21,532 research outputs found
On the equivalence of Lambda(t) and gravitationally induced particle production cosmologies
The correspondence between cosmological models powered by a decaying vacuum
energy density and gravitationally induced particle production is investigated.
Although being physically different in the physics behind them we show that
both classes of cosmologies under certain conditions can exhibit the same
dynamic and thermodynamic behavior. Our method is applied to obtain three
specific models that may be described either as Lambda(t)CDM or gravitationally
induced particle creation cosmologies. In the point of view of particle
production models, the later class of cosmologies can be interpreted as a kind
of one-component unification of the dark sector. By using current type Ia
supernovae data, recent estimates of the cosmic microwave background shift
parameter and baryon acoustic oscillations measurements we also perform a
statistical analysis to test the observational viability within the two
equivalent classes of models and we obtain the best-fit of the free parameters.
By adopting the Akaike information criterion we also determine the rank of the
models considered here. Finally, the particle production cosmologies (and the
associated decaying Lambda(t)-models) are modeled in the framework of field
theory by a phenomenological scalar field model.Comment: 9 pages, 3 figures, new comments and 8 references added. Accepted for
publication in Physics Letters
Accessing the Acceleration of the Universe with Sunyaev-Zel'dovich and X-ray Data from Galaxy Clusters
By using exclusively the Sunyaev-Zel'dovich effect and X-ray surface
brightness data from 25 galaxy clusters in the redshift range 0.023< z < 0.784
we access cosmic acceleration employing a kinematic description. Such result is
fully independent on the validity of any metric gravity theory, the possible
matter-energy contents filling the Universe, as well as on the SNe Ia Hubble
diagram.Comment: 3 pages, 4 figures, To appear in the Proceedings of the Twelfth
Marcel Grossmann Meeting on General Relativit
Are Galaxy Clusters Suggesting an Accelerating Universe?
The present cosmic accelerating stage is discussed through a new kinematic
method based on the Sunyaev- Zel'dovich effect (SZE) and X-ray surface
brightness data from galaxy clusters. By using the SZE/X-ray data from 38
galaxy clusters in the redshift range [Bonamente et
al., Astrop. J. {\bf 647}, 25 (2006)] it is found that the present Universe is
accelerating and that the transition from an earlier decelerating to a late
time accelerating regime is relatively recent. The ability of the ongoing
Planck satellite mission to obtain tighter constraints on the expansion history
through SZE/X-ray angular diameters is also discussed. Our results are fully
independent on the validity of any metric gravity theory, the possible matter-
energy contents filling the Universe, as well as on the SNe Ia Hubble diagram
from which the presenting accelerating stage was inferred.Comment: 6 pages, 6 figures, AIP Conf. Proc. Invisible Universe: Proceedings
of the Conferenc
Modeling Financial Volatility: Extreme Observations, Nonlinearities and Nonstationarities
This paper presents a selective survey of volatility topics, with emphasis on the measurement of volatility and a discussion of some of the most important time series models commonly employed in its modelling. In particular, the paper details the long memory characteristics of volatility, and discusses its possible origins and impact on option pricing. To conclude, the paper discusses statistical tools that discriminate between nonlinearity and nonstationarity.long memory; nonstationarity; nonlinearity; option pricing, volatility
Formation of Dark Matter Haloes in a Homogeneous Dark Energy Universe
Several independent cosmological tests have shown evidences that the energy
density of the Universe is dominated by a dark energy component, which cause
the present accelerated expansion. The large scale structure formation can be
used to probe dark energy models, and the mass function of dark matter haloes
is one of the best statistical tools to perform this study. We present here a
statistical analysis of mass functions of galaxies under a homogeneous dark
energy model, proposed in the work of Percival (2005), using an observational
flux-limited X-ray cluster survey, and CMB data from WMAP. We compare, in our
analysis, the standard Press-Schechter (PS) approach (where a Gaussian
distribution is used to describe the primordial density fluctuation field of
the mass function), and the PL (Power Law) mass function (where we apply a
nonextensive q-statistical distribution to the primordial density field). We
conclude that the PS mass function cannot explain at the same time the X-ray
and the CMB data (even at 99% confidence level), and the PS best fit dark
energy equation of state parameter is , which is distant from the
cosmological constant case. The PL mass function provides better fits to the
HIFLUGCS X-ray galaxy data and the CMB data; we also note that the
parameter is very sensible to modifications in the PL free parameter, ,
suggesting that the PL mass function could be a powerful tool to constrain dark
energy models.Comment: 4 pages, 2 figures, Latex. Accepted for publication in the
International Journal of Modern Physics D (IJMPD)
Non-nequilibrium model on Apollonian networks
We investigate the Majority-Vote Model with two states () and a noise
on Apollonian networks. The main result found here is the presence of the
phase transition as a function of the noise parameter . We also studies de
effect of redirecting a fraction of the links of the network. By means of
Monte Carlo simulations, we obtained the exponent ratio ,
, and for several values of rewiring probability . The
critical noise was determined and also was calculated. The
effective dimensionality of the system was observed to be independent on ,
and the value is observed for these networks. Previous
results on the Ising model in Apollonian Networks have reported no presence of
a phase transition. Therefore, the results present here demonstrate that the
Majority-Vote Model belongs to a different universality class as the
equilibrium Ising Model on Apollonian Network.Comment: 5 pages, 5 figure
q-Deformed Kink Solutions
The q-deformed kink of the model is obtained via the
normalisable ground state eigenfunction of a fluctuation operator associated
with the q-deformed hyperbolic functions. From such a bosonic zero-mode the
q-deformed potential in 1+1 dimensions is found, and we show that the
q-deformed kink solution is a kink displaced away from the origin.Comment: REvtex, 11 pages, 2 figures. Preprint CBPF-NF-005/03, site at
http://www.cbpf.br. Revised version to appear in International Journal of
Modern Physics
Spin Disorder and Magnetic Anisotropy in Fe3O4 Nanoparticles
We have studied the magnetic behavior of dextran-coated magnetite
(FeO) nanoparticles with median particle size \left=8 .
Magnetization curves and in-field M\"ossbauer spectroscopy measurements showed
that the magnetic moment of the particles was much smaller than the bulk
material. However, we found no evidence of magnetic irreversibility or
non-saturating behavior at high fields, usually associated to spin canting. The
values of magnetic anisotropy from different techniques indicate that
surface or shape contributions are negligible. It is proposed that these
particles have bulk-like ferrimagnetic structure with ordered A and B
sublattices, but nearly compensated magnetic moments. The dependence of the
blocking temperature with frequency and applied fields, ,
suggests that the observed non-monotonic behavior is governed by the strength
of interparticle interactions.Comment: 11 pages, 7 figures, 3 Table
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