24,286 research outputs found
Cold Flows and Large Scale Tides
Several studies have indicated that the local cosmic velocity field is rather
cold, in particular in the regions outside the massive, virialized clusters of
galaxies. If our local cosmic environment is taken to be a representative
volume of the Universe, the repercussion of this finding is that either we live
in a low- Universe and/or that the galaxy distribution is a biased
reflection of the underlying mass distribution. Otherwise, the pronounced
nature of the observed galaxy distribution would be irreconcilable with the
relatively quiet flow of the galaxies.
Here we propose a different view on this cosmic dilemma, stressing the fact
that our cosmic neighbourhood embodies a region of rather particular dynamical
properties, and henceforth we are apt to infer flawed conclusions with respect
to the global Universe. Suspended between two huge mass concentrations, the
Great Attractor region and the Perseus-Pisces chain, we find ourselves in a
region of relatively low density yet with a very strong tidal shear. This tidal
field induces a local velocity field with a significant large-scale bulk flow
but a low small-scale velocity dispersion. By means of constrained realizations
of our local Universe, consisting of Wiener-filtered reconstructions inferred
from the Mark III catalogue of galaxy peculiar velocities in combination with
appropriate spectrally determined fluctuations, we study the implications for
our local velocity field. We find that we live near a local peak in the
distribution of the cosmic Mach number, , and that our
local cosmic niche is located in the tail of the Mach number distribution
function.Comment: Contribution to `Evolution of Large Scale Structure', MPA/ESO
Conference, August 1997, eds. A. Banday & R. Sheth, Twin Press. 5 pages of
LaTeX including 3 postscript figures. Uses tp.sty and psfi
Portfolio Allocation for Bayesian Optimization
Bayesian optimization with Gaussian processes has become an increasingly
popular tool in the machine learning community. It is efficient and can be used
when very little is known about the objective function, making it popular in
expensive black-box optimization scenarios. It uses Bayesian methods to sample
the objective efficiently using an acquisition function which incorporates the
model's estimate of the objective and the uncertainty at any given point.
However, there are several different parameterized acquisition functions in the
literature, and it is often unclear which one to use. Instead of using a single
acquisition function, we adopt a portfolio of acquisition functions governed by
an online multi-armed bandit strategy. We propose several portfolio strategies,
the best of which we call GP-Hedge, and show that this method outperforms the
best individual acquisition function. We also provide a theoretical bound on
the algorithm's performance.Comment: This revision contains an updated the performance bound and other
minor text change
New Maser Emission from Nonmetastable Ammonia in NGC 7538. II. Green Bank Telescope Observations Including Water Masers
We present new maser emission from ^{14}NH_3 (9,6) in NGC 7538. Our
observations include the known spectral features near v_LSR = -60 km/s and -57
km/s and several more features extending to -46 km/s. In three epochs of
observation spanning two months we do not detect any variability in the ammonia
masers, in contrast to the >10-fold variability observed in other ^{14}NH_3
(9,6) masers in the Galaxy over comparable timescales. We also present
observations of water masers in all three epochs for which emission is observed
over the velocity range -105 km/s < v_LSR < -4 km/s, including the highest
velocity water emission yet observed from NGC 7538. Of the remarkable number of
maser species in IRS 1, H_2O and, now, ^{14}NH_3 are the only masers known to
exhibit emission outside of the velocity range -62 km/s < v_LSR < -51 km/s.
However, we find no significant intensity or velocity correlations between the
water emission and ammonia emission. We also present a non-detection in the
most sensitive search to date toward any source for emission from the CC^{32}S
and CC^{34}S molecules, indicating an age greater than \approx 10^4 yr for IRS
1-3. We discuss these findings in the context of embedded stellar cores and
recent models of the region.Comment: 7 pages, 4 figures, 3 tables; accepted to AJ; color figures only on
arxiv; revised to include references and minor proof change
Constrained simulations of the local universe: I. Mass and motion in the Local Volume
It has been recently claimed that there is no correlation between the
distribution of galaxies and their peculiar velocities within the Local Volume
(LV), namely a sphere of R=7/h Mpc around the Local Group (LG). It has been
then stated that this implies that either locally dark matter is not
distributed in the same way as luminous matter, or peculiar velocities are not
due to fluctuations in mass. To test that statement a set of constrained N-body
cosmological simulations, designed to reproduce the main observed large scale
structure, have been analyzed. The simulations were performed within the
flat-Lambda, open and flat matter only CDM cosmogonies. Two unconstrained
simulations of the flat-Lambda and open CDM models were performed for
comparison. LG-like objects have been selected so as to mimic the real LG
environment. The local gravitational field due to all halos found within each
LV is compared with the exact gravitational field induced by all matter in the
simulation. We conclude that there is no correlation between the exact and the
local gravitational field obtained by pairwise newtonian forces between halos.
