26,327 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
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
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
Cosmic voids in modified gravity scenarios
Modified gravity (MG) theories aim to reproduce the observed acceleration of
the Universe by reducing the dark sector while simultaneously recovering
General Relativity (GR) within dense environments. Void studies appear to be a
suitable scenario to search for imprints of alternative gravity models on
cosmological scales. Voids cover an interesting range of density scales where
screening mechanisms fade out, which reaches from a density contrast close to their centers to close to their
boundