37,460 research outputs found
Spatial curvature effects on molecular transport by diffusion
For a substance diffusing on a curved surface, we obtain an explicit relation
valid for very small values of the time, between the local concentration, the
diffusion coefficient, the intrinsic spatial curvature and the time. We recover
the known solution of Fick's law of diffusion in the flat space limit. In the
biological context, this result would be useful in understanding the variations
in the diffusion rates of integral proteins and other molecules on membranes.Comment: 10 page
An investigation of the basement complex aquifer system in Lofa county, Liberia, for the purpose of siting boreholes
Liberia is recovering from a 14 year civil war and only 51% of the rural population has access to safe drinking water. Little hydrogeological knowledge survives in Liberia, increasing the difficulty in successfully siting new boreholes. An understanding of the local hydrogeological environment is therefore needed to improve borehole site selection and increase success rates. This research provides a semi-quantitative characterization of the hydrogeological environment of the basement aquifer in Lofa county, Liberia. Based on literature review and analysis of borehole logs, the study has developed a conceptual hydrogeological model for the local conditions, which is further characterized using 2D geoelectrical sections. Groundwater is predominantly obtained from the saprolite and underlying fractured bedrock, but specific capacities (median 281 l h-1 m-1; 25th and 75th percentile of 179 and 490 l h-1 m-1, respectively) are constrained by the limited thickness of the saturated saprolite. This study has shown that the groundwater resources in the crystalline basement in this part of Liberia conform to the general conceptual model, allowing standard techniques used elsewhere for siting and developing groundwater to be used
Acceleration of the universe, vacuum metamorphosis, and the large-time asymptotic form of the heat kernel
We investigate the possibility that the late acceleration observed in the
rate of expansion of the universe is due to vacuum quantum effects arising in
curved spacetime. The theoretical basis of the vacuum cold dark matter (VCDM),
or vacuum metamorphosis, cosmological model of Parker and Raval is revisited
and improved. We show, by means of a manifestly nonperturbative approach, how
the infrared behavior of the propagator (related to the large-time asymptotic
form of the heat kernel) of a free scalar field in curved spacetime causes the
vacuum expectation value of its energy-momentum tensor to exhibit a resonance
effect when the scalar curvature R of the spacetime reaches a particular value
related to the mass of the field. we show that the back reaction caused by this
resonance drives the universe through a transition to an accelerating expansion
phase, very much in the same way as originally proposed by Parker and Raval.
Our analysis includes higher derivatives that were neglected in the earlier
analysis, and takes into account the possible runaway solutions that can follow
from these higher-derivative terms. We find that the runaway solutions do not
occur if the universe was described by the usual classical FRW solution prior
to the growth of vacuum energy-density and negative pressure (i.e., vacuum
metamorphosis) that causes the transition to an accelerating expansion of the
universe in this theory.Comment: 33 pages, 3 figures. Submitted to Physical Review D15 (Dec 23, 2003).
v2: 1 reference added. No other change
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Stable isotope component
For Bays and Estuaries Program, Environmental Systems Section, Texas Water Development BoardDecember 1989The purpose of this study was to evaluate the use of stable carbon and nitrogen isotope ratio variations for detecting and quantifying the impact of freshwater inflow on three Texas bays. The study was divided into three objectives. ... Objective 1 was accomplished and a large database reported in final reports to the Texas Water Development Board (TWDB Contracts #55-61011 and #55-71003). Objectives 2 and 3 are reported in this document.Marine Scienc
Hawking radiation by Kerr black holes and conformal symmetry
The exponential blueshift associated with the event horizon of a black hole
makes conformal symmetry play a fundamental role in accounting for its thermal
properties. Using a derivation based on two-point functions, we show that the
spectrum of thermal radiation of scalar particles by Kerr (and Schwarzschild)
black holes can be explicitly derived on the basis of a -dimensional
conformal symmetry arising in the wave equation near the horizon. This result
reinforces the recently conjectured relation between Kerr geometry and a
-dimensional conformal field theory.Comment: Version published in Phys. Rev. Let
On the observational determination of squeezing in relic gravitational waves and primordial density perturbations
We develop a theory in which relic gravitational waves and primordial density
perturbations are generated by strong variable gravitational field of the early
Universe. The generating mechanism is the superadiabatic (parametric)
amplification of the zero-point quantum oscillations. The generated fields have
specific statistical properties of squeezed vacuum quantum states.
