1,890 research outputs found
Measurement of the small-scale structure of the intergalactic medium using close quasar pairs
The distribution of diffuse gas in the intergalactic medium (IGM) imprints a
series of hydrogen absorption lines on the spectra of distant background
quasars known as the Lyman- forest. Cosmological hydrodynamical
simulations predict that IGM density fluctuations are suppressed below a
characteristic scale where thermal pressure balances gravity. We measured this
pressure-smoothing scale by quantifying absorption correlations in a sample of
close quasar pairs. We compared our measurements to hydrodynamical simulations,
where pressure smoothing is determined by the integrated thermal history of the
IGM. Our findings are consistent with standard models for photoionization
heating by the ultraviolet radiation backgrounds that reionized the universe.Comment: Accepted for publication on Scienc
Feasibility of satellite-to-ground continuous-variable quantum key distribution
Establishing secure communication links at a global scale is a major
potential application of quantum information science but also extremely
challenging for the underlying technology. While milestone experiments using
satellite-to-ground links and exploiting singe-photon encoding for implementing
quantum key distribution have shown recently that this goal is achievable, it
is still necessary to further investigate practical solutions compatible with
classical optical communication systems. Here we examine the feasibility of
establishing secret keys in a satellite-to-ground downlink configuration using
continuous-variable encoding, which can be implemented using standard
telecommunication components certified for space environment and able to
operate at high symbol rates. Considering a realistic channel model and
state-of-the-art technology, and exploiting an orbit subdivision technique for
mitigating fluctuations in the transmission efficiency, we find positive secret
key rates for a low-Earth-orbit scenario, while finite-size effects can be a
limiting factor for higher orbits. Our analysis determines regions of values
for important experimental parameters where secret key exchange is possible and
can be used as a guideline for experimental efforts in this direction.Comment: 13 pages, 11 figure
PRIYA: A New Suite of Lyman-alpha Forest Simulations for Cosmology
We present the PRIYA suite of cosmological simulations, based on the code and
hydrodynamic model of the ASTRID simulation, and designed for cosmological
analyses of the Lyman- forest. Our simulation suite spans a
-dimensional parameter space, including cosmological parameters and
astrophysical/thermal parameters. We have run low fidelity simulations
with particles in a Mpc/h box and high fidelity simulations
with particles in a Mpc/h box. All our simulations include a
full physics model for galaxy formation, including supernova and AGN feedback,
and thus also contain a realistic population of DLAs. We advance on earlier
simulations suites by larger particle loads, by incorporating new physical
models for patchy hydrogen and helium reionization, and by self-consistently
incorporating a model for AGN feedback. We show that patchy helium reionization
imprints an excess in the 1D flux power spectrum on large scales, which may
allow future measurements of helium reionization bubble sizes. Simulation
parameters are chosen based on a Latin hypercube design and a Gaussian process
is used to interpolate to arbitrary parameter combinations. We build a
multi-fidelity emulator for the 1D flux power spectrum and the mean IGM
temperature. We show that our final interpolation error is and that our
simulations produce a flux power spectrum converged at the percent level for
- . Our simulation suite will be used to interpret Lyman-
forest 1D flux power spectra from SDSS and future DESI data releases.Comment: 24 pages, 11 figures, submitted to JCA
Euclidean reconstruction of natural underwater scenes using optic imagery sequence
The development of maritime applications require monitoring, studying and preserving of detailed and close observation on the underwater seafloor and objects. Stereo vision offers advanced technologies to build 3D models from 2D still overlapping images in a relatively inexpensive way. However, while image stereo matching is a necessary step in 3D reconstruction procedure, even the most robust dense matching techniques are not guaranteed to work for underwater images due to the challenging aquatic environment. In this thesis, in addition to a detailed introduction and research on the key components of building 3D models from optic images, a robust modified quasi-dense matching algorithm based on correspondence propagation and adaptive least square matching for underwater images is proposed and applied to some typical underwater image datasets. The experiments demonstrate the robustness and good performance of the proposed matching approach
Controlled-Stress Large-Area Pulsed Laser Deposition of Yttria Stabilized Zirconia
The US Air Force has need of parabolic-shaped membrane mirrors for surveillance satellites. The current polymer membrane technology has been unable to overcome shape deformation problems caused by intrinsic stresses from the membrane casting and mounting processes. One proposed solution was to coat the membrane mirrors with a stressed coating to compensate for shape deformations. Thus, the research presented in this dissertation produced controlled-stress large-area pulsed laser deposition (PLD) grown thin films on polymer substrates and investigated optical time-of-flight (TOF) sensor systems and Raman spectroscopy for control for the PLD process with respect to thin film stress. Initially, the PLD-grown film stress was controlled using a constant combination of deposition parameters. Also, the velocity was extracted from the optical TOF data and indicated film stress. As such, the velocity was used to control the laser fluence in order to compensate for slight variations in deposition conditions, which improved the film stress run-to-run stability. Additionally, iterative and theoretical experiments produced large-area YSZ films with less than ten percent total thickness variations. Combining the controlled-stress and large-area aspects produced desirable compensations in shape to the polymer substrates. Finally, Raman spectroscopy was shown to be compatible with the PLD
- …