13,968 research outputs found
Model Independent Primordial Power Spectrum from Maxima, Boomerang, and DASI Data
A model-independent determination of the primordial power spectrum of matter
density fluctuations could uniquely probe physics of the very early universe,
and provide powerful constraints on inflationary models. We parametrize the
primordial power spectrum as an arbitrary function, and deduce its
binned amplitude from the cosmic microwave background radiation anisotropy
(CMB) measurements of Maxima, Boomerang, and DASI. We find that for a flat
universe with (scale-invariant) for scales h/Mpc, the
primordial power spectrum is marginally consistent with a scale-invariant
Harrison-Zeldovich spectrum. However, we deduce a rise in power compared to a
scale-invariant power spectrum for 0.001 h/{Mpc} \la k \la 0.01 h/{Mpc}. Our
results are consistent with large-scale structure data, and seem to suggest
that the current observational data allow for the possibility of unusual
physics in the very early universe.Comment: substantially revised and final version, accepted by Ap
Ring Formation in Magnetically Subcritical Clouds and Multiple Star Formation
We study numerically the ambipolar diffusion-driven evolution of
non-rotating, magnetically subcritical, disk-like molecular clouds, assuming
axisymmetry. Previous similar studies have concentrated on the formation of
single magnetically supercritical cores at the cloud center, which collapse to
form isolated stars. We show that, for a cloud with many Jeans masses and a
relatively flat mass distribution near the center, a magnetically supercritical
ring is produced instead. The supercritical ring contains a mass well above the
Jeans limit. It is expected to break up, through both gravitational and
possibly magnetic interchange instabilities, into a number of supercritical
dense cores, whose dynamic collapse may give rise to a burst of star formation.
Non-axisymmetric calculations are needed to follow in detail the expected ring
fragmentation into multiple cores and the subsequent core evolution.
Implications of our results on multiple star formation in general and the
northwestern cluster of protostars in the Serpens molecular cloud core in
particular are discussed.Comment: 25 pages, 4 figures, to appear in Ap
Spectral dimensions of hierarchical scale-free networks with shortcuts
The spectral dimension has been widely used to understand transport
properties on regular and fractal lattices. Nevertheless, it has been little
studied for complex networks such as scale-free and small world networks. Here
we study the spectral dimension and the return-to-origin probability of random
walks on hierarchical scale-free networks, which can be either fractals or
non-fractals depending on the weight of shortcuts. Applying the renormalization
group (RG) approach to the Gaussian model, we obtain the spectral dimension
exactly. While the spectral dimension varies between and for the
fractal case, it remains at , independent of the variation of network
structure for the non-fractal case. The crossover behavior between the two
cases is studied through the RG flow analysis. The analytic results are
confirmed by simulation results and their implications for the architecture of
complex systems are discussed.Comment: 10 pages, 3 figure
Feasibility of diffusion and probabilistic white matter analysis in patients implanted with a deep brain stimulator.
Deep brain stimulation (DBS) for Parkinson\u27s disease (PD) is an established advanced therapy that produces therapeutic effects through high frequency stimulation. Although this therapeutic option leads to improved clinical outcomes, the mechanisms of the underlying efficacy of this treatment are not well understood. Therefore, investigation of DBS and its postoperative effects on brain architecture is of great interest. Diffusion weighted imaging (DWI) is an advanced imaging technique, which has the ability to estimate the structure of white matter fibers; however, clinical application of DWI after DBS implantation is challenging due to the strong susceptibility artifacts caused by implanted devices. This study aims to evaluate the feasibility of generating meaningful white matter reconstructions after DBS implantation; and to subsequently quantify the degree to which these tracts are affected by post-operative device-related artifacts. DWI was safely performed before and after implanting electrodes for DBS in 9 PD patients. Differences within each subject between pre- and post-implantation FA, MD, and RD values for 123 regions of interest (ROIs) were calculated. While differences were noted globally, they were larger in regions directly affected by the artifact. White matter tracts were generated from each ROI with probabilistic tractography, revealing significant differences in the reconstruction of several white matter structures after DBS. Tracts pertinent to PD, such as regions of the substantia nigra and nigrostriatal tracts, were largely unaffected. The aim of this study was to demonstrate the feasibility and clinical applicability of acquiring and processing DWI post-operatively in PD patients after DBS implantation. The presence of global differences provides an impetus for acquiring DWI shortly after implantation to establish a new baseline against which longitudinal changes in brain connectivity in DBS patients can be compared. Understanding that post-operative fiber tracking in patients is feasible on a clinically-relevant scale has significant implications for increasing our current understanding of the pathophysiology of movement disorders, and may provide insights into better defining the pathophysiology and therapeutic effects of DBS
Axion detection in the milli-eV mass range
We propose an experimental scheme to search for galactic halo axions with
mass eV, which is above the range accessible with cavity
techniques. The detector consists of a large number of parallel superconducting
wires embedded in a material transparent to microwave radiation. The wires
carry a current configuration which produces a static, inhomogeneous magnetic
field within the detector volume. Axions which enter this
volume may convert to photons. We discuss the feasibility of the detector and
its sensitivity.Comment: LaTex, 9 pages, 4 figures (sent upon request), UFIFT-HEP-93--
The Extremely Luminous Quasar Survey in the Pan-STARRS 1 Footprint (PS-ELQS)
We present the results of the Extremely Luminous Quasar Survey in the
survey of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS;
PS1). This effort applies the successful quasar selection strategy of the
Extremely Luminous Survey in the Sloan Digital Sky Survey footprint
() to a much larger area
(). This spectroscopic survey targets the most
luminous quasars (; ) at intermediate redshifts
(). Candidates are selected based on a near-infrared JKW2 color cut
using WISE AllWISE and 2MASS photometry to mainly reject stellar contaminants.
Photometric redshifts () and star-quasar classifications for each
candidate are calculated from near-infrared and optical photometry using the
supervised machine learning technique random forests. We select 806 quasar
candidates at from a parent sample of 74318 sources. After
exclusion of known sources and rejection of candidates with unreliable
photometry, we have taken optical identification spectra for 290 of our 334
good PS-ELQS candidates. We report the discovery of 190 new quasars
and an additional 28 quasars at lower redshifts. A total of 44 good PS-ELQS
candidates remain unobserved. Including all known quasars at , our
quasar selection method has a selection efficiency of at least . At lower
declinations we approximately triple the known
population of extremely luminous quasars. We provide the PS-ELQS quasar catalog
with a total of 592 luminous quasars (, ). This unique
sample will not only be able to provide constraints on the volume density and
quasar clustering of extremely luminous quasars, but also offers valuable
targets for studies of the intergalactic medium.Comment: 34 pages, 10 figures, accepted to ApJ
Generation of terahertz pulses with arbitrary elliptical polarization
We employ two different methods to generate controllable elliptical polarization of teraherz (THz) pulses. First, THz pulses are generated via optical rectification in nonlinear crystals using a pair of temporally separated and perpendicularly polarized optical pulses. The THz ellipticity is controlled by adjusting the relative time delay and polarization of the two optical pulses. We generate mixed polarization states of single-cycle THz pulses using ZnTe, and elliptically polarized multicycle THz pulses in periodically poled lithium niobate crystals. Second, we generate elliptically polarized THz pulses by making a THz “wave plate” using a combination of a wire-grid THz polarizer and a mirror to transform linearly polarized multicycle THz pulses into elliptical polarization.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87838/2/221111_1.pd
Genome-wide meta-analysis of 241,258 adults accounting for smoking behaviour identifies novel loci for obesity traits
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