20,145 research outputs found
Non-Line-of-Sight Passive Acoustic Localization Around Corners
Non-line-of-sight (NLoS) imaging is an important challenge in many fields
ranging from autonomous vehicles and smart cities to defense applications.
Several recent works in optics and acoustics tackle the challenge of imaging
targets hidden from view (e.g. placed around a corner) by measuring
time-of-flight (ToF) information using active SONAR/LiDAR techniques,
effectively mapping the Green functions (impulse responses) from several
sources to an array of detectors. Here, leveraging passive correlations-based
imaging techniques, we study the possibility of acoustic NLoS target
localization around a corner without the use of controlled active sources. We
demonstrate localization and tracking of a human subject hidden around the
corner in a reverberating room, using Green functions retrieved from
correlations of broadband noise in multiple detectors. Our results demonstrate
that the controlled active sources can be replaced by passive detectors as long
as a sufficiently broadband noise is present in the scene.Comment: 6 pages, 3 figure
Surface magnetic field effects in local helioseismology
Using helioseismic holography strong evidence is presented that the phase (or
equivalent travel-time) of helioseismic signatures in Dopplergrams within
sunspots depend upon the line-of-sight angle in the plane containing the
magnetic field and vertical directions. This is shown for the velocity signal
in the penumbrae of two sunspots at 3, 4 and 5 mHz. Phase-sensitive holography
demonstrates that they are significantly affected in a strong, moderately
inclined magnetic field. This research indicates that the effects of the
surface magnetic field are potentially very significant for local helioseismic
analysis of active regions.Comment: 6 pages, 8 figure
High Resolution Ionization of Ultracold Neutral Plasmas
Collective effects, such as waves and instabilities, are integral to our
understanding of most plasma phenomena. We have been able to study these in
ultracold neutral plasmas by shaping the initial density distribution through
spatial modulation of the ionizing laser intensity. We describe a relay imaging
system for the photoionization beam that allows us to create higher resolution
features and its application to extend the observation of ion acoustic waves to
shorter wavelengths. We also describe the formation of sculpted density
profiles to create fast expansion of plasma into vacuum and streaming plasmas
Acoustic scale from the angular power spectra of SDSS-III DR8 photometric luminous galaxies
We measure the acoustic scale from the angular power spectra of the Sloan
Digital Sky Survey III (SDSS-III) Data Release 8 imaging catalog that includes
872,921 galaxies over ~ 10,000 deg^2 between 0.45<z<0.65. The extensive
spectroscopic training set of the Baryon Oscillation Spectroscopic Survey
(BOSS) luminous galaxies allows precise estimates of the true redshift
distributions of galaxies in our imaging catalog. Utilizing the redshift
distribution information, we build templates and fit to the power spectra of
the data, which are measured in our companion paper, Ho et al. 2011, to derive
the location of Baryon acoustic oscillations (BAO) while marginalizing over
many free parameters to exclude nearly all of the non-BAO signal. We derive the
ratio of the angular diameter distance to the sound horizon scale D_A/r_s=
9.212 + 0.416 -0.404 at z=0.54, and therefore, D_A= 1411+- 65 Mpc at z=0.54;
the result is fairly independent of assumptions on the underlying cosmology.
Our measurement of angular diameter distance D_A is 1.4 \sigma higher than what
is expected for the concordance LCDM (Komatsu et al. 2011), in accordance to
the trend of other spectroscopic BAO measurements for z >~ 0.35. We report
constraints on cosmological parameters from our measurement in combination with
the WMAP7 data and the previous spectroscopic BAO measurements of SDSS
(Percival et al. 2010) and WiggleZ (Blake et al. 2011). We refer to our
companion papers (Ho et al. 2011; de Putter et al. 2011) for investigations on
information of the full power spectrum.Comment: 16 pages, 14 figures, 3 tables, submitted to Ap
Observational Evidence of the Accelerated Expansion of the Universe
The discovery of cosmic acceleration is one of the most important
developments in modern cosmology. The observation, thirteen years ago, that
type Ia supernovae appear dimmer that they would have been in a decelerating
universe followed by a series of independent observations involving galaxies
and cluster of galaxies as well as the cosmic microwave background, all point
in the same direction: we seem to be living in a flat universe whose expansion
is currently undergoing an acceleration phase. In this paper, we review the
various observational evidences, most of them gathered in the last decade, and
the improvements expected from projects currently collecting data or in
preparation.Comment: Accepted review article to appear in a special volume of the "Comptes
Rendus de l'Acad\'emie des Sciences" about Dark Energy and Dark Matte
Probing Dark Energy with Baryonic Acoustic Oscillations from Future Large Galaxy Redshift Surveys
We show that the measurement of the baryonic acoustic oscillations in large
high redshift galaxy surveys offers a precision route to the measurement of
dark energy. The cosmic microwave background provides the scale of the
oscillations as a standard ruler that can be measured in the clustering of
galaxies, thereby yielding the Hubble parameter and angular diameter distance
as a function of redshift. This, in turn, enables one to probe dark energy. We
use a Fisher matrix formalism to study the statistical errors for redshift
surveys up to z=3 and report errors on cosmography while marginalizing over a
large number of cosmological parameters including a time-dependent equation of
state. With redshifts surveys combined with cosmic microwave background
satellite data, we achieve errors of 0.037 on Omega_x, 0.10 on w(z=0.8), and
0.28 on dw(z)/dz for cosmological constant model. Models with less negative
w(z) permit tighter constraints. We test and discuss the dependence of
performance on redshift, survey conditions, and fiducial model. We find results
that are competitive with the performance of future supernovae Ia surveys. We
conclude that redshift surveys offer a promising independent route to the
measurement of dark energy.Comment: submitted to ApJ, 24 pages, LaTe
Large Scale Structure Observations
Galaxy Surveys are enjoying a renaissance thanks to the advent of
multi-object spectrographs on ground-based telescopes. The last 15 years have
seen the fruits of this experimental advance, including the 2-degree Field
Galaxy Redshift Survey (2dFGRS; Colless et al. 2003) and the Sloan Digital Sky
Survey (SDSS; York et al. 2000). Most recently, the Baryon Oscillation
Spectroscopic Survey (BOSS; Dawson et al. 2013), part of the SDSS-III project
(Eisenstein et al. 2011), has provided the largest volume of the low-redshift
Universe ever surveyed with a galaxy density useful for high-precision
cosmology. This set of lecture notes looks at some of the physical processes
that underpin these measurements, the evolution of measurements themselves, and
looks ahead to the next 15 years and the advent of surveys such as the enhanced
Baryon Oscillation Spectroscopic Survey (eBOSS), the Dark Energy Spectroscopic
Instrument (DESI) and the ESA Euclid satellite mission.Comment: Lectures given at Post-Planck Cosmology, Ecole de Physique des
Houches, Les Houches, July 8-Aug 2, 2013, eds. B. Wandelt, C. Deffayet, P.
Peter, to be published by Oxford University Press, and New Horizons for
Observational Cosmology, International School of Physics Enrico Fermi,
Varenna, July 1-6, 2013, eds. A. Melchiorri, A. Cooray, E. Komatsu, to be
published by the Italian Society of Physic
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