3,196 research outputs found
Reproducing Cosmic Microwave Background anisotropies with mixed isocurvature perturbations
Recently high quality data of the cosmic microwave background anisotropies
have been published. In this work we study to which extent the cosmological
parameters determined by using this data depend on assumptions about the
initial conditions. We show that for generic initial conditions, not only the
best fit values are very different but, and this is our main result, the
allowed parameter range enlarges dramatically.Comment: 4 pages, 5 figures, submitted to PRL; Major changes following
referees suggestions; the allowed cosmological parameter range enlarges
dramaticall
The power of Bayesian evidence in astronomy
We discuss the use of the Bayesian evidence ratio, or Bayes factor, for model
selection in astronomy. We treat the evidence ratio as a statistic and
investigate its distribution over an ensemble of experiments, considering both
simple analytical examples and some more realistic cases, which require
numerical simulation. We find that the evidence ratio is a noisy statistic, and
thus it may not be sensible to decide to accept or reject a model based solely
on whether the evidence ratio reaches some threshold value. The odds suggested
by the evidence ratio bear no obvious relationship to the power or Type I error
rate of a test based on the evidence ratio. The general performance of such
tests is strongly affected by the signal to noise ratio in the data, the
assumed priors, and the threshold in the evidence ratio that is taken as
`decisive'. The comprehensiveness of the model suite under consideration is
also very important. The usefulness of the evidence ratio approach in a given
problem can be assessed in advance of the experiment, using simple models and
numerical approximations. In many cases, this approach can be as informative as
a much more costly full-scale Bayesian analysis of a complex problem.Comment: 11 pages; MNRAS in pres
The effect of local optically thick regions in the long-wave emission of young circumstellar disks
Multi-wavelength observations of protoplanetary disks in the sub-millimeter
continuum have measured spectral indices values which are significantly lower
than what is found in the diffuse interstellar medium. Under the assumption
that mm-wave emission of disks is mostly optically thin, these data have been
generally interpreted as evidence for the presence of mm/cm-sized pebbles in
the disk outer regions. In this work we investigate the effect of possible
local optically thick regions on the mm-wave emission of protoplanetary disks
without mm/cm-sized grains. A significant local increase of the optical depth
in the disk can be caused by the concentration of solid particles, as predicted
to result from a variety of proposed physical mechanisms. We calculate the
filling factors and implied overdensities these optically thick regions would
need to significantly affect the millimeter fluxes of disks, and we discuss
their plausibility. We find that optically thick regions characterized by
relatively small filling factors can reproduce the mm-data of young disks
without requesting emission from mm/cm-sized pebbles. However, these optically
thick regions require dust overdensities much larger than what predicted by any
of the physical processes proposed in the literature to drive the concentration
of solids. We find that only for the most massive disks it is possible and
plausible to imagine that the presence of optically thick regions in the disk
is responsible for the low measured values of the mm spectral index. For the
majority of the disk population, optically thin emission from a population of
large mm-sized grains remains the most plausible explanation. The results of
this analysis further strengthen the scenario for which the measured low
spectral indices of protoplanetary disks at mm wavelengths are due to the
presence of large mm/cm-sized pebbles in the disk outer regions.Comment: 13 pages, 2 figures, A&A in pres
Hunting Down the Best Model of Inflation with Bayesian Evidence
We present the first calculation of the Bayesian evidence for different
prototypical single field inflationary scenarios, including representative
classes of small field and large field models. This approach allows us to
compare inflationary models in a well-defined statistical way and to determine
the current "best model of inflation". The calculation is performed numerically
by interfacing the inflationary code FieldInf with MultiNest. We find that
small field models are currently preferred, while large field models having a
self-interacting potential of power p>4 are strongly disfavoured. The class of
small field models as a whole has posterior odds of approximately 3:1 when
compared with the large field class. The methodology and results presented in
this article are an additional step toward the construction of a full numerical
pipeline to constrain the physics of the early Universe with astrophysical
observations. More accurate data (such as the Planck data) and the techniques
introduced here should allow us to identify conclusively the best inflationary
model.Comment: 12 pages, 2 figures, uses RevTeX. Misprint corrected, references
added. Matches published versio
Pippi - painless parsing, post-processing and plotting of posterior and likelihood samples
Interpreting samples from likelihood or posterior probability density
functions is rarely as straightforward as it seems it should be. Producing
publication-quality graphics of these distributions is often similarly painful.
