11,926 research outputs found
Collective excitation frequencies and stationary states of trapped dipolar Bose-Einstein condensates in the Thomas-Fermi regime
We present a general method for obtaining the exact static solutions and
collective excitation frequencies of a trapped Bose-Einstein condensate (BEC)
with dipolar atomic interactions in the Thomas-Fermi regime. The method
incorporates analytic expressions for the dipolar potential of an arbitrary
polynomial density profile, thereby reducing the problem of handling non-local
dipolar interactions to the solution of algebraic equations.
We comprehensively map out the static solutions and excitation modes,
including non-cylindrically symmetric traps, and also the case of negative
scattering length where dipolar interactions stabilize an otherwise unstable
condensate. The dynamical stability of the excitation modes gives insight into
the onset of collapse of a dipolar BEC. We find that global collapse is
consistently mediated by an anisotropic quadrupolar collective mode, although
there are two trapping regimes in which the BEC is stable against quadrupole
fluctuations even as the ratio of the dipolar to s-wave interactions becomes
infinite. Motivated by the possibility of fragmented BEC in a dipolar Bose gas
due to the partially attractive interactions, we pay special attention to the
scissors modes, which can provide a signature of superfluidity, and identify a
long-range restoring force which is peculiar to dipolar systems. As part of the
supporting material for this paper we provide the computer program used to make
the calculations, including a graphical user interface.Comment: 23 pages, 11 figure
Reduced grapevine canopy size post-flowering via mechanical trimming alters ripening and yield of 'Pinot noir'
The degree and time of canopy trimming can alter phenology, rates of increase or decrease in berry components during grape ripening, and may influence yield and its components. The objective of this study was to investigate the extent to which reducing canopy size, by mechanical trimming post-flowering, changed Vitis vinifera L. 'Pinot noir' fruit yield and composition. Vines were mechanically trimmed to three different canopy heights at fruitset: 1000 mm (100 % canopy height), 600 mm (60 % canopy height relative to the control treatment) and 300 mm (30 % canopy height relative to the control treatment). Total soluble solids concentration and content, titratable acidity, pH and fresh berry mass were measured throughout ripening, and yield and leaf area were measured at harvest.Reduced canopy size via trimming to 30 and 60 % of the control treatment height slowed total soluble solids accumulation and in some cases increased titratable acidity and increased pH. The total soluble solids-titratable acidity ratio was therefore reduced throughout ripening by these trimming treatments relative to the full canopy height. Trimming to reduce canopy size had two effects on the source-sink ratio; it reduced the source (canopy) but increased fruit yield, an important sink. Therefore, the time of trimming is an important management consideration because it can delay and slow ripening due to reduced source leaves but could potentially accentuate the delay via increasing yield (sink). This technique may represent a way to offset the acceleration of phenology and grape ripening that has been observed to occur as a result of warmer seasons
Spatial and temporal variability of groundwater recharge in a sandstone aquifer in a semiarid region
With the aim to understand the spatial and temporal variability of
groundwater recharge, a high-resolution, spatially distributed numerical
model (MIKE SHE) representing surface water and groundwater was used to
simulate responses to precipitation in a 2.16 km2 upland catchment on
fractured sandstone near Los Angeles, California. Exceptionally high
temporal and spatial resolution was used for this catchment modeling: hourly
climate data, a 20 m×20 m grid in the horizontal plane, and 240 numerical
layers distributed vertically within the thick vadose zone and in the upper
part of the groundwater zone. The finest practical spatial and temporal
resolutions were selected to accommodate the large degree of surface and
subsurface variability of catchment features. Physical property values for
the different lithologies were assigned based on previous on-site
investigations, whereas the parameters controlling streamflow and
evapotranspiration were derived from calibration to continuous streamflow at
the outfall and to average hydraulic heads from 17 wells. Confidence in the
calibrated model was enhanced by validation through (i) comparison of
simulated average recharge to estimates based on the applications of the
chloride mass-balance method to data from the groundwater and vadose zones
within and beyond the catchment, (ii) comparison of the water isotope
signature (18O and 2H) in shallow groundwater to the variability
of isotope signatures for precipitation events over an annual cycle, and
(iii) comparison of simulated recharge time series and observed fluctuation
of water levels. The average simulated recharge across the catchment for the
period 1995–2014 is 16 mm yr−1 (4 % of the average annual
precipitation), which is consistent with previous estimates obtained by
using the chloride mass balance method (4.2 % of the average
precipitation). However, one of the most unexpected results was that local
recharge was simulated to vary from 0 to >1000 mm yr−1 due
to episodic precipitation and overland runoff effects. This recharge occurs
episodically with the major flux events at the bottom of the
evapotranspiration zone, as simulated by MIKE SHE and confirmed by the
isotope signatures, occurring only at the end of the rainy season. This is
the first study that combines MIKE SHE simulations with the analysis of
water isotopes in groundwater and rainfall to determine the timing of
recharge in a sedimentary bedrock aquifer in a semiarid region. The study
advances the understanding of recharge and unsaturated flow processes and
enhances our ability to predict the effects of surface and subsurface
features on recharge rates. This is crucial in highly heterogeneous
contaminated sites because different contaminant source areas have widely
varying recharge and, hence, groundwater fluxes impacting their mobility.</p
Loop Quantum Gravity and the The Planck Regime of Cosmology
The very early universe provides the best arena we currently have to test
quantum gravity theories. The success of the inflationary paradigm in
accounting for the observed inhomogeneities in the cosmic microwave background
already illustrates this point to a certain extent because the paradigm is
based on quantum field theory on the curved cosmological space-times. However,
this analysis excludes the Planck era because the background space-time
satisfies Einstein's equations all the way back to the big bang singularity.
