234 research outputs found
Bayesian Calibrated Significance Levels Applied to the Spectral Tilt and Hemispherical Asymmetry
Bayesian model selection provides a formal method of determining the level of
support for new parameters in a model. However, if there is not a specific
enough underlying physical motivation for the new parameters it can be hard to
assign them meaningful priors, an essential ingredient of Bayesian model
selection. Here we look at methods maximizing the prior so as to work out what
is the maximum support the data could give for the new parameters. If the
maximum support is not high enough then one can confidently conclude that the
new parameters are unnecessary without needing to worry that some other prior
may make them significant. We discuss a computationally efficient means of
doing this which involves mapping p-values onto upper bounds of the Bayes
factor (or odds) for the new parameters. A p-value of 0.05 ()
corresponds to odds less than or equal to 5:2 which is below the `weak' support
at best threshold. A p-value of 0.0003 () corresponds to odds of
less than or equal to 150:1 which is the `strong' support at best threshold.
Applying this method we find that the odds on the scalar spectral index being
different from one are 49:1 at best. We also find that the odds that there is
primordial hemispherical asymmetry in the cosmic microwave background are 9:1
at best.Comment: 5 pages. V2: clarifying comments added in response to referee report.
Matches version to appear in MNRA
Vision and Learning for Deliberative Monocular Cluttered Flight
Cameras provide a rich source of information while being passive, cheap and
lightweight for small and medium Unmanned Aerial Vehicles (UAVs). In this work
we present the first implementation of receding horizon control, which is
widely used in ground vehicles, with monocular vision as the only sensing mode
for autonomous UAV flight in dense clutter. We make it feasible on UAVs via a
number of contributions: novel coupling of perception and control via relevant
and diverse, multiple interpretations of the scene around the robot, leveraging
recent advances in machine learning to showcase anytime budgeted cost-sensitive
feature selection, and fast non-linear regression for monocular depth
prediction. We empirically demonstrate the efficacy of our novel pipeline via
real world experiments of more than 2 kms through dense trees with a quadrotor
built from off-the-shelf parts. Moreover our pipeline is designed to combine
information from other modalities like stereo and lidar as well if available
Non-detection of a statistically anisotropic power spectrum in large-scale structure
We search a sample of photometric luminous red galaxies (LRGs) measured by
the Sloan Digital Sky Survey (SDSS) for a quadrupolar anisotropy in the
primordial power spectrum, in which P(\vec{k}) is an isotropic power spectrum
P(k) multiplied by a quadrupolar modulation pattern. We first place limits on
the 5 coefficients of a general quadrupole anisotropy. We also consider
axisymmetric quadrupoles of the form P(\vec{k}) = P(k){1 +
g_*[(\hat{k}\cdot\hat{n})^2-1/3]} where \hat{n} is the axis of the anisotropy.
When we force the symmetry axis \hat{n} to be in the direction (l,b)=(94
degrees,26 degrees) identified in the recent Groeneboom et al. analysis of the
cosmic microwave background, we find g_*=0.006+/-0.036 (1 sigma). With uniform
priors on \hat{n} and g_* we find that -0.41<g_*<+0.38 with 95% probability,
with the wide range due mainly to the large uncertainty of asymmetries aligned
with the Galactic Plane. In none of these three analyses do we detect evidence
for quadrupolar power anisotropy in large scale structure.Comment: 23 pages; 10 figures; 3 tables; replaced with version published in
JCAP (added discussion of scale-varying quadrupolar anisotropy
Frictional fluid instabilities shaped by viscous forces
Multiphase flows involving granular materials are complex and prone to pattern formation caused by competing mechanical and hydrodynamic interactions. Here we study the interplay between granular bulldozing and the stabilising effect of viscous pressure gradients in the invading fluid. Injection of aqueous solutions into layers of dry, hydrophobic grains represent a viscously stable scenario where we observe a transition from growth of a single frictional finger to simultaneous growth of multiple fingers as viscous forces are increased. The pattern is made more compact by the internal viscous pressure gradient, ultimately resulting in a fully stabilised front of frictional fingers advancing as a radial spoke pattern
