16,671 research outputs found
Stable and Unstable Circular Strings in Inflationary Universes
It was shown by Garriga and Vilenkin that the circular shape of nucleated
cosmic strings, of zero loop-energy in de Sitter space, is stable in the sense
that the ratio of the mean fluctuation amplitude to the loop radius is
constant. This result can be generalized to all expanding strings (of non-zero
loop-energy) in de Sitter space. In other curved spacetimes the situation,
however, may be different.
In this paper we develop a general formalism treating fluctuations around
circular strings embedded in arbitrary spatially flat FRW spacetimes. As
examples we consider Minkowski space, de Sitter space and power law expanding
universes. In the special case of power law inflation we find that in certain
cases the fluctuations grow much slower that the radius of the underlying
unperturbed circular string. The inflation of the universe thus tends to wash
out the fluctuations and to stabilize these strings.Comment: 15 pages Latex, NORDITA 94/14-
Sulfate Reduction in Sediments Produces High Levels of Chromophoric Dissolved Organic Matter
Sulfate reduction plays an important role in altering dissolved organic matter (DOM) in estuarine and coastal sediments, although its role in the production of optically active chromophoric DOM (CDOM) and a subset of fluorescent DOM (FDOM) has not been previously investigated in detail. Freshwater sediment slurries were incubated anaerobically with added sulfate and acetate to promote sulfate-reducing bacteria. Ultraviolet visible (UV-Vis) absorbance and 3-dimensional excitation emission matrix (EEM) fluorescence spectra were measured over a five weeks anaerobic dark incubation period. Parallel Factor Analysis (PARAFAC) of FDOM determined components that increased significantly during dark and anaerobic incubation matching three components previously considered of terrestrially-derived or humic-like origin published in the OpenFluor database. The observed FDOM increase was strongly correlated (R2 = 0.96) with the reduction of sulfate. These results show a direct experimental link between sulfate reduction and FDOM production, which impacts our understanding of coastal FDOM sources and early sediment diagenesis. As 3D fluorescence techniques are commonly applied to diverse systems, these results provide increasing support that FDOM can have many diverse sources not consistently captured by common classifications such as “humic-like” fluorescence
Evidence of a structural anomaly at 14 K in polymerised CsC60
We report the results of a high-resolution synchrotron X-ray powder
diffraction study of polymerised CsC in the temperature range 4 to 40 K.
Its crystal structure is monoclinic (space group I2/m), isostructural with
RbC. Below 14 K, a spontaneous thermal contraction is observed along
both the polymer chain axis, and the interchain separation along [111],
. This structural anomaly could trigger the occurrence of the spin-singlet
ground state, observed by NMR at the same temperature.Comment: 8 pages, 5 figures, submitte
Dioxins and dioxin-like compounds in domestic meat
Persistent environmental pollutants including polychlorinated dibenzo-p-dioxins, dibenzofurans (PCDD/Fs), coplanar polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PDBEs) were analysed in 65 meat samples collected from supermarkets across the U.S. in 2001
Refinements to data acquired by 2-dimensional video disdrometers
The 2-Dimensional Video Disdrometer (2DVD) is a commonly used tool for exploring rain microphysics and for validating remotely sensed rain retrievals. Recent work has revealed a persistent anomaly in 2DVD data. Early investigations of this anomaly concluded that the resulting errors in rain measurement were modest, but the methods used to flag anomalous data were not optimized, and related considerations associated with the sample sensing area were not fully investigated. Here, we (i) refine the anomaly-detecting algorithm for increased sensitivity and reliability and (ii) develop a related algorithm for refining the estimate of sample sensing area for all detected drops, including those not directly impacted by the anomaly. Using these algorithms, we explore the corrected data to measure any resulting changes to estimates of bulk rainfall statistics from two separate 2DVDs deployed in South Carolina combining for approximately 10 total years of instrumental uptime. Analysis of this data set consisting of over 200 million drops shows that the error induced in estimated total rain accumulations using the manufacturer-reported area is larger than the error due to considerations related to the anomaly. The algorithms presented here imply that approximately 4.2% of detected drops are spurious and the mean reported effective sample area for drops believed to be correctly detected is overestimated by ~8.5%. Simultaneously accounting for all of these effects suggests that the total accumulated rainfall in the data record is approximately 1.1% larger than the raw data record suggests
Circular String-Instabilities in Curved Spacetime
We investigate the connection between curved spacetime and the emergence of
string-instabilities, following the approach developed by Loust\'{o} and
S\'{a}nchez for de Sitter and black hole spacetimes. We analyse the linearised
equations determining the comoving physical (transverse) perturbations on
circular strings embedded in Schwarzschild, Reissner-Nordstr\"{o}m and de
Sitter backgrounds. In all 3 cases we find that the "radial" perturbations grow
infinitely for (ring-collapse), while the "angular"
perturbations are bounded in this limit. For we find that
the perturbations in both physical directions (perpendicular to the string
world-sheet in 4 dimensions) blow up in the case of de Sitter space. This
confirms results recently obtained by Loust\'{o} and S\'{a}nchez who considered
perturbations around the string center of mass.Comment: 24 pages Latex + 2 figures (not included). Observatoire de Paris,
Meudon No. 9305
The Holographic Universe
We present a holographic description of four-dimensional single-scalar
inflationary universes in terms of a three-dimensional quantum field theory.
The holographic description correctly reproduces standard inflationary
predictions in their regime of applicability. In the opposite case, wherein
gravity is strongly coupled at early times, we propose a holographic
description in terms of perturbative QFT and present models capable of
satisfying the current observational constraints while exhibiting a
phenomenology distinct from standard inflation. This provides a qualitatively
new method for generating a nearly scale-invariant spectrum of primordial
cosmological perturbations.Comment: 20 pages, 5 figs; extended version of arXiv:0907.5542 including
background material and detailed derivations. To appear in Proceedings of 1st
Mediterranean Conference on Classical and Quantum Gravit
Effects of caffeine on reading performance on the Visagraph2
Caffeine is a drug that wakes people up and stimulates the central nervous system. It is frequently found in many beverages and other consumed products. It also comes as an over-the-counter supplement. Since it is so common and gets used to help people stay awake during reading tasks, we wanted to see what effect it has on comprehension, attention and eye movements when reading. To do this we screened 27 optometry students for health problems and tested them twice on the Visagraph2, an instrument that objectively monitors eye movements and tests comprehension of material read. Tests were conducted when subjects had no caffeine in their system and when caffeine was at its highest concentration in their blood. Some started on caffeine and others did not in an attempt to limit a learning affect. In an analysis of our data, we found better performance when not on caffeine that was statistically significant in the number of fixations, regressions and percent of directional attack. Span of recognition, comprehension, rate with comprehension and grade level efficiency were also better when not on caffeine, but they were not statistically significant. Reading rate without comprehension was slower and the average duration of fixation was longer when not on caffeine. However, since comprehension was better and their were fewer fixations when not on caffeine it can be concluded that caffeine made reading more erratic and less efficient. In short, not using caffeine makes reading more efficient and improves comprehension. Using caffeine makes reading quicker, but less efficient and decreases comprehension. This means that caffeine keeps the mind and body awake, but may not make you a better reader
A Comment on the Geometric Entropy and Conical Space
It has been recently pointed out that a definition of the geometric entropy
using the partition function in a conical space does not in general lead to a
positive definite quantity. For a scalar field model with a non-minimal
coupling we clarify the origin of the anomalous behavior from the viewpoint of
the canonical formulation.Comment: No Figures. To appear in Classical and Quantum Gravit
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