7,343 research outputs found
Similar phenomena at different scales: Black Holes, the Sun, Gamma-ray Bursts, Supernovae, Galaxies and Galaxy Clusters
Many similar phenomena occur in astrophysical systems with spatial and mass
scales different by many orders of magnitudes. For examples, collimated
outflows are produced from the Sun, proto-stellar systems, gamma-ray bursts,
neutron star and black hole X-ray binaries, and supermassive black holes;
various kinds of flares occur from the Sun, stellar coronae, X-ray binaries and
active galactic nuclei; shocks and particle acceleration exist in supernova
remnants, gamma-ray bursts, clusters of galaxies, etc. In this report I
summarize briefly these phenomena and possible physical mechanisms responsible
for them. I emphasize the importance of using the Sun as an astrophysical
laboratory in studying these physical processes, especially the roles magnetic
fields play in them; it is quite likely that magnetic activities dominate the
fundamental physical processes in all of these systems.
As a case study, I show that X-ray lightcurves from solar flares, black hole
binaries and gamma-ray bursts exhibit a common scaling law of non-linear
dynamical properties, over a dynamical range of several orders of magnitudes in
intensities, implying that many basic X-ray emission nodes or elements are
inter-connected over multi-scales. A future high timing and imaging resolution
solar X-ray instrument, aimed at isolating and resolving the fundamental
elements of solar X-ray lightcurves, may shed new lights onto the fundamental
physical mechanisms, which are common in astrophysical systems with vastly
different mass and spatial scales. Using the Sun as an astrophysical
laboratory, "Applied Solar Astrophysics" will deepen our understanding of many
important astrophysical problems.Comment: 22 pages, 13 figures, invited discourse for the 26th IAU GA, Prague,
Czech Republic, Aug. 2006, to be published in Vol. 14 IAU Highlights of
Astronomy, Ed. K.A. van der Hucht. Revised slightly to match the final
submitted version, after incorporating comments and suggestions from several
colleagues. A full-resolution version is available on request from the author
at [email protected]
A Survey on Bayesian Deep Learning
A comprehensive artificial intelligence system needs to not only perceive the
environment with different `senses' (e.g., seeing and hearing) but also infer
the world's conditional (or even causal) relations and corresponding
uncertainty. The past decade has seen major advances in many perception tasks
such as visual object recognition and speech recognition using deep learning
models. For higher-level inference, however, probabilistic graphical models
with their Bayesian nature are still more powerful and flexible. In recent
years, Bayesian deep learning has emerged as a unified probabilistic framework
to tightly integrate deep learning and Bayesian models. In this general
framework, the perception of text or images using deep learning can boost the
performance of higher-level inference and in turn, the feedback from the
inference process is able to enhance the perception of text or images. This
survey provides a comprehensive introduction to Bayesian deep learning and
reviews its recent applications on recommender systems, topic models, control,
etc. Besides, we also discuss the relationship and differences between Bayesian
deep learning and other related topics such as Bayesian treatment of neural
networks.Comment: To appear in ACM Computing Surveys (CSUR) 202
Deep Exponential Families
We describe \textit{deep exponential families} (DEFs), a class of latent
variable models that are inspired by the hidden structures used in deep neural
networks. DEFs capture a hierarchy of dependencies between latent variables,
and are easily generalized to many settings through exponential families. We
perform inference using recent "black box" variational inference techniques. We
then evaluate various DEFs on text and combine multiple DEFs into a model for
pairwise recommendation data. In an extensive study, we show that going beyond
one layer improves predictions for DEFs. We demonstrate that DEFs find
interesting exploratory structure in large data sets, and give better
predictive performance than state-of-the-art models
Automatic Differentiation Variational Inference
Probabilistic modeling is iterative. A scientist posits a simple model, fits
it to her data, refines it according to her analysis, and repeats. However,
fitting complex models to large data is a bottleneck in this process. Deriving
algorithms for new models can be both mathematically and computationally
challenging, which makes it difficult to efficiently cycle through the steps.
To this end, we develop automatic differentiation variational inference (ADVI).
Using our method, the scientist only provides a probabilistic model and a
dataset, nothing else. ADVI automatically derives an efficient variational
inference algorithm, freeing the scientist to refine and explore many models.
ADVI supports a broad class of models-no conjugacy assumptions are required. We
study ADVI across ten different models and apply it to a dataset with millions
of observations. ADVI is integrated into Stan, a probabilistic programming
system; it is available for immediate use
Notes on the Riemann Hypothesis
These notes were written from a series of lectures given in March 2010 at the
Universidad Complutense of Madrid and then in Barcelona for the centennial
anniversary of the Spanish Mathematical Society (RSME). Our aim is to give an
introduction to the Riemann Hypothesis and a panoramic view of the world of
zeta and L-functions. We first review Riemann's foundational article and
discuss the mathematical background of the time and his possible motivations
for making his famous conjecture. We discuss some of the most relevant
developments after Riemann that have contributed to a better understanding of
the conjecture.Comment: 2 sections added, 55 pages, 6 figure
Uncertainty in Economic Growth and Inequality
A step to consilience, starting with a deconstruction of the causality of
uncertainty that is embedded in the fundamentals of growth and inequality,
following a construction of aggregation laws that disclose the invariance
principle across heterogeneous individuals, ending with a reconstruction of
metric models that yields deeper structural connections via U.S. GDP and income
data
Rhythms of the nervous system: mathematical themes and variations
The nervous system displays a variety of rhythms in both waking and sleep. These rhythms have been closely associated with different behavioral and cognitive states, but it is still unknown how the nervous system makes use of these rhythms to perform functionally important tasks. To address those questions, it is first useful to understood in a mechanistic way the origin of the rhythms, their interactions, the signals which create the transitions among rhythms, and the ways in which rhythms filter the signals to a network of neurons. This talk discusses how dynamical systems have been used to investigate the origin, properties and interactions of rhythms in the nervous system. It focuses on how the underlying physiology of the cells and synapses of the networks shape the dynamics of the network in different contexts, allowing the variety of dynamical behaviors to be displayed by the same network. The work is presented using a series of related case studies on different rhythms. These case studies are chosen to highlight mathematical issues, and suggest further mathematical work to be done. The topics include: different roles of excitation and inhibition in creating synchronous assemblies of cells, different kinds of building blocks for neural oscillations, and transitions among rhythms. The mathematical issues include reduction of large networks to low dimensional maps, role of noise, global bifurcations, use of probabilistic formulations.Published versio
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