194,359 research outputs found
Cosmic String Spacetime in Dilaton Gravity and Flat Rotation Curves
In dilaton gravity theories, we consider a string-like topological defect
formed during U(1) gauge symmetry-breaking phase transition in the early
Universe, and far from the cosmic string we have vacuum solutions of the
generalized Einstein equation. We discuss how they can be related to the
flatness of galactic rotation curves.Comment: 9 pages, RevTeX4 fil
Persistence in systems with algebraic interaction
Persistence in coarsening 1D spin systems with a power law interaction
is considered. Numerical studies indicate that for sufficiently
large values of the interaction exponent ( in our
simulations), persistence decays as an algebraic function of the length scale
, . The Persistence exponent is found to be
independent on the force exponent and close to its value for the
extremal () model, . For smaller
values of the force exponent (), finite size effects prevent the
system from reaching the asymptotic regime. Scaling arguments suggest that in
order to avoid significant boundary effects for small , the system size
should grow as .Comment: 4 pages 4 figure
First passage time for random walks in heterogeneous networks
The first passage time (FPT) for random walks is a key indicator of how fast
information diffuses in a given system. Despite the role of FPT as a
fundamental feature in transport phenomena, its behavior, particularly in
heterogeneous networks, is not yet fully understood. Here, we study, both
analytically and numerically, the scaling behavior of the FPT distribution to a
given target node, averaged over all starting nodes. We find that random walks
arrive quickly at a local hub, and therefore, the FPT distribution shows a
crossover with respect to time from fast decay behavior (induced from the
attractive effect to the hub) to slow decay behavior (caused by the exploring
of the entire system). Moreover, the mean FPT is independent of the degree of
the target node in the case of compact exploration. These theoretical results
justify the necessity of using a random jump protocol (empirically used in
search engines) and provide guidelines for designing an effective network to
make information quickly accessible.Comment: 5 pages, 3 figure
Chiron: A Robust Recommendation System with Graph Regularizer
Recommendation systems have been widely used by commercial service providers
for giving suggestions to users. Collaborative filtering (CF) systems, one of
the most popular recommendation systems, utilize the history of behaviors of
the aggregate user-base to provide individual recommendations and are effective
when almost all users faithfully express their opinions. However, they are
vulnerable to malicious users biasing their inputs in order to change the
overall ratings of a specific group of items. CF systems largely fall into two
categories - neighborhood-based and (matrix) factorization-based - and the
presence of adversarial input can influence recommendations in both categories,
leading to instabilities in estimation and prediction. Although the robustness
of different collaborative filtering algorithms has been extensively studied,
designing an efficient system that is immune to manipulation remains a
significant challenge. In this work we propose a novel "hybrid" recommendation
system with an adaptive graph-based user/item similarity-regularization -
"Chiron". Chiron ties the performance benefits of dimensionality reduction
(through factorization) with the advantage of neighborhood clustering (through
regularization). We demonstrate, using extensive comparative experiments, that
Chiron is resistant to manipulation by large and lethal attacks
Gauge Theory of Gravity Requires Massive Torsion Field
One of the greatest unsolved issues of the physics of this century is to find
a quantum field theory of gravity. According to a vast amount of literature
unification of quantum field theory and gravitation requires a gauge theory of
gravity which includes torsion and an associated spin field. Various models
including either massive or massless torsion fields have been suggested. We
present arguments for a massive torsion field, where the probable rest mass of
the corresponding spin three gauge boson is the Planck mass.Comment: 3 pages, Revte
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