17,760 research outputs found
Studying and Modeling the Connection between People's Preferences and Content Sharing
People regularly share items using online social media. However, people's
decisions around sharing---who shares what to whom and why---are not well
understood. We present a user study involving 87 pairs of Facebook users to
understand how people make their sharing decisions. We find that even when
sharing to a specific individual, people's own preference for an item
(individuation) dominates over the recipient's preferences (altruism). People's
open-ended responses about how they share, however, indicate that they do try
to personalize shares based on the recipient. To explain these contrasting
results, we propose a novel process model of sharing that takes into account
people's preferences and the salience of an item. We also present encouraging
results for a sharing prediction model that incorporates both the senders' and
the recipients' preferences. These results suggest improvements to both
algorithms that support sharing in social media and to information diffusion
models.Comment: CSCW 201
The power spectrum of galaxies in the 2dF 100k redshift survey
We compute the real-space power spectrum and the redshift-space distortions
of galaxies in the 2dF 100k galaxy redshift survey using pseudo-Karhunen-Loeve
eigenmodes and the stochastic bias formalism. Our results agree well with those
published by the 2dFGRS team, and have the added advantage of producing
easy-to-interpret uncorrelated minimum-variance measurements of the
galaxy-galaxy, galaxy-velocity and velocity-velocity power spectra in 27
k-bands, with narrow and well-behaved window functions in the range 0.01h/Mpc <
k < 0.8h/Mpc. We find no significant detection of baryonic wiggles, although
our results are consistent with a standard flat Omega_Lambda=0.7
``concordance'' model and previous tantalizing hints of baryonic oscillations.
We measure the galaxy-matter correlation coefficient r > 0.4 and the
redshift-distortion parameter beta=0.49+/-0.16 for r=1 (beta=0.47+/- 0.16
without finger-of-god compression). Since this is an apparent-magnitude limited
sample, luminosity-dependent bias may cause a slight red-tilt in the power
spectum. A battery of systematic error tests indicate that the survey is not
only impressive in size, but also unusually clean, free of systematic errors at
the level to which our tests are sensitive. Our measurements and window
functions are available at http://www.hep.upenn.edu/~max/2df.html together with
the survey mask, radial selection function and uniform subsample of the survey
that we have constructed.Comment: Replaced to match accepted MNRAS version, with new radial/angular
systematics plot and sigma8 typo corrected. High-res figures, power spectra,
windows and our uniform galaxy subsample with mask at
http://www.hep.upenn.edu/~max/2df.html or from [email protected]. 26
journal pages, 28 fig
Applications of Wavelets to the Analysis of Cosmic Microwave Background Maps
We consider wavelets as a tool to perform a variety of tasks in the context
of analyzing cosmic microwave background (CMB) maps. Using Spherical Haar
Wavelets we define a position and angular-scale-dependent measure of power that
can be used to assess the existence of spatial structure. We apply planar
Daubechies wavelets for the identification and removal of points sources from
small sections of sky maps. Our technique can successfully identify virtually
all point sources which are above 3 sigma and more than 80% of those above 1
sigma. We discuss the trade-offs between the levels of correct and false
detections. We denoise and compress a 100,000 pixel CMB map by a factor of
about 10 in 5 seconds achieving a noise reduction of about 35%. In contrast to
Wiener filtering the compression process is model independent and very fast. We
discuss the usefulness of wavelets for power spectrum and cosmological
parameter estimation. We conclude that at present wavelet functions are most
suitable for identifying localized sources.Comment: 10 pages, 6 figures. Submitted to MNRA
Graviton mass and total relative density of mass Omega_tot in Universe
It is noticed that the total relative density of mass in the Universe
Omega_tot should exceed 1, i.e. Omega_tot=1+f^2/6 according to the field
relativistic theory of gravity (RTG), which is free of the cosmological
singularity and which provides the Euclidean character for the 3-dimensional
space. Here f is the ratio of the graviton mass m_g to the contemporary value
of the ``Hubble mass'' m^0_H=\hbar H_0/c^2\simeq 3,8\cdot 10^{-66}h(g)
(h=0,71\pm0,07). Applying results of the experimental data processing presented
in [1] an upper limit for the graviton mass is established as m_g\leq 3,2\cdot
10^{-66}g at the 95% confidence level.Comment: 8 pages, latex fil
A Way to Dynamically Overcome the Cosmological Constant Problem
The Cosmological Constant problem can be solved once we require that the full
standard Einstein Hilbert lagrangian, gravity plus matter, is multiplied by a
total derivative. We analyze such a picture writing the total derivative as the
covariant gradient of a new vector field (b_mu). The dynamics of this b_mu
field can play a key role in the explanation of the present cosmological
acceleration of the Universe.Comment: 5 page
The Evolution of the Cosmic Microwave Background
We discuss the time dependence and future of the Cosmic Microwave Background
(CMB) in the context of the standard cosmological model, in which we are now
entering a state of endless accelerated expansion. The mean temperature will
simply decrease until it reaches the effective temperature of the de Sitter
vacuum, while the dipole will oscillate as the Sun orbits the Galaxy. However,
the higher CMB multipoles have a richer phenomenology. The CMB anisotropy power
spectrum will for the most part simply project to smaller scales, as the
comoving distance to last scattering increases, and we derive a scaling
relation that describes this behaviour. However, there will also be a dramatic
increase in the integrated Sachs-Wolfe contribution at low multipoles. We also
discuss the effects of tensor modes and optical depth due to Thomson
scattering. We introduce a correlation function relating the sky maps at two
times and the closely related power spectrum of the difference map. We compute
the evolution both analytically and numerically, and present simulated future
sky maps.Comment: 23 pages, 11 figures; references added; one figure dropped and minor
changes to match published version. For high-resolution versions of figures
and animations, see http://www.astro.ubc.ca/people/scott/future.htm
Strengthening impact assessment: a call for integration and focus
We suggest that the impact assessment community has lost its way based on our observation that impact assessment is under attack because of a perceived lack of efficiency. Specifically, we contend that the proliferation of different impact assessment types creates separate silos of expertise and feeds arguments for not only a lack of efficiency but also a lack of effectiveness of the process through excessive specialisation and a lack of interdisciplinary practice. We propose that the solution is a return to the basics of impact assessment with a call for increased integration around the goal of sustainable development and focus through better scoping. We rehearse and rebut counter arguments covering silo-based expertise, advocacy, democracy, sustainability understanding and communication. We call on the impact assessment community to rise to the challenge of increasing integration and focus, and to engage in the debate about the means of strengthening impact assessment
Cosmic Microwave Background Anisotropy Window Functions Revisited
The primary results of most observations of cosmic microwave background (CMB)
anisotropy are estimates of the angular power spectrum averaged through some
broad band, called band-powers. These estimates are in turn what are used to
produce constraints on cosmological parameters due to all CMB observations.
Essential to this estimation of cosmological parameters is the calculation of
the expected band-power for a given experiment, given a theoretical power
spectrum. Here we derive the "band power" window function which should be used
for this calculation, and point out that it is not equivalent to the window
function used to calculate the variance. This important distinction has been
absent from much of the literature: the variance window function is often used
as the band-power window function. We discuss the validity of this assumed
equivalence, the role of window functions for experiments that constrain the
power in {\it multiple} bands, and summarize a prescription for reporting
experimental results. The analysis methods detailed here are applied in a
companion paper to three years of data from the Medium Scale Anisotropy
Measurement.Comment: 5 pages, 1 included .eps figure, PRD in press---final published
versio
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