32,174 research outputs found
Remarks on a Decrumpling Model of the Universe
It is argued that when the dimension of space is a constant integer the full
set of Einstein's field equations has more information than the spatial
components of Einstein's equation plus the energy conservation law. Applying
the former approach to the decrumpling FRW cosmology recently proposed, it is
shown that the spacetime singularity cannot be avoided and that turning points
are absent. This result is in contrast to the decrumpling nonsingular spacetime
model with turning points previously obtained using the latter approach.Comment: 8 pages, latex, no figure
Thermodynamics of Decaying Vacuum Cosmologies
The thermodynamic behavior of vacuum decaying cosmologies is investigated
within a manifestly covariant formulation. Such a process corresponds to a
continuous irreversible energy flow from the vacuum component to the created
matter constituents. It is shown that if the specific entropy per particle
remains constant during the process, the equilibrium relations are preserved.
In particular, if the vacuum decays into photons, the energy density and
average number density of photons scale with the temperature as and . The temperature law is determined and a generalized
Planckian type form of the spectrum, which is preserved in the course of the
evolution, is also proposed. Some consequences of these results for decaying
vacuum FRW type cosmologies as well as for models with ``adiabatic'' photon
creation are discussed.Comment: 21 pages, uses LATE
Magnetic braking in young late-type stars: the effect of polar spots
The concentration of magnetic flux near the poles of rapidly rotating cool
stars has been recently proposed as an alternative mechanism to dynamo
saturation in order to explain the saturation of angular momentum loss. In this
work we study the effect of magnetic surface flux distribution on the coronal
field topology and angular momentum loss rate. We investigate if magnetic flux
concentration towards the pole is a reasonable alternative to dynamo
saturation. We construct a 1D wind model and also apply a 2-D self-similar
analytical model, to evaluate how the surface field distribution affects the
angular momentum loss of the rotating star. From the 1D model we find that, in
a magnetically dominated low corona, the concentrated polar surface field
rapidly expands to regions of low magnetic pressure resulting in a coronal
field with small latitudinal variation. We also find that the angular momentum
loss rate due to a uniform field or a concentrated field with equal total
magnetic flux is very similar. From the 2D wind model we show that there are
several relevant factors to take into account when studying the angular
momentum loss from a star. In particular, we show that the inclusion of force
balance across the field in a wind model is fundamental if realistic
conclusions are to be drawn from the effect of non-uniform surface field
distribution on magnetic braking. This model predicts that a magnetic field
concentrated at high latitudes leads to larger Alfven radii and larger braking
rates than a smoother field distribution. From the results obtained, we argue
that the magnetic surface field distribution towards the pole does not directly
limit the braking efficiency of the wind.Comment: 11 pages, 10 figures, accepted in A&
New Cosmic Accelerating Scenario without Dark Energy
We propose an alternative, nonsingular, cosmic scenario based on
gravitationally induced particle production. The model is an attempt to evade
the coincidence and cosmological constant problems of the standard model
(CDM) and also to connect the early and late time accelerating stages
of the Universe. Our space-time emerges from a pure initial de Sitter stage
thereby providing a natural solution to the horizon problem. Subsequently, due
to an instability provoked by the production of massless particles, the
Universe evolves smoothly to the standard radiation dominated era thereby
ending the production of radiation as required by the conformal invariance.
Next, the radiation becomes sub-dominant with the Universe entering in the cold
dark matter dominated era. Finally, the negative pressure associated with the
creation of cold dark matter (CCDM model) particles accelerates the expansion
and drives the Universe to a final de Sitter stage. The late time cosmic
expansion history of the CCDM model is exactly like in the standard
CDM model, however, there is no dark energy. This complete scenario is
fully determined by two extreme energy densities, or equivalently, the
associated de Sitter Hubble scales connected by , a result that has no correlation with the cosmological constant
problem. We also study the linear growth of matter perturbations at the final
accelerating stage. It is found that the CCDM growth index can be written as a
function of the growth index, . In this
framework, we also compare the observed growth rate of clustering with that
predicted by the current CCDM model. Performing a statistical test
we show that the CCDM model provides growth rates that match sufficiently well
with the observed growth rate of structure.Comment: 12 pages, 3 figures, accepted for publication by Phys. Rev. D. (final
version, some references have corrected). arXiv admin note: substantial text
overlap with arXiv:1106.193
Asteroseismology and Magnetic Cycles
Small cyclic variations in the frequencies of acoustic modes are expected to
be a common phenomenon in solar-like pulsators, as a result of stellar magnetic
activity cycles. The frequency variations observed throughout the solar and
stellar cycles contain information about structural changes that take place
inside the stars as well as about variations in magnetic field structure and
intensity. The task of inferring and disentangling that information is,
however, not a trivial one. In the sun and solar-like pulsators, the direct
effect of the magnetic field on the oscillations might be significantly
important in regions of strong magnetic field (such as solar- / stellar-spots),
where the Lorentz force can be comparable to the gas-pressure gradient. Our aim
is to determine the sun- / stellar-spots effect on the oscillation frequencies
and attempt to understand if this effect contributes strongly to the frequency
changes observed along the magnetic cycle. The total contribution of the spots
to the frequency shifts results from a combination of direct and indirect
effects of the magnetic field on the oscillations. In this first work we
considered only the indirect effect associated with changes in the
stratification within the starspot. Based on the solution of the wave equation
and the variational principle we estimated the impact of these stratification
changes on the oscillation frequencies of global modes in the sun and found
that the induced frequency shifts are about two orders of magnitude smaller
than the frequency shifts observed over the solar cycle.Comment: 4 pages, 6 figures, ESF Conference: The Modern Era of Helio- and
Asteroseismology, to be published on 3 December 2012 at Astronomische
Nachrichten 333, No. 10, 1032-103
Effect of particle polydispersity on the irreversible adsorption of fine particles on patterned substrates
We performed extensive Monte Carlo simulations of the irreversible adsorption
of polydispersed disks inside the cells of a patterned substrate. The model
captures relevant features of the irreversible adsorption of spherical
colloidal particles on patterned substrates. The pattern consists of (equal)
square cells, where adsorption can take place, centered at the vertices of a
square lattice. Two independent, dimensionless parameters are required to
control the geometry of the pattern, namely, the cell size and cell-cell
distance, measured in terms of the average particle diameter. However, to
describe the phase diagram, two additional dimensionless parameters, the
minimum and maximum particle radii are also required. We find that the
transition between any two adjacent regions of the phase diagram solely depends
on the largest and smallest particle sizes, but not on the shape of the
distribution function of the radii. We consider size dispersions up-to 20% of
the average radius using a physically motivated truncated Gaussian-size
distribution, and focus on the regime where adsorbing particles do not interact
with those previously adsorbed on neighboring cells to characterize the jammed
state structure. The study generalizes previous exact relations on monodisperse
particles to account for size dispersion. Due to the presence of the pattern,
the coverage shows a non-monotonic dependence on the cell size. The pattern
also affects the radius of adsorbed particles, where one observes preferential
adsorption of smaller radii particularly at high polydispersity.Comment: 9 pages, 5 figure
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