672 research outputs found
Mean Field Dynamos with Algebraic and Dynamic alpha-Quenchings
Calculations for mean field dynamo models (in both full spheres and spherical
shells), with both algebraic and dynamic --quenchings, show qualitative
as well as quantitative differences and similarities in the dynamical behaviour
of these models. We summarise and enhance recent results with extra examples.
Overall, the effect of using a dynamic appears to be complicated and
is affected by the region of parameter space examined.Comment: 6 pages, 2 postscript figures, also available at
http://www.maths.qmw.ac.uk/~eo
A model-independent dark energy reconstruction scheme using the geometrical form of the luminosity-distance relation
We put forward a new model-independent reconstruction scheme for dark energy
which utilises the expected geometrical features of the luminosity-distance
relation. The important advantage of this scheme is that it does not assume
explicit ansatzes for cosmological parameters but only some very general
cosmological properties via the geometrical features of the reconstructed
luminosity-distance relation. Using the recently released supernovae data by
the Supernova Legacy Survey together with a phase space representation, we show
that the reconstructed luminosity-distance curves best fitting the data
correspond to a slightly varying dark energy density with the Universe
expanding slightly slower than the Lambda CDM model. However, the Lambda CDM
model fits the data at 1 sigma significance level and the fact that our best
fitting luminosity-distance curve is lower than that of the corresponding
Lambda CDM model could be due to systematics. The transition from an
accelerating to a decelerating expansion occurs at a redshift larger than
z=0.35. Interpreting the dark energy as a minimally coupled scalar field we
also reconstruct the scalar field and its potential. We constrain
using the baryon acoustic oscillation peak in the SDSS luminous
red galaxy sample and find that the best fit is obtained with
, in agreement with the CMB data.Comment: 10 pages, 18 figure
Effects of boundary conditions on the dynamics of the solar convection zone
Recent analyses of the helioseismic data have produced evidence for a variety of interesting dynamical behaviour associated with torsional oscillations. What is not so far clear is whether these oscillations extend all the way to the bottom of the convection zone and, if so, whether the oscillatory behaviour at the top and the bottom of the convection zone is different. Attempts have been made to understand such modes of behaviour within the framework of nonlinear dynamo models which include the nonlinear action of the Lorentz force of the dynamo generated magnetic field on the solar angular velocity. One aspect of these models that remains uncertain is the nature of the boundary conditions on the magnetic field. Here by employing a range of physically plausible boundary conditions, we show that for near-critical and moderately supercritical dynamo regimes, the oscillations extend all the way down to the bottom of the convection zone. Thus, such penetration is an extremely robust feature of the models considered. We also find parameter ranges for which the supercritical models show spatiotemporal fragmentation for a range of choices of boundary conditions. Given their observational importance, we also make a comparative study of the amplitude of torsional oscillations as a function of the boundary conditions
In--out intermittency in PDE and ODE models
We find concrete evidence for a recently discovered form of intermittency,
referred to as in--out intermittency, in both PDE and ODE models of mean field
dynamos. This type of intermittency (introduced in Ashwin et al 1999) occurs in
systems with invariant submanifolds and, as opposed to on--off intermittency
which can also occur in skew product systems, it requires an absence of skew
product structure. By this we mean that the dynamics on the attractor
intermittent to the invariant manifold cannot be expressed simply as the
dynamics on the invariant subspace forcing the transverse dynamics; the
transverse dynamics will alter that tangential to the invariant subspace when
one is far enough away from the invariant manifold.
Since general systems with invariant submanifolds are not likely to have skew
product structure, this type of behaviour may be of physical relevance in a
variety of dynamical settings.
The models employed here to demonstrate in--out intermittency are
axisymmetric mean--field dynamo models which are often used to study the
observed large scale magnetic variability in the Sun and solar-type stars. The
occurrence of this type of intermittency in such models may be of interest in
understanding some aspects of such variabilities.Comment: To be published in Chaos, June 2001, also available at
http://www.eurico.web.co
Second Order Perturbations of Flat Dust FLRW Universes with a Cosmological Constant
We summarize recent results concerning the evolution of second order
perturbations in flat dust irrotational FLRW models with . We
show that asymptotically these perturbations tend to constants in time, in
agreement with the cosmic no-hair conjecture. We solve numerically the second
order scalar perturbation equation, and very briefly discuss its all time
behaviour and some possible implications for the structure formation.Comment: 6 pages, 1 figure. to be published in "Proceedings of the 5th
Alexander Friedmann Seminar on Gravitation and Cosmology", Int. Journ. Mod.
Phys. A (2002). Macros: ws-ijmpa.cls, ws-p9-75x6-50.cl
A note on the large-angle anisotropies in the WMAP cut-sky maps
Recent analyses of the WMAP data seem to indicate the possible presence of
large-angle anisotropy in the Universe. If confirmed, these can have important
consequences for our understanding of the Universe. A number of attempts have
recently been made to establish the reality and nature of such anisotropies in
the CMB data. Among these is a directional indicator recently proposed by the
authors. A distinctive feature of this indicator is that it can be used to
generate a sky map of the large-scale anisotropies of the CMB maps. Applying
this indicator to full-sky temperature maps we found a statistically
significant preferred direction. The full-sky maps used in these analyses are
known to have residual foreground contamination as well as complicated noise
properties. Thus, here we performed the same analysis for a map where regions
with high foreground contamination were removed. We find that the main feature
of the full-sky analysis, namely the presence of a significant axis of
asymmetry, is robust with respect to this masking procedure. Other subtler
anomalies of the full-sky are on the other hand no longer present.Comment: 10 pages, 3 figeres. We performed a similar analysis of
arXiv:astro-ph/0511666 by considering the LILC map with a Kp2 sky cut, and
find that the presence of a significant axis of asymmetry is robust with
respect to this masking procedur
Evolution of the density contrast in inhomogeneous dust models
With the help of families of density contrast indicators, we study the
tendency of gravitational systems to become increasingly lumpy with time.
Depending upon their domain of definition, these indicators could be local or
global. We make a comparative study of these indicators in the context of
inhomogeneous cosmological models of Lemaitre--Tolman and Szekeres. In
particular, we look at the temporal asymptotic behaviour of these indicators
and ask under what conditions, and for which class of models, they evolve
monotonically in time. We find that for the case of ever-expanding models,
there is a larger class of indicators that grow monotonically with time,
whereas the corresponding class for the recollapsing models is more restricted.
Nevertheless, in the absence of decaying modes, indicators exist which grow
monotonically with time for both ever-expanding and recollapsing models
simultaneously. On the other hand, no such indicators may found which grow
monotonically if the decaying modes are allowed to exist. We also find the
conditions for these indicators to be non-divergent at the initial singularity
in both models. Our results can be of potential relevance for understanding
structure formation in inhomogeneous settings and in debates regarding
gravitational entropy and arrow of time. In particular, the spatial dependence
of turning points in inhomogeneous cosmologies may result in multiple density
contrast arrows in recollapsing models over certain epochs. We also find that
different notions of asymptotic homogenisation may be deduced, depending upon
the density contrast indicators used.Comment: 22 pages, 1 figure. To be published in Classical and Quantum Gravit
Optical response of supported gold nanodisks
It is shown that the ellipsometric spectra of short range ordered
planar arrays of gold nanodisks supported on glass substrates can be
described by modeling the nanostructured arrays as uniaxial homogeneous
layers with dielectric functions of the Lorentz type. However, appreciable
deviations from experimental data are observed in calculated spectra of
irradiance measurements. A qualitative and quantitative description of all
measured spectra is obtained with a uniaxial effective medium dielectric
function in which the nanodisks are modeled as oblate spheroids. Dynamic
depolarization factors in the long-wavelength approximation and interaction
with the substrate are considered. Similar results are obtained calculating the
optical spectra using the island-film theory. Nevertheless, a small in-plane
anisotropy and quadrupolar coupling effects reveal a very complex optical
response of the nanostructured arrays
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