2,764 research outputs found
Accelerating expansion and change of signature
We show that some types of sudden singularities admit a natural explanation
in terms of regular changes of signature on brane-worlds in AdS. The
present accelerated expansion of the Universe and its possible ending at a
sudden singularity may therefore simply be an indication that our braneworld is
about to change its Lorentzian signature to an Euclidean one, while remaining
fully regular. An explicit example of this behaviour satisfying the weak and
strong energy conditions is presented.Comment: LaTeX, 4 pages. Uses the eas.cls class. To appear in the proceedings
of the Spanish Relativity Meeting ERE'0
An investigation into the use of 3G mobile communications to provide telehealth services in rural KwaZulu-Natal
This article has been made available through the Brunel Open Access Publishing Fund.Abstract Background: We investigated the use of third-generation (3G) mobile communications to provide telehealth services in remote health clinics in rural KwaZulu-Natal, South Africa. Materials and Methods: We specified a minimal set of services as our use case that would be representative of typical activity and to provide a baseline for analysis of network performance. Services included database access to manage chronic disease, local support and management of patients (to reduce unnecessary travel to the hospital), emergency care (up to 8 h for an ambulance to arrive), e-mail, access to up-to-date information (Web), and teleclinics. We made site measurements at a representative set of health clinics to determine the type of coverage (general packet radio service [GPRS]/3G), its capabilities to support videoconferencing (H323 and Skype™ [Microsoft, Redmond, WA]) and audio (Skype), and throughput for transmission control protocol (TCP) to gain a measure of application performance. Results: We found that none of the remote health clinics had 3G service. The GPRS service provided typical upload speed of 44 kilobits per second (Kbps) and download speed of 64 Kbps. This was not sufficient to support any form of videoconferencing. We also observed that GPRS had significant round trip time (RTT), in some cases in excess of 750 ms, and this led to slow start-up for TCP applications. Conclusions: We found audio was always so broken as to be unusable and further observed that many applications such as Web access would fail under conditions of very high RTT. We found some health clinics were so remote that they had no mobile service. 3G, where available, had measured upload speed of 331 Kbps and download speed of 446 Kbps and supported videoconferencing and audio at all sites, but we frequently experienced 3G changing to GPRS. We conclude that mobile communications currently provide insufficient coverage and capability to provide reliable clinical services and would advocate dedicated wireless services where reliable communication is essential and use of store and forward for mobile applications.The Royal Society, United Kingdom
The adsorption of sulfur by microporous materials
The sorption of sulfur by the zeolites NaX (= 13X) and CaA (= 5A) and an activated charcoal prepared from sugar was investigated at temperatures between 150 and 350°C and relative sulfur pressures between 10−4 and 10−1. The adsorbate-adsorbate interaction indicated by the S-shaped isotherm for the zeolite NaX points to physical or chemical interaction of sulfur molecules in neighboring supercages. In CaA adsorbate-adsorbate interaction between sulfur species in different supercages is negligible. Below 200°C the rate of sulfur uptake by the zeolite CaA is determined by the diffusion rate of a sulfur species through the zeolitic framework. In activated charcoal a strong adsorbate-adsorbent interaction is present and part of the sulfur is chemisorbed at 350°C. The differences in the density of the adsorbed sulfur determined with three different methods indicate that even at full saturation of the micropore volume with sulfur, there is still some residual adsorption volume
The role of sulfur trapped in micropores in the catalytic partial oxidation of hydrogen sulfide with oxygen
The catalytic oxidation of hydrogen sulfide into sulfur with molecular oxygen has been studied in the temperature range 130–200 °C. Active carbon, molecular sieve 13X and liquid sulfur were used as catalysts. Sulfur is adsorbed in the micropores (3 < r < 40 Å) of the catalysts. Experiments with a surface of liquid sulfur demonstrated that sulfur is a catalyst for H2S oxidation.\ud
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This catalytic function reflects itself in the fact that H2S oxidation rate showed a maximum as a function of the amount of sulfur present in the pores of active carbon and molecular sieve 13X. The kinetics of the reaction and the activation energy are equal on catalysts of different chemical composition.\ud
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The mechanism of the catalysis by sulfur is discussed as well as the function of traces of iron oxide, present in most catalysts
Slowly rotating charged fluid balls and their matching to an exterior domain
The slow-rotation approximation of Hartle is developed to a setting where a
charged rotating fluid is present. The linearized Einstein-Maxwell equations
are solved on the background of the Reissner-Nordstrom space-time in the
exterior electrovacuum region. The theory is put to action for the charged
generalization of the Wahlquist solution found by Garcia. The Garcia solution
is transformed to coordinates suitable for the matching and expanded in powers
of the angular velocity. The two domains are then matched along the zero
pressure surface using the Darmois-Israel procedure. We prove a theorem to the
effect that the exterior region is asymptotically flat if and only if the
parameter C_{2}, characterizing the magnitude of an external magnetic field,
vanishes. We obtain the form of the constant C_{2} for the Garcia solution. We
conjecture that the Garcia metric cannot be matched to an asymptotically flat
exterior electrovacuum region even to first order in the angular velocity. This
conjecture is supported by a high precision numerical analysis.Comment: 11 pages, 2 figure
On the construction of global models describing rotating bodies; uniqueness of the exterior gravitational field
The problem of constructing global models describing isolated axially
symmetric rotating bodies in equilibrium is analyzed. It is claimed that,
whenever the global spacetime is constructed by giving boundary data on the
limiting surface of the body and integrating Einstein's equations both inside
and ouside the body, the problem becomes overdetermined. Similarly, when the
spacetime describing the interior of the body is explicitly given,the problem
of finding the exterior vacuum solution becomes overdetermined. We discuss in
detail the procedure to be followed in order to construct the exterior vacuum
field created by a given but arbitrary distribution of matter. Finally, the
uniqueness of the exterior vacuum gravitational field is proven by exploiting
the harmonic map formulation of the vacuum equations and the boundary
conditions prescribed from the matching.Comment: Latex, 10 pages, no figure
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