21,626 research outputs found
Measurement potential of laser speckle velocimetry
Laser speckle velocimetry, the measurement of fluid velocity by measuring the translation of speckle pattern or individual particles that are moving with the fluid, is described. The measurement is accomplished by illuminating the fluid with consecutive pulses of Laser Light and recording the images of the particles or the speckles on a double exposed photographic plate. The plate contains flow information throughout the image plane so that a single double exposure may provide data at hundreds or thousands of points in the illuminated region of the fluid. Conventional interrogation of the specklegram involves illuminating the plate to form Young's fringes, whose spacing is inversely proportional to the speckle separation. Subsequently the fringes are digitized and analyzed in a computer to determine their frequency and orientation, yielding the velocity magnitude and orientation. The Young's fringe technique is equivalent to performing a 2-D spatial correlation of the double exposed specklegram intensity pattern, and this observation suggests that correlation should be considered as an alternative processing method. The principle of the correlation technique is examined
Scalar potentials, propagators and global symmetries in AdS/CFT
We study the transition of a scalar field in a fixed background
between an extremum and a minimum of a potential. We first prove that two
conditions must be met for the solution to exist. First, the potential involved
cannot be generic, i.e. a fine-tuning of their parameters is mandatory. Second,
at least in some region its second derivative must have a negative upper limit
which depends only on the dimensionality . We then calculate the boundary
propagator for small momenta in two different ways: first in a WKB
approximation, and second with the usual matching method, generalizing the
known calculation to arbitrary order. Finally, we study a system with
spontaneously broken non-Abelian global symmetry, and show in the holographic
language why the Goldstone modes appear.Comment: 26 pages - Invited contribution for the Central European Journal of
Physics, topical issue devoted to "Cosmology and Particle Physics beyond
Standard Models". Some parts overlap with 1304.3051v1, which has been
replaced by the published versio
On the matching method and the Goldstone theorem in holography
We study the transition of a scalar field in a fixed background
between an extremum and a minimum of a potential. We compute analytically the
solution to the perturbation equation for the vev deformation case by
generalizing the usual matching method to higher orders and find the propagator
of the boundary theory operator defined through the AdS-CFT correspondence. We
show that, contrary to what happens at the leading order of the matching
method, the next-to-leading order presents a simple pole at in
accordance with the Goldstone theorem applied to a spontaneously broken
dilatation invariance.Comment: 16 pages, 1 figure, published versio
Constraining cosmology and ionization history with combined 21 cm power spectrum and global signal measurements
Improvements in current instruments and the advent of next-generation
instruments will soon push observational 21 cm cosmology into a new era, with
high significance measurements of both the power spectrum and the mean
("global") signal of the 21 cm brightness temperature. In this paper we use the
recently commenced Hydrogen Epoch of Reionization Array as a worked example to
provide forecasts on astrophysical and cosmological parameter constraints. In
doing so we improve upon previous forecasts in a number of ways. First, we
provide updated forecasts using the latest best-fit cosmological parameters
from the Planck satellite, exploring the impact of different Planck datasets on
21 cm experiments. We also show that despite the exquisite constraints that
other probes have placed on cosmological parameters, the remaining
uncertainties are still large enough to have a non-negligible impact on
upcoming 21 cm data analyses. While this complicates high-precision constraints
on reionization models, it provides an avenue for 21 cm reionization
measurements to constrain cosmology. We additionally forecast HERA's ability to
measure the ionization history using a combination of power spectrum
measurements and semi-analytic simulations. Finally, we consider ways in which
21 cm global signal and power spectrum measurements can be combined, and
propose a method by which power spectrum results can be used to train a compact
parameterization of the global signal. This parameterization reduces the number
of parameters needed to describe the global signal, increasing the likelihood
of a high significance measurement.Comment: 16 pages, 8 figures. Revised to match accepted MNRAS version:
expanded discussion of covariances between astrophysics and cosmology in
Section 2.2, including two new figures; short discussion relating to KL modes
added to Section 4; final results unchange
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