12,184 research outputs found
Cosmological Acceleration Through Transition to Constant Scalar Curvature
As shown by Parker and Raval, quantum field theory in curved spacetime gives
a possible mechanism for explaining the observed recent acceleration of the
universe. This mechanism, which differs in its dynamics from quintessence
models, causes the universe to make a transition to an accelerating expansion
in which the scalar curvature, R, of spacetime remains constant. This
transition occurs despite the fact that we set the renormalized cosmological
constant to zero. We show that this model agrees very well with the current
observed type-Ia supernova (SNe-Ia) data. There are no free parameters in this
fit, as the relevant observables are determined independently by means of the
current cosmic microwave background radiation (CMBR) data. We also give the
predicted curves for number count tests and for the ratio, w(z), of the dark
energy pressure to its density, as well as for dw(z)/dz versus w(z). These
curves differ significantly from those obtained from a cosmological constant,
and will be tested by planned future observations.Comment: 31 pages, 7 figures; to appear in ApJ. Corrected numerical results;
described quantum basis of theory; 18 references added; 2 figures adde
Constraining Isocurvature Initial Conditions with WMAP 3-year data
We present constraints on the presence of isocurvature modes from the
temperature and polarization CMB spectrum data from the WMAP satellite alone,
and in combination with other datasets including SDSS galaxy survey and SNLS
supernovae. We find that the inclusion of polarization data allows the WMAP
data alone, as well as in combination with complementary observations, to place
improved limits on the contribution of CDM and neutrino density isocurvature
components individually. With general correlations, the upper limits on these
sub-dominant isocurvature components are reduced to ~60% of the first year WMAP
results, with specific limits depending on the type of fluctuations. If
multiple isocurvature components are allowed, however, we find that the data
still allow a majority of the initial power to come from isocurvature modes. As
well as providing general constraints we also consider their interpretation in
light of specific theoretical models like the curvaton and double inflation.Comment: 8 pages, 7 figures. Revised Sec 4 and Figs 3-4 post-publication to
correct an error for models with varying isocurvature spectral inde
Vector magnetic hysteresis of hard superconductors
Critical state problems which incorporate more than one component for the
magnetization vector of hard superconductors are investigated. The theory is
based on the minimization of a cost functional
which weighs the changes of the magnetic field vector within the sample. We
show that Bean's simplest prescription of choosing the correct sign for the
critical current density in one dimensional problems is just a particular
case of finding the components of the vector . is
determined by minimizing under the constraint , with a bounded set. Upon the selection of
different sets we discuss existing crossed field measurements and
predict new observable features. It is shown that a complex behavior in the
magnetization curves may be controlled by a single external parameter, i.e.:
the maximum value of the applied magnetic field .Comment: 10 pages, 9 figures, accepted in Phys. Rev.
The Adiabatic Instability on Cosmology's Dark Side
We consider theories with a nontrivial coupling between the matter and dark
energy sectors. We describe a small scale instability that can occur in such
models when the coupling is strong compared to gravity, generalizing and
correcting earlier treatments. The instability is characterized by a negative
sound speed squared of an effective coupled dark matter/dark energy fluid. Our
results are general, and applicable to a wide class of coupled models and
provide a powerful, redshift-dependent tool, complementary to other
constraints, with which to rule many of them out. A detailed analysis and
applications to a range of models are presented in a longer companion paper.Comment: 4 pages, 1 figur
Product development within the framework of a National Casting Technology Centre
Published ArticleThe need for a state of the art advanced National Casting Technology Centre (NCTC) has been widely supported throughout industry and recognised as an important facilitator in the growth of the foundry industry. This initiative also aligns itself with the government's Advanced Manufacturing Technology Strategy (AMTS), which is an implementation strategy in support of the South African government's Integrated Manufacturing Strategy (IMS) and National R&D Strategy (NRDS). The AMTS aims at supporting and developing the downstream high technology manufacturing industry, inter alia through the aerospace, automotive and metals sectors. In light of the above and in an effort to retain and expand the current national skills, expertise and facilities in advanced casting technologies, the National Product Development Centre at the CSIR has initiated a process of establishing a National Casting Technology Centre (NCTC). The establishment of the NCTC provides a supportive technology platform for the Advanced Metals Initiative (AMI), which was launched in 2003. The primary objective of the NCTC is to preserve and expand the national expertise and capabilities in cast metals manufacturing by supporting the local casting industry with process development, technology transfer and skills enhancement in order to increase their global competitiveness
Irreversible magnetization in thin YBCO films rotated in external magnetic field
The magnetization M of a thin YBaCuO film is measured as a function of the
angle between the applied field H and the c-axis. For fields above
the first critical field, but below the Bean's field for first penetration H*,
M is symmetric with respect to and the magnetization curves for
forward and backward rotation coincide. For H>H* the curves are asymmetric and
they do not coincide. These phenomena have a simple explanation in the
framework of the Bean critical state model.Comment: 14 pages, 7 PostScript figure
Field Induced Nodal Order Parameter in the Tunneling Spectrum of YBaCuO Superconductor
We report planar tunneling measurements on thin films of
YBaCuO at various doping levels under magnetic fields. By
choosing a special setup configuration, we have probed a field induced energy
scale that dominates in the vicinity of a node of the d-wave superconducting
order parameter. We found a high doping sensitivity for this energy scale. At
Optimum doping this energy scale is in agreement with an induced
order parameter. We found that it can be followed down to low fields at optimum
doping, but not away from it.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.
Autonomous clustering using rough set theory
This paper proposes a clustering technique that minimises the need for subjective
human intervention and is based on elements of rough set theory. The proposed algorithm is
unified in its approach to clustering and makes use of both local and global data properties to
obtain clustering solutions. It handles single-type and mixed attribute data sets with ease and
results from three data sets of single and mixed attribute types are used to illustrate the
technique and establish its efficiency
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