Using techniques from loop quantum gravity, the paradigm has now been extended
to a self-consistent theory from the Planck regime to the onset of inflation,
covering some 11 orders of magnitude in curvature. In addition, for a narrow
window of initial conditions, there are departures from the standard paradigm,
with novel effects, such as a modification of the consistency relation
involving the scalar and tensor power spectra and a new source for
non-Gaussianities. Thus, the genesis of the large scale structure of the
universe can be traced back to quantum gravity fluctuations \emph{in the Planck
regime}. This report provides a bird's eye view of these developments for the
general relativity community.Comment: 23 pages, 4 figures. Plenary talk at the Conference: Relativity and
Gravitation: 100 Years after Einstein in Prague. To appear in the Proceedings
to be published by Edition Open Access. Summarizes results that appeared in
journal articles [2-13
Thermal radiation in non-static curved spacetimes: quantum mechanical path integrals and configuration space topology
A quantum mechanical path integral derivation is given of a thermal
propagator in non-static Gui spacetime. The thermal nature of the propagator is
understood in terms of homotopically non-trivial paths in the configuration
space appropriate to tortoise coordinates. The connection to thermal emission
from collapsing black holes is discussed.Comment: 20 pages, major revised version, 9 figures, new titl
Random noise in Diffusion Tensor Imaging, its Destructive Impact and Some Corrections
The empirical origin of random noise is described, its influence on DTI variables is illustrated by a review of numerical and in vivo studies supplemented by new simulations investigating high noise levels. A stochastic model of noise propagation is presented to structure noise impact in DTI. Finally, basics of voxelwise and spatial denoising procedures are presented. Recent denoising procedures are reviewed and consequences of the stochastic model for convenient denoising strategies are discussed
Noise Kernel in Stochastic Gravity and Stress Energy Bi-Tensor of Quantum Fields in Curved Spacetimes
The noise kernel is the vacuum expectation value of the (operator-valued)
stress-energy bi-tensor which describes the fluctuations of a quantum field in
curved spacetimes. It plays the role in stochastic semiclassical gravity based
on the Einstein-Langevin equation similar to the expectation value of the
stress-energy tensor in semiclassical gravity based on the semiclassical
Einstein equation. According to the stochastic gravity program, this two point
function (and by extension the higher order correlations in a hierarchy) of the
stress energy tensor possesses precious statistical mechanical information of
quantum fields in curved spacetime and, by the self-consistency required of
Einstein's equation, provides a probe into the coherence properties of the
gravity sector (as measured by the higher order correlation functions of
gravitons) and the quantum nature of spacetime. It reflects the low and medium
energy (referring to Planck energy as high energy) behavior of any viable
theory of quantum gravity, including string theory. It is also useful for
calculating quantum fluctuations of fields in modern theories of structure
formation and for backreaction problems in cosmological and black holes
spacetimes.
We discuss the properties of this bi-tensor with the method of
point-separation, and derive a regularized expression of the noise-kernel for a
scalar field in general curved spacetimes. One collorary of our finding is that
for a massless conformal field the trace of the noise kernel identically
vanishes. We outline how the general framework and results derived here can be
used for the calculation of noise kernels for Robertson-Walker and
Schwarzschild spacetimes.Comment: 22 Pages, RevTeX; version accepted for publication in PR
Monopole-antimonopole bound states as a source of ultra-high-energy cosmic rays
The electromagnetic decay and final annihilation of magnetic
monopole-antimonopole pairs formed in the early universe has been proposed as a
possible mechanism to produce the highest energy cosmic rays. We show that for
a monopole abundance saturating the Parker limit, the density of magnetic
monopolonium formed is many orders of magnitude less than that required to
explain the observed cosmic ray flux. We then propose a different scenario in
which the monopoles and antimonopoles are connected by strings formed at a low
energy phase transition (~ 100 GeV). The bound states decay by gravitational
radiation, with lifetimes comparable with the age of the universe. This
mechanism avoids the problems of the standard monopolonium scenario, since the
binding of monopoles and antimonopoles is perfectly efficient.Comment: 10 pages, RevTeX, no figure
microRNA expression in the prefrontal cortex of individuals with schizophrenia and schizoaffective disorder
BACKGROUND: microRNAs (miRNAs) are small, noncoding RNA molecules that are now thought to regulate the expression of many mRNAs. They have been implicated in the etiology of a variety of complex diseases, including Tourette's syndrome, Fragile × syndrome, and several types of cancer. RESULTS: We hypothesized that schizophrenia might be associated with altered miRNA profiles. To investigate this possibility we compared the expression of 264 human miRNAs from postmortem prefrontal cortex tissue of individuals with schizophrenia (n = 13) or schizoaffective disorder (n = 2) to tissue of 21 psychiatrically unaffected individuals using a custom miRNA microarray. Allowing a 5% false discovery rate, we found that 16 miRNAs were differentially expressed in prefrontal cortex of patient subjects, with 15 expressed at lower levels (fold change 0.63 to 0.89) and 1 at a higher level (fold change 1.77) than in the psychiatrically unaffected comparison subjects. The expression levels of 12 selected miRNAs were also determined by quantitative RT-PCR in our lab. For the eight miRNAs distinguished by being expressed at lower microarray levels in schizophrenia samples versus comparison samples, seven were also expressed at lower levels with quantitative RT-PCR. CONCLUSION: This study is the first to find altered miRNA profiles in postmortem prefrontal cortex from schizophrenia patients
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