59 research outputs found
On plane gravitational waves in real connection variables
We investigate using plane fronted gravitational wave space-times as model
systems to study loop quantization techniques and dispersion relations. In this
classical analysis, we start with planar symmetric space-times in the real
connection formulation. We reduce via Dirac constraint analysis to a final form
with one canonical pair and one constraint, equivalent to the metric and
Einstein equations of plane-fronted with parallel rays waves. Due to the
symmetries and use of special coordinates general covariance is broken.
However, this allows us to simply express the constraints of the consistent
system. A recursive construction of Dirac brackets results in non-local
brackets, analogous to those of self-dual fields, for the triad variables
chosen in this approach.Comment: v2: Matches published version, up to minor stylistic change
Factor ordering in standard quantum cosmology
The Wheeler-DeWitt equation of Friedmann models with a massless quantum field
is formulated with arbitrary factor ordering of the Hamiltonian constraint
operator. A scalar product of wave functions is constructed, giving rise to a
probability interpretation and making comparison with the classical solution
possible. In general the bahaviour of the wave function of the model depends on
a critical energy of the matter field, which, in turn, depends on the chosen
factor ordering. By certain choices of the ordering the critical energy can be
pushed down to zero.Comment: 15 pages, 3 figure
Wavelength-selected Neutron Pulses Formed by a Spatial Magnetic Neutron Spin Resonator
AbstractWe present a novel type of spatial magnetic neutron spin resonator whose time and wavelength resolution can be de- coupled from each other by means of a travelling wave mode of operation. Combined with a pair of highly efficient polarisers such a device could act simultaneously as monochromator and chopper, able to produce short neutron pulses, whose wavelength, spectral width and duration could be varied almost instantaneously by purely electronic means with- out any mechanical modification of the experimental setup. To demonstrate the practical feasibility of this technique we have designed and built a first prototype resonator consisting of ten individually switchable modules which allows to produce neutron pulses in the microsecond regime. It was installed at a polarised 2.6Å neutron beamline at the 250kW TRIGA research reactor of the Vienna University of Technology where it could deliver pulses of 55μs duration, which is about three times less than the passage time of the neutrons through the resonator itself. In order to further improve the achievable wavelength resolution to about 3% a second prototype resonator, consisting of 48 individual modules with optimised field homogeneity and enlarged beam cross-section of 6 × 6cm2 was developed. We present the results of first measurements which demonstrate the successful operation of this device
Radiation from a uniformly accelerating harmonic oscillator
We consider a radiation from a uniformly accelerating harmonic oscillator
whose minimal coupling to the scalar field changes suddenly. The exact time
evolutions of the quantum operators are given in terms of a classical solution
of a forced harmonic oscillator. After the jumping of the coupling constant
there occurs a fast absorption of energy into the oscillator, and then a slow
emission follows. Here the absorbed energy is independent of the acceleration
and proportional to the log of a high momentum cutoff of the field. The emitted
energy depends on the acceleration and also proportional to the log of the
cutoff. Especially, if the coupling is comparable to the natural frequency of
the detector () enormous energies are radiated away
from the oscillator.Comment: 26 pages, 1 eps figure, RevTeX, minor correction in grammar, add a
discussio
Stochastic Theory of Accelerated Detectors in a Quantum Field
We analyze the statistical mechanical properties of n-detectors in arbitrary
states of motion interacting with each other via a quantum field. We use the
open system concept and the influence functional method to calculate the
influence of quantum fields on detectors in motion, and the mutual influence of
detectors via fields. We discuss the difference between self and mutual
impedance and advanced and retarded noise. The mutual effects of detectors on
each other can be studied from the Langevin equations derived from the
influence functional, as it contains the backreaction of the field on the
system self-consistently. We show the existence of general fluctuation-
dissipation relations, and for trajectories without event horizons,
correlation-propagation relations, which succinctly encapsulate these quantum
statistical phenomena. These findings serve to clarify some existing confusions
in the accelerated detector problem. The general methodology presented here
could also serve as a platform to explore the quantum statistical properties of
particles and fields, with practical applications in atomic and optical physics
problems.Comment: 32 pages, Late
Interaction of Hawking radiation with static sources outside a Schwarzschild black hole
We show that the response rate of (i) a static source interacting with
Hawking radiation of massless scalar field in Schwarzschild spacetime (with the
Unruh vacuum) and that of (ii) a uniformly accelerated source with the same
proper acceleration in Minkowski spacetime (with the Minkowski vacuum) are
equal. We show that this equality will not hold if the Unruh vacuum is replaced
by the Hartle-Hawking vacuum. It is verified that the source responds to the
Hawking radiation near the horizon as if it were at rest in a thermal bath in
Minkowski spacetime with the same temperature. It is also verified that the
response rate in the Hartle-Hawking vacuum approaches that in Minkowski
spacetime with the same temperature far away from the black hole. Finally, we