In this short note I describe pippi, a simple, publicly-available package for
parsing and post-processing such samples, as well as generating high-quality
PDF graphics of the results. Pippi is easily and extensively configurable and
customisable, both in its options for parsing and post-processing samples, and
in the visual aspects of the figures it produces. I illustrate some of these
using an existing supersymmetric global fit, performed in the context of a
gamma-ray search for dark matter. Pippi can be downloaded and followed at
http://github.com/patscott/pippi .Comment: 4 pages, 1 figure. v3: Updated for pippi 2.0. New features include
hdf5 support, out-of-core processing, inline post-processing with arbitrary
Python code in the input file, and observable-specific data cuts. Pippi can
be downloaded from http://github.com/patscott/pipp
Aerosol Extinction Uncertainty Determination for a Laser-Based Detecting System
Quantitative measurement of optic and aerosolic property of troposphere may be carried out from lidar system more easily and with low marginal cost than other system (i.e. satellite observation). The aim of this work is to show how extinction coefficient and water vapor concentration in troposphere may be educed from lidar signals at several altitudes and how uncertainty is close to acceptable value. Real data observations will be presented to demostrate uncertainty range on aerosolic extinction coefficient function
Solving Cosmological Problems of Supersymmetric Axion Models in an Inflationary Universe
We revisit inflationary cosmology of axion models in the light of recent
developments on the inflaton decay in supergravity. We find that all the
cosmological difficulties, including gravitino, axino overproduction and
axionic isocurvature fluctuation, can be avoided if the saxion field has large
initial amplitude during inflation and decays before big-bang nucleosynthesis.Comment: 19 pages, 4 figure
Rate-Control or Rhythm-Contol: Where do we stand?
Atrial fibrillation is the most common sustained rhythm disturbance and its prevalence is increasing worldwide due to the progressive aging of the population. Current guidelines clearly depict the gold standard management of acute symptomatic atrial fibrillation but the best-long term approach for first or recurrent atrial fibrillation is still debated with regard to quality of life, risk of new hospitalizations, and possible disabling complications, such as thromboembolic stroke, major bleeds and death. Some authors propose that regaining sinus rhythm in all cases, thus re-establishing a physiologic cardiac function not requiring a prolonged antithrombotic therapy, avoids the threat of intracranial or extracranial haemorrhages due to Vitamin K antagonists or aspirin. On the contrary, advocates of a rate control approach with an accurate antithrombotic prophylaxis propose that such a strategy may avoid the risk of cardiovascular and non cardiovascular side effects related to antiarrhythmic drugs. This review aims to explore the state of our knowledge in order to summarize evidences and issues that need to be furthermore clarified
Phonon-Assisted Two-Photon Interference from Remote Quantum Emitters
Photonic quantum technologies are on the verge offinding applications in everyday life with quantum cryptography andquantum simulators on the horizon. Extensive research has beencarried out to identify suitable quantum emitters and single epitaxialquantum dots have emerged as near-optimal sources of bright, on-demand, highly indistinguishable single photons and entangledphoton-pairs. In order to build up quantum networks, it is essentialto interface remote quantum emitters. However, this is still anoutstanding challenge, as the quantum states of dissimilarâartificialatomsâhave to be prepared on-demand with highfidelity and thegenerated photons have to be made indistinguishable in all possibledegrees of freedom. Here, we overcome this major obstacle and show an unprecedented two-photon interference (visibility of 51±5%) from remote strain-tunable GaAs quantum dots emitting on-demand photon-pairs. We achieve this result by exploiting forthefirst time the full potential of a novel phonon-assisted two-photon excitation scheme, which allows for the generation ofhighly indistinguishable (visibility of 71±9%) entangled photon-pairs (fidelity of 90±2%), enables push-button biexciton statepreparation (fidelity of 80±2%) and outperforms conventional resonant two-photon excitation schemes in terms of robustnessagainst environmental decoherence. Our results mark an important milestone for the practical realization of quantum repeatersand complex multiphoton entanglement experiments involving dissimilar artificial atom
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