526 research outputs found
Zeldovich flow on cosmic vacuum background: new exact nonlinear analytical solution
A new exact nonlinear Newtonian solution for a plane matter flow superimposed
on the isotropic Hubble expansion is reported. The dynamical effect of cosmic
vacuum is taken into account. The solution describes the evolution of nonlinear
perturbations via gravitational instability of matter and the termination of
the perturbation growth by anti-gravity of vacuum at the epoch of transition
from matter domination to vacuum domination. On this basis, an `approximate' 3D
solution is suggested as an analog of the Zeldovich ansatz.Comment: 9 pages, 1 figure
Self Interacting Dark Matter in the Solar System
Weakly coupled, almost massless, spin 0 particles have been predicted by many
extensions of the standard model of particle physics. Recently, the PVLAS group
observed a rotation of polarization of electromagnetic waves in vacuum in the
presence of transverse magnetic field. This phenomenon is best explained by the
existence of a weakly coupled light pseudoscalar particle. However, the
coupling required by this experiment is much larger than the conventional
astrophysical limits. Here we consider a hypothetical self-interacting
pseudoscalar particle which couples weakly with visible matter.
Assuming that these pseudoscalars pervade the galaxy, we show that the solar
limits on the pseudoscalar-photon coupling can be evaded.Comment: 17 pages, 2 figure
Meningococcal disease in children in Merseyside, England:a 31 year descriptive study
Meningococcal disease (MCD) is the leading infectious cause of death in early childhood in the United Kingdom, making it a public health priority. MCD most commonly presents as meningococcal meningitis (MM), septicaemia (MS), or as a combination of the two syndromes (MM/MS). We describe the changing epidemiology and clinical presentation of MCD, and explore associations with socioeconomic status and other risk factors. A hospital-based study of children admitted to a tertiary children's centre, Alder Hey Children's Foundation Trust, with MCD, was undertaken between 1977 to 2007 (n = 1157). Demographics, clinical presentations, microbiological confirmation and measures of deprivation were described. The majority of cases occurred in the 1-4 year age group and there was a dramatic fall in serogroup C cases observed with the introduction of the meningococcal C conjugate (MCC) vaccine. The proportion of MS cases increased over the study period, from 11% in the first quarter to 35% in the final quarter. Presentation with MS (compared to MM) and serogroup C disease (compared to serogroup B) were demonstrated to be independent risk factors for mortality, with odds ratios of 3.5 (95% CI 1.18 to 10.08) and 2.18 (95% CI 1.26 to 3.80) respectively. Cases admitted to Alder Hey were from a relatively more deprived population (mean Townsend score 1.25, 95% CI 1.09 to 1.41) than the Merseyside reference population. Our findings represent one of the largest single-centre studies of MCD. The presentation of MS is confirmed to be a risk factor of mortality from MCD. Our study supports the association between social deprivation and MCD
Occupationally related bilateral calcific tendonitis of Flexor carpi ulnaris: case report
We present a case of bilateral calcific tendonitis of the Flexor Carpi Ulnaris attributable to repetitive wrist action which was occupationally related. This was treated conservatively with avoidance of aggravating movement, resting splints and anti inflammatory medication when acute flare ups occurred. Since avoidance of repetitive strain on the wrists he has had no further flare ups in over 2 years. This is the only case of bilateral calcific tendonitis of Flexor Carpi Ulnaris that has been reported in the literature, further more it is the only one which has been attributed to occupation and settled following a change of career
The optical system of the H.E.S.S. imaging atmospheric Cherenkov telescopes, Part II: mirror alignment and point spread function
Mirror facets of the H.E.S.S. imaging atmospheric Cherenkov telescopes are
aligned using stars imaged onto the closed lid of the PMT camera, viewed by a
CCD camera. The alignment procedure works reliably and includes the automatic
analysis of CCD images and control of the facet alignment actuators. On-axis,
80% of the reflected light is contained in a circle of less than 1 mrad
diameter. The spot widens with increasing angle to the telescope axis. In
accordance with simulations, the spot size has roughly doubled at an angle of
1.4 degr. from the axis. The expected variation of spot size with elevation due
to deformations of the support structure is visible, but is completely
non-critical over the usual working range. Overall, the optical quality of the
telescope exceeds the specifications.Comment: 23 pages, 13 figure
Supporting the game construction process: development of artefacts in the context of a Toolkit to Game Design
The Gamers4Nature project aims to deliver a set of strategies to empower and en-courage youngsters (upper-secondary and undergraduate students) to actively participate in games creation while raising knowledge about environmental preservation and biodiversity conservation. To accomplish these goals, a Toolkit to Game Design is being created, containing a set of resources and tools aiming to help in the creation of mobile digital games, namely a Game Construction Cards Set. This paper presents the creation process of the projectâs general identi-ty and its concept adaptation to the Game Construction Cards Set, which required a deep understanding of the target audience and the development of a strategy to engage the participants in its activities. A general identity was defined, where a specific âlanguageâ to reach this audience was designed and a brand to support it was created. This âlanguageâ was applied in all graphic materials developed since day one, namely to the Game Construction Cards Set â focus of this paper. The validation of the prototypes was made through focus groups, using an iterative design approach. The focus groups participantsâ inputs were integrated in the de-sign and helped with the graphic elementsâ evolution, allowing the project to maintain a coherent strategy in all its representations that exceeds its graphic lan-guage and a deeper identification with the developed artefacts.publishe
Higher spin fields and the problem of cosmological constant
The cosmological evolution of free massless vector or tensor (but not gauge)
fields minimally coupled to gravity is analyzed. It is shown that there are
some unstable solutions for these fields in De Sitter background. The back
reaction of the energy-momentum tensor of such solutions to the original
cosmological constant exactly cancels the latter and the expansion regime
changes from the exponential to the power law one. In contrast to the
adjustment mechanism realized by a scalar field the gravitational coupling
constant in this model is time-independent and the resulting cosmology may
resemble the realistic one.Comment: 15 pages, Latex twic
The supersymmetric modified Poschl-Teller and delta-well potentials
New supersymmetric partners of the modified Poschl-Teller and the Dirac's
delta well potentials are constructed in closed form. The resulting
one-parametric potentials are shown to be interrelated by a limiting process.
The range of values of the parameters for which these potentials are free of
singularities is exactly determined. The construction of higher order
supersymmetric partner potentials is also investigated.Comment: 20 pages, LaTeX file, 4 eps figure
Exact solutions to chaotic and stochastic systems
We investigate functions that are exact solutions to chaotic dynamical
systems. A generalization of these functions can produce truly random numbers.
For the first time, we present solutions to random maps. This allows us to
check, analytically, some recent results about the complexity of random
dynamical systems. We confirm the result that a negative Lyapunov exponent does
not imply predictability in random systems. We test the effectiveness of
forecasting methods in distinguishing between chaotic and random time-series.
Using the explicit random functions, we can give explicit analytical formulas
for the output signal in some systems with stochastic resonance. We study the
influence of chaos on the stochastic resonance. We show, theoretically, the
existence of a new type of solitonic stochastic resonance, where the shape of
the kink is crucial. Using our models we can predict specific patterns in the
output signal of stochastic resonance systems.Comment: 31 pages, 18 figures (.eps). To appear in Chaos, March 200
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