Dynamic in situ chromosome immobilisation and DNA extraction using localized poly(N-isopropylacrylamide) phase transition
A method of in situ chromosome immobilisation and DNA extraction in a microfluidic polymer chip was presented. Light-induced local heating was used to induce poly(N-isopropylacrylamide) phase transition in order to create a hydrogel and embed a single chromosome such that it was immobilised. This was achieved with the use of a near-infrared laser focused on an absorption layer integrated in the polymer chip in close proximity to the microchannel. It was possible to proceed to DNA extraction while holding on the chromosome at an arbitrary location by introducing protease K into the microchannel
Constraints on cosmic hemispherical power anomalies from quasars
Recent analyses of the cosmic microwave background (CMB) maps from the WMAP
satellite have uncovered evidence for a hemispherical power anomaly, i.e. a
dipole modulation of the CMB power spectrum at large angular scales with an
amplitude of +/-14 percent. Erickcek et al have put forward an inflationary
model to explain this anomaly. Their scenario is a variation on the curvaton
scenario in which the curvaton possesses a large-scale spatial gradient that
modulates the amplitude of CMB fluctuations. We show that this scenario would
also lead to a spatial gradient in the amplitude of perturbations sigma_8, and
hence to a dipole asymmetry in any highly biased tracer of the underlying
density field. Using the high-redshift quasars from the Sloan Digital Sky
Survey, we find an upper limit on such a gradient of |nabla
sigma_8|/sigma_8<0.027/r_{lss} (99% posterior probability), where r_{lss} is
the comoving distance to the last-scattering surface. This rules out the
simplest version of the curvaton spatial gradient scenario.Comment: matches JCAP accepted version (minor revisions
A refined model for spinning dust radiation
We present a comprehensive treatment of the spectrum of electric dipole
emission from spinning dust grains, updating the commonly used model of Draine
and Lazarian. Grain angular velocity distributions are computed using the
Fokker-Planck equation; we revisit the drift and diffusion coefficients for the
major torques on the grain, including collisions, grain-plasma interactions,
and infrared emission. We use updated grain optical properties and size
distributions. The theoretical formalism is implemented in the companion code,
SPDUST, which is publicly available. The effect of some environmental and grain
parameters on the emissivity is shown and analysed.Comment: Minor corrections. Matches the MNRAS published version (except for a
typo in Eq.(74) corrected here). The companion code, SPDUST, can be
downloaded from http://www.tapir.caltech.edu/~yacine/spdust/spdust.htm
Tracing the Filamentary Structure of the Galaxy Distribution at z~0.8
We study filamentary structure in the galaxy distribution at z ~ 0.8 using
data from the Deep Extragalactic Evolutionary Probe 2 (DEEP2) Redshift Survey
and its evolution to z ~ 0.1 using data from the Sloan Digital Sky Survey
(SDSS). We trace individual filaments for both surveys using the Smoothed
Hessian Major Axis Filament Finder, an algorithm which employs the Hessian
matrix of the galaxy density field to trace the filamentary structures in the
distribution of galaxies. We extract 33 subsamples from the SDSS data with a
geometry similar to that of DEEP2. We find that the filament length
distribution has not significantly changed since z ~ 0.8, as predicted in a
previous study using a \LamdaCDM cosmological N-body simulation. However, the
filament width distribution, which is sensitive to the non-linear growth of
structure, broadens and shifts to smaller widths for smoothing length scales of
5-10 Mpc/h from z ~ 0.8 to z ~ 0.1, in accord with N-body simulations.Comment: 10 pages, 8 figures, accepted for the publication in MNRA
Electron coherence at low temperatures: The role of magnetic impurities
We review recent experimental progress on the saturation problem in metallic
quantum wires. In particular, we address the influence of magnetic impurities
on the electron phase coherence time. We also present new measurements of the
phase coherence time in ultra-clean gold and silver wires and analyse the
saturation of \tauphi in these samples, cognizant of the role of magnetic
scattering. For the cleanest samples, Kondo temperatures below 1 mK and
extremely-small magnetic-impurity concentration levels of less than 0.08 ppm
have to be assumed to attribute the observed saturation to the presence of
magnetic impurities.Comment: review article, 14 pages, 11 figures. Physica E (in press
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