Macroscopically, squeezing manifests itself in a non-stationary character of
variances and correlation functions of the fields, the periodic structures of
the metric power spectra, and, as a consequence, in oscillatory behavior of the
higher order multipoles C_l of the cosmic microwave background anisotropy. We
start with the gravitational wave background and then apply the theory to
primordial density perturbations. We derive an analytical formula for the
positions of peaks and dips in the angular power spectrum l(l+1)C_l as a
function of l. This formula shows that the values of l at the peak positions
are ordered in the proportion 1:3:5:..., whereas at the dips they are ordered
as 1:2:3:.... We compare the derived positions with the actually observed
features, and find them to be in reasonably good agreement. It appears that the
observed structure is better described by our analytical formula based on the
(squeezed) metric perturbations associated with the primordial density
perturbations, rather than by the acoustic peaks reflecting the existence of
plasma sound waves at the last scattering surface. We formulate a forecast for
other features in the angular power spectrum, that may be detected by the
advanced observational missions, such as MAP and PLANCK. We tentatively
conclude that the observed structure is a macroscopic manifestation of
squeezing in the primordial metric perturbations.Comment: 34 pages, 3 figures; to appear in Phys. Rev. D66, 0435XX (2002);
includes Note Added in Proofs: "The latest CBI observations (T.J.Pearson et
al., astro-ph/0205388) have detected four peaks, at l ~ 550, 800, 1150, 1500,
and four dips, at l ~ 400, 700, 1050, 1400. These positions are in a very
good agreement with the theoretical formula (6.35) of the present paper. We
interpret this data as confirmation of our conclusion that it is gravity, and
not acoustics, that is responsible for the observed structure.
Particle production from nonlocal gravitational effective action
In this paper we show how the nonlocal effective action for gravity, obtained
after integrating out the matter fields, can be used to compute particle
production and spectra for different space-time metrics. Applying this
technique to several examples, we find that the perturbative calculation of the
effective action up to second order in curvatures yields exactly the same
results for the total number of particles as the Bogolyubov transformations
method, in the case of masless scalar fields propagating in a Robertson-Walker
space-time. Using an adiabatic approximation we also obtain the corresponding
spectra and compare the results with the traditional WKB approximation.Comment: 22 pages, LaTeX, no figures. Corrected version with new comments and
results. To appear in Phys. Rev.
Individual patient data meta-analysis of randomized controlled trials of community occupational therapy for stroke patients
<p><b>Background and Purpose:</b> Trials of occupational therapy for stroke patients living in the community have varied in their findings. It is unclear why these discrepancies have occurred.</p>
<p><b>Methods:</b> Trials were identified from searches of the Cochrane Library and other sources. The primary outcome measure was the Nottingham Extended Activities of Daily Living (NEADL) score at the end of intervention. Secondary outcome measures included the Barthel Index or the Rivermead ADL (Personal ADL), General Health Questionnaire (GHQ), Nottingham Leisure Questionnaire (NLQ), and death. Data were analyzed using linear or logistic regression with a random effect for trial and adjustment for age, gender, baseline dependency, and method of follow-up. Subgroup analyses compared any occupational therapy intervention with control.</p>
<p><b>Results:</b> We included 8 single-blind randomized controlled trials incorporating 1143 patients. Occupational therapy was associated with higher NEADL scores at the end of intervention (weighted mean difference [WMD], 1.30 points, 95% confidence intervals [CI], 0.47 to 2.13) and higher leisure scores at the end of intervention (WMD, 1.51 points; 95% CI, 0.24 to 2.79). Occupational therapy emphasizing activities of daily living (ADL) was associated with improved end of intervention NEADL (WMD, 1.61 points; 95% CI, 0.72 to 2.49) and personal activities of daily living (odds ratio [OR], 0.65; 95% CI, 0.46 to 0.91), but not NLQ. Leisure-based occupational therapy improved end of intervention NLQ (WMD, 1.96 points; 95% CI, 0.27 to 3.66) but not NEADL or PADL.</p>
<p><b>Conclusions:</b> Community occupational therapy significantly improved personal and extended activities of daily living and leisure activity in patients with stroke. Better outcomes were found with targeted interventions.</p>
Involving users in the evaluation of apps for specific health conditions
With rapid growth of Internet accessibility over recent years, the way in which we engage with healthcare services and make decisions about our own healthcare has changed. One form of engagement with the Internet is through the use of mobile applications (apps) via mobile devices such as smartphones and tablets. As a result of this, many apps have been developed to target people with varying long term conditions. These apps may aim to help educate individuals about their condition or help them to manage it. Assessing the quality of these apps is crucial especially when considering the risks associated with providing misleading information or unsubstantiated claims. This has led to a growing body of research assessing the quality of apps aimed at people living with varying long term conditions. These attempts have not involved service users to address the suitability of apps for their intended target audiences. This paper presents two examples of how service users and associated healthcare professionals can be involved in the suitability assessment of mobile apps for children and young people with Attention Deficit Hyperactivity Disorder and those living with dementia
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