Moreover, the local gravitational field is uncorrelated with the peculiar
velocities of halos. The exact gravitational field has a linear correlation
with peculiar velocities but the proportionality constant relating the velocity
with gravitational field falls below the prediction of the linear theory. Upon
considering all matter inside the LVs, the exact and local gravitational
accelerations show a much better correlation, but with a considerable scatter
independent on the cosmological models. The main conclusion is that the lack of
correlation between the local gravitation and the peculiar velocity fields
around LG-like objects is naturally expected in the CDM cosmologies.Comment: 10 pages, 19 figures. Accepted for publication in MNRA
An Entropy Search Portfolio for Bayesian Optimization
Bayesian optimization is a sample-efficient method for black-box global
optimization. How- ever, the performance of a Bayesian optimization method very
much depends on its exploration strategy, i.e. the choice of acquisition
function, and it is not clear a priori which choice will result in superior
performance. While portfolio methods provide an effective, principled way of
combining a collection of acquisition functions, they are often based on
measures of past performance which can be misleading. To address this issue, we
introduce the Entropy Search Portfolio (ESP): a novel approach to portfolio
construction which is motivated by information theoretic considerations. We
show that ESP outperforms existing portfolio methods on several real and
synthetic problems, including geostatistical datasets and simulated control
tasks. We not only show that ESP is able to offer performance as good as the
best, but unknown, acquisition function, but surprisingly it often gives better
performance. Finally, over a wide range of conditions we find that ESP is
robust to the inclusion of poor acquisition functions.Comment: 10 pages, 5 figure
Contemporary splinting practice in the UK for adults with neurological dysfunction: A cross-sectional survey
This article is made available through the Brunel Open Access Publishing Fund.Aim: To explore the contemporary splinting practice of UK occupational therapists and physiotherapists for adults with neurological dysfunction.
Method: Cross-sectional online survey of members of the Association of Chartered Physiotherapists in Neurology and College of Occupational Therapists Specialist Section Neurological Practice.
Results: Four hundred and twenty therapists completed the survey. Contracture management is the most common rationale for therapists splinting adults with neurological dysfunction. Other shared therapeutic goals of splinting include maintaining muscle and joint alignment, spasticity management, function, pain management and control of oedema. Considerable clinical uncertainty was uncovered in practice particularly around wearing regimens of splints. Most therapists have access to locally-derived splinting guidelines, which may contribute to this diversity of practice.
Conclusions: This study provides a unique insight into aspects of contemporary splinting practice among UK therapists, who belong to a specialist neurological professional network and work in a number of different health-care settings with adults who have a neurological condition. Study findings show a wide variation in splinting practice, thereby indicating a potential need for national guidance to assist therapists in this area of clinical uncertainty. Further research is required to establish best practice parameters for splinting in neurological rehabilitation
Supportive but wary. How Europeans feel about the EU 60 years after the Treaty of Rome. eupinions #2017/1
The Eurozone crisis has pushed reform of the European Union (EU) to the forefront
of political debate. How can a Union of 28 states with a population of over half a
billion be reformed to weather future economic crises and political challenges?
Finding an answer to this question is extremely difficult not only because current
reform proposals are so varied, but even more so because we lack insights into the
preferences for reform amongst national elites and publics. Although EU support
has interested scholars for over three decades now, we virtually know nothing
about public support for EU reform. Current research focuses almost exclusively
on the causes of support for the current project and fails to provide a sufficient
basis for effective reform decisions. Surely, the feasibility and sustainability of
EU reform crucially hinges on the support amongst national publics. eupinions
examines public support for EU reform by developing a theoretical model and
employing cutting-edge data collection techniques. Our findings will aid policy
makers to craft EU reform proposals that can secure widespread public support
On the Complexity of Hilbert Refutations for Partition
Given a set of integers W, the Partition problem determines whether W can be
divided into two disjoint subsets with equal sums. We model the Partition
problem as a system of polynomial equations, and then investigate the
complexity of a Hilbert's Nullstellensatz refutation, or certificate, that a
given set of integers is not partitionable. We provide an explicit construction
of a minimum-degree certificate, and then demonstrate that the Partition
problem is equivalent to the determinant of a carefully constructed matrix
called the partition matrix. In particular, we show that the determinant of the
partition matrix is a polynomial that factors into an iteration over all
possible partitions of W.Comment: Final versio
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