802 research outputs found
Large-scale Bias and Efficient Generation of Initial Conditions for Non-Local Primordial Non-Gaussianity
We study the scale-dependence of halo bias in generic (non-local) primordial
non-Gaussian (PNG) initial conditions of the type motivated by inflation,
parametrized by an arbitrary quadratic kernel. We first show how to generate
non-local PNG initial conditions with minimal overhead compared to local PNG
models for a general class of primordial bispectra that can be written as
linear combinations of separable templates. We run cosmological simulations for
the local, and non-local equilateral and orthogonal models and present results
on the scale-dependence of halo bias. We also derive a general formula for the
Fourier-space bias using the peak-background split (PBS) in the context of the
excursion set approach to halos and discuss the difference and similarities
with the known corresponding result from local bias models. Our PBS bias
formula generalizes previous results in the literature to include non-Markovian
effects and non-universality of the mass function and are in better agreement
with measurements in numerical simulations than previous results for a variety
of halo masses, redshifts and halo definitions. We also derive for the first
time quadratic bias results for arbitrary non-local PNG, and show that
non-linear bias loops give small corrections at large-scales. The resulting
well-behaved perturbation theory paves the way to constrain non-local PNG from
measurements of the power spectrum and bispectrum in galaxy redshift surveys.Comment: 43 pages, 10 figures. v2: references added. 2LPT parallel code for
generating non-local PNG initial conditions available at
http://cosmo.nyu.edu/roman/2LP
Floodlight quantum key distribution: Demonstrating a framework for high-rate secure communication
Floodlight quantum key distribution (FL-QKD) is a radically different QKD paradigm that can achieve gigabit-per-second secret-key rates over metropolitan area distances without multiplexing [Q. Zhuang et al., Phys. Rev. A 94, 012322 (2016)]. It is a two-way protocol that transmits many photons per bit duration and employs a high-gain optical amplifier, neither of which can be utilized by existing QKD protocols, to mitigate channel loss. FL-QKD uses an optical bandwidth that is substantially larger than the modulation rate and performs decoding with a unique broadband homodyne receiver. Essential to FL-QKD is Alice's injection of photons from a photon-pair source—in addition to the light used for key generation—into the light she sends to Bob. This injection enables Alice and Bob to quantify Eve's intrusion and thus secure FL-QKD against collective attacks. Our proof-of-concept experiment included 10 dB propagation loss—equivalent to 50 km of low-loss fiber—and achieved a 55 Mbit/s secret-key rate (SKR) for a 100 Mbit/s modulation rate, as compared to the state-of-the-art system's 1 Mbit/s SKR for a 1 Gbit/s modulation rate [M. Lucamarini et al., Opt. Express 21, 24550 (2013)], representing ∼500-fold and ∼50 fold improvements in secret-key efficiency (bits per channel use) and SKR (bits per second), respectively.United States. Office of Naval Research (Grant N00014-13-1-0774)United States. Air Force Office of Scientific Research (Grant FA9550-14-1-0052)United States. Army Research Office (United States. Defense Advanced Research Projects Agency. Quiness Program. Grant W31P4Q-12-1-0019)United States. Office of Naval Research. Defense University Research Instrumentation Program (Grant N00014-14-1-0808
Large-Scale Structure in Brane-Induced Gravity II. Numerical Simulations
We use N-body simulations to study the nonlinear structure formation in
brane-induced gravity, developing a new method that requires alternate use of
Fast Fourier Transforms and relaxation. This enables us to compute the
nonlinear matter power spectrum and bispectrum, the halo mass function, and the
halo bias. From the simulation results, we confirm the expectations based on
analytic arguments that the Vainshtein mechanism does operate as anticipated,
with the density power spectrum approaching that of standard gravity within a
modified background evolution in the nonlinear regime. The transition is very
broad and there is no well defined Vainshtein scale, but roughly this
corresponds to k_*~ 2 at redshift z=1 and k_*~ 1 at z=0. We checked that while
extrinsic curvature fluctuations go nonlinear, and the dynamics of the
brane-bending mode C receives important nonlinear corrections, this mode does
get suppressed compared to density perturbations, effectively decoupling from
the standard gravity sector. At the same time, there is no violation of the
weak field limit for metric perturbations associated with C. We find good
agreement between our measurements and the predictions for the nonlinear power
spectrum presented in paper I, that rely on a renormalization of the linear
spectrum due to nonlinearities in the modified gravity sector. A similar
prediction for the mass function shows the right trends. Our simulations also
confirm the induced change in the bispectrum configuration dependence predicted
in paper I.Comment: 19 pages, 13 figures. v2: corrected typos, added more simulations,
better test of predictions in large mass regime. v3: minor changes, published
versio
Work-related injuries among physiotherapists in public hospitals—a Southeast Asian picture
OBJECTIVES: A cross-sectional study was conducted to measure the prevalence of work-related injuries among physiotherapists in Malaysia and to explore the influence of factors such as gender, body mass index, years of work experience and clinical placement areas on the occurrence of work-related musculoskeletal disorders. METHODS: Self-administered questionnaires adapted from the Nordic Musculoskeletal Questionnaire were sent to 105 physiotherapists at three main public hospitals in Kuala Lumpur, Malaysia. The questionnaire had 12 items that covered demographic information, areas of musculoskeletal problems and physiotherapy techniques that could contribute to work-related musculoskeletal disorders. The data obtained were analyzed using the Statistical Package for Social Science version 14 software. RESULTS: The overall prevalence of work-related injuries during the past 12 months was 71.6%. Female therapists reported a significantly higher prevalence of work-related musculoskeletal disorders than the male therapists (73.0%, p,0.001). Significant differences were observed between the proportion of therapists who had work-related musculoskeletal disorders and those who did not for the group with a body mass index (BMI) .25 (x ² = 9.0, p = 0.003) and the group with a BMI of 18-25 (x ² = 7.8, p = 0.006). Manual therapy (58.6%) and lifting/transfer tasks (41.3%) were the two physiotherapy techniques that most often contributed to work-related musculoskeletal disorders. CONCLUSION: Work-related injuries are significantly higher among the physiotherapists in Malaysia compared with many other countries. Female therapists reported a higher incidence of work-related musculoskeletal disorders in this study, and work-related musculoskeletal disorders were more common among therapists working in the pediatric specialty. This study contributes to the understanding of work-related disorders among physiotherapists from a southeast Asian perspective where the profession is in its development stage
Halo Sampling, Local Bias and Loop Corrections
We develop a new test of local bias, by constructing a locally biased halo
density field from sampling the dark matter-halo distribution. Our test differs
from conventional tests in that it preserves the full scatter in the bias
relation and it does not rely on perturbation theory. We put forward that bias
parameters obtained using a smoothing scale R can only be applied to computing
the halo power spectrum at scales k ~ 1/R. Our calculations can automatically
include the running of bias parameters and give vanishingly small loop
corrections at low-k. Our proposal results in much better agreement of the
sampling and perturbation theory results with simulations. In particular,
unlike the standard interpretation of local bias in the literature, our
treatment of local bias does not generate a constant power in the low-k limit.
We search for extra noise in the Poisson corrected halo power spectrum at
wavenumbers below its turn-over and find no evidence of significant positive
noise (as predicted by the standard interpretation) while we find evidence of
negative noise coming from halo exclusion for very massive halos. Using
perturbation theory and our non-perturbative sampling technique we also
demonstrate that nonlocal bias effects discovered recently in simulations
impact the power spectrum only at the few percent level in the weakly nonlinear
regime.Comment: 25 pages, 14 figures; V2: significant revision including more details
about halo exclusion and low-k noise. Conclusions unchange
Gravity and Large-Scale Non-local Bias
The relationship between galaxy and matter overdensities, bias, is most often
assumed to be local. This is however unstable under time evolution, we provide
proofs under several sets of assumptions. In the simplest model galaxies are
created locally and linearly biased at a single time, and subsequently move
with the matter (no velocity bias) conserving their comoving number density (no
merging). We show that, after this formation time, the bias becomes unavoidably
non-local and non-linear at large scales. We identify the non-local
gravitationally induced fields in which the galaxy overdensity can be expanded,
showing that they can be constructed out of the invariants of the deformation
tensor (Galileons). In addition, we show that this result persists if we
include an arbitrary evolution of the comoving number density of tracers. We
then include velocity bias, and show that new contributions appear, a dipole
field being the signature at second order. We test these predictions by
studying the dependence of halo overdensities in cells of fixed matter density:
measurements in simulations show that departures from the mean bias relation
are strongly correlated with the non-local gravitationally induced fields
identified by our formalism. The effects on non-local bias seen in the
simulations are most important for the most biased halos, as expected from our
predictions. The non-locality seen in the simulations is not fully captured by
assuming local bias in Lagrangian space. Accounting for these effects when
modeling galaxy bias is essential for correctly describing the dependence on
triangle shape of the galaxy bispectrum, and hence constraining cosmological
parameters and primordial non-Gaussianity. We show that using our formalism we
remove an important systematic in the determination of bias parameters from the
galaxy bispectrum, particularly for luminous galaxies. (abridged)Comment: 26 pages, 9 figures. v2: improved appendix
Polyphenols from Grape Pomace: Functionalization of Chitosan-Coated Hydroxyapatite for Modulated Swelling and Release of Polyphenols
[Image: see text] Chitosan is known for its specific antibacterial mechanism and biodegradability, while polyphenols are known for their antioxidant and anti-inflammatory properties: coupling these properties on a surface for bone contact, such as hydroxyapatite, is of great interest. The system developed here allows the combination of hydroxyapatite, chitosan, and polyphenol properties in the same multifunctional biomaterial in order to modulate the host response after implantation. Crosslinked chitosan is used in this research to create a stable coating on hydroxyapatite, and then it is functionalized for a smart release of the polyphenols. The release is higher in inflammatory conditions and lower in physiological conditions. The properties of the coated and functionalized samples are characterized on the as-prepared samples and after the samples are immersed (for 24 h) in solutions, which simulate the inflammatory and physiological conditions. Characterization is performed in order to confirm the presence of polyphenols grafted within the chitosan coating, the stability of grafting as a function of pH, the morphology of the coating and distribution of polyphenols on the surface, and the redox reactivity and radical scavenging activity of the functionalized coating. All the results are in line with previous results, which show a successful coating with chitosan and functionalization with polyphenols. Moreover, the polyphenols have a different release kinetics that is faster in a simulated inflammatory environment compared to that in the physiological environment. Even after the release tests, a fraction of polyphenols are still bound on the surface, maintaining the antioxidant and radical scavenging activity for a longer time. An electrostatic bond occurs between the negative-charged polar groups of polyphenols (carboxyls and/or phenols) and the positive amide groups of the chitosan coating, and the substitution of the crosslinker by the polyphenols occurs during the functionalization process
Triplicity and Physical Characteristics of Asteroid (216) Kleopatra
To take full advantage of the September 2008 opposition passage of the M-type
asteroid (216) Kleopatra, we have used near-infrared adaptive optics (AO)
imaging with the W.M. Keck II telescope to capture unprecedented high
resolution images of this unusual asteroid. Our AO observations with the W.M.
Keck II telescope, combined with Spitzer/IRS spectroscopic observations and
past stellar occultations, confirm the value of its IRAS radiometric radius of
67.5 km as well as its dog-bone shape suggested by earlier radar observations.
Our Keck AO observations revealed the presence of two small satellites in orbit
about Kleopatra (see Marchis et al., 2008). Accurate measurements of the
satellite orbits over a full month enabled us to determine the total mass of
the system to be 4.64+/-0.02 10^18 Kg. This translates into a bulk density of
3.6 +/-0.4 g/cm3, which implies a macroscopic porosity for Kleopatra of ~
30-50%, typical of a rubble-pile asteroid. From these physical characteristics
we measured its specific angular momentum, very close to that of a spinning
equilibrium dumbbell.Comment: 35 pages, 3 Tables, 9 Figures. In press to Icaru
Photon-efficient imaging with a single-photon camera
Reconstructing a scene’s 3D structure and reflectivity accurately with an active imaging system operating in low-light-level conditions has wide-ranging applications, spanning biological imaging to remote sensing. Here we propose and experimentally demonstrate a depth and reflectivity imaging system with a single-photon camera that generates high-quality images from ∼1 detected signal photon per pixel. Previous achievements of similar photon efficiency have been with conventional raster-scanning data collection using single-pixel photon counters capable of ∼10-ps time tagging. In contrast, our camera’s detector array requires highly parallelized time-to-digital conversions with photon time-tagging accuracy limited to ∼ns. Thus, we develop an array-specific algorithm that converts coarsely time-binned photon detections to highly accurate scene depth and reflectivity by exploiting both the transverse smoothness and longitudinal sparsity of natural scenes. By overcoming the coarse time resolution of the array, our framework uniquely achieves high photon efficiency in a relatively short acquisition time.National Science Foundation (U.S.) (1161413)National Science Foundation (U.S.) (1422034)Lincoln LaboratorySamsung (Firm
Real-time Digital Double Framework to Predict Collapsible Terrains for Legged Robots
Inspired by the digital twinning systems, a novel real-time digital double
framework is developed to enhance robot perception of the terrain conditions.
Based on the very same physical model and motion control, this work exploits
the use of such simulated digital double synchronized with a real robot to
capture and extract discrepancy information between the two systems, which
provides high dimensional cues in multiple physical quantities to represent
differences between the modelled and the real world. Soft, non-rigid terrains
cause common failures in legged locomotion, whereby visual perception solely is
insufficient in estimating such physical properties of terrains. We used
digital double to develop the estimation of the collapsibility, which addressed
this issue through physical interactions during dynamic walking. The
discrepancy in sensory measurements between the real robot and its digital
double are used as input of a learning-based algorithm for terrain
collapsibility analysis. Although trained only in simulation, the learned model
can perform collapsibility estimation successfully in both simulation and real
world. Our evaluation of results showed the generalization to different
scenarios and the advantages of the digital double to reliably detect nuances
in ground conditions.Comment: IEEE/RSJ International Conference on Intelligent Robots and Systems
(IROS). Preprint version. Accepted June 202
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