compare our results with others in the literature.Comment: 18 pages (REVTEX
Big Crunch Avoidance in k = 1 Semi-Classical Loop Quantum Cosmology
It is well known that a closed universe with a minimally coupled massive
scalar field always collapses to a singularity unless the initial conditions
are extremely fine tuned. We show that the corrections to the equations of
motion for the massive scalar field, given by loop quantum gravity in high
curvature regime, always lead to a bounce independently of the initial
conditions. In contrast to the previous works in loop quantum cosmology, we
note that the singularity can be avoided even at the semi-classical level of
effective dynamical equations with non-perturbative quantum gravity
modifications, without using a discrete quantum evolution.Comment: Minor changes, To appear in Physical Review
Sediment and nutrient delivery from thermokarst features in the foothills of the North Slope, Alaska : potential impacts on headwater stream ecosystems
Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): G02026, doi:10.1029/2007JG000470.Permafrost is a defining characteristic of the Arctic environment. However, climate warming is thawing permafrost in many areas leading to failures in soil structure called thermokarst. An extensive survey of a 600 km2 area in and around the Toolik Lake Natural Research Area (TLNRA) revealed at least 34 thermokarst features, two thirds of which were new since ∼1980 when a high resolution aerial survey of the area was done. Most of these thermokarst features were associated with headwater streams or lakes. We have measured significantly increased sediment and nutrient loading from thermokarst features to streams in two well-studied locations near the TLNRA. One small thermokarst gully that formed in 2003 on the Toolik River in a 0.9 km2 subcatchment delivered more sediment to the river than is normally delivered in 18 years from 132 km2 in the adjacent upper Kuparuk River basin (a long-term monitoring reference site). Ammonium, nitrate, and phosphate concentrations downstream from a thermokarst feature on Imnavait Creek increased significantly compared to upstream reference concentrations and the increased concentrations persisted over the period of sampling (1999–2005). The downstream concentrations were similar to those we have used in a long-term experimental manipulation of the Kuparuk River and that have significantly altered the structure and function of that river. A subsampling of other thermokarst features from the extensive regional survey showed that concentrations of ammonium, nitrate, and phosphate were always higher downstream of the thermokarst features. Our previous research has shown that even minor increases in nutrient loading stimulate primary and secondary production. However, increased sediment loading could interfere with benthic communities and change the responses to increased nutrient delivery. Although the terrestrial area impacted by thermokarsts is limited, the aquatic habitat altered by these failures can be extensive. If warming in the Arctic foothills accelerates thermokarst formation, there may be substantial and wide-spread impacts on arctic stream ecosystems that are currently poorly understood.The results
presented in this report are based upon work supported by the U.S. National
Science Foundation under grants to the Arctic Hyporheic project (OPP-
0327440) and the Arctic Long-Term Ecological Research Program (DEB-
9810222)
Impact of COVID-19 on cardiovascular testing in the United States versus the rest of the world
Objectives: This study sought to quantify and compare the decline in volumes of cardiovascular procedures between the United States and non-US institutions during the early phase of the coronavirus disease-2019 (COVID-19) pandemic.
Background: The COVID-19 pandemic has disrupted the care of many non-COVID-19 illnesses. Reductions in diagnostic cardiovascular testing around the world have led to concerns over the implications of reduced testing for cardiovascular disease (CVD) morbidity and mortality.
Methods: Data were submitted to the INCAPS-COVID (International Atomic Energy Agency Non-Invasive Cardiology Protocols Study of COVID-19), a multinational registry comprising 909 institutions in 108 countries (including 155 facilities in 40 U.S. states), assessing the impact of the COVID-19 pandemic on volumes of diagnostic cardiovascular procedures. Data were obtained for April 2020 and compared with volumes of baseline procedures from March 2019. We compared laboratory characteristics, practices, and procedure volumes between U.S. and non-U.S. facilities and between U.S. geographic regions and identified factors associated with volume reduction in the United States.
Results: Reductions in the volumes of procedures in the United States were similar to those in non-U.S. facilities (68% vs. 63%, respectively; p = 0.237), although U.S. facilities reported greater reductions in invasive coronary angiography (69% vs. 53%, respectively; p < 0.001). Significantly more U.S. facilities reported increased use of telehealth and patient screening measures than non-U.S. facilities, such as temperature checks, symptom screenings, and COVID-19 testing. Reductions in volumes of procedures differed between U.S. regions, with larger declines observed in the Northeast (76%) and Midwest (74%) than in the South (62%) and West (44%). Prevalence of COVID-19, staff redeployments, outpatient centers, and urban centers were associated with greater reductions in volume in U.S. facilities in a multivariable analysis.
Conclusions: We observed marked reductions in U.S. cardiovascular testing in the early phase of the pandemic and significant variability between U.S. regions. The association between reductions of volumes and COVID-19 prevalence in the United States highlighted the need for proactive efforts to maintain access to cardiovascular testing in areas most affected by outbreaks of COVID-19 infection
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