typical users than to adapt existing apps for use in real-world clinical populations.
TRIAL REGISTRATION NUMBER: ISRCTN10925965
Geometry of River Networks II: Distributions of Component Size and Number
The structure of a river network may be seen as a discrete set of nested
sub-networks built out of individual stream segments. These network components
are assigned an integral stream order via a hierarchical and discrete ordering
method. Exponential relationships, known as Horton's laws, between stream order
and ensemble-averaged quantities pertaining to network components are observed.
We extend these observations to incorporate fluctuations and all higher moments
by developing functional relationships between distributions. The relationships
determined are drawn from a combination of theoretical analysis, analysis of
real river networks including the Mississippi, Amazon and Nile, and numerical
simulations on a model of directed, random networks. Underlying distributions
of stream segment lengths are identified as exponential. Combinations of these
distributions form single-humped distributions with exponential tails, the sums
of which are in turn shown to give power law distributions of stream lengths.
Distributions of basin area and stream segment frequency are also addressed.
The calculations identify a single length-scale as a measure of size
fluctuations in network components. This article is the second in a series of
three addressing the geometry of river networks.Comment: 16 pages, 13 figures, 4 tables, Revtex4, submitted to PR
Space Shuttle program communication and tracking systems interface analysis
The Space Shuttle Program Communications and Tracking Systems Interface Analysis began April 18, 1983. During this time, the shuttle communication and tracking systems began flight testing. Two areas of analysis documented were a result of observations made during flight tests. These analyses involved the Ku-band communication system. First, there was a detailed analysis of the interface between the solar max data format and the Ku-band communication system including the TDRSS ground station. The second analysis involving the Ku-band communication system was an analysis of the frequency lock loop of the Gunn oscillator used to generate the transmit frequency. The stability of the frequency lock loop was investigated and changes to the design were reviewed to alleviate the potential loss of data due the loop losing lock and entering the reacquisition mode. Other areas of investigation were the S-band antenna analysis and RF coverage analysis
In situ characterization of mixing and sedimentation dynamics in an impinging jet ballast tank via acoustic backscatter
Impinging jets are utilized in numerous applications, including nuclear waste treatment, for both the erosion of sediment beds and maintaining particulates in suspension. Pulse-echo ultrasonic methods offer great potential for the in situ monitoring of critical mixing and settling dynamics, in concentrated dispersions. A non-active scaled version of a Highly Active Storage Tank at Sellafield, UK, was profiled with an acoustic backscatter system under various jet firing conditions. An advanced analysis technique enabled the direct quantification of dispersion concentration changes from the converted backscatter attenuation. Hence, the erosion and mixing capability of the jets, and settling kinetics were characterized. It was found that jet operation alone provided inadequate localized mixing of eroded sediment. An additional air-lift process operation was required to hinder the rapid re-settling of dispersed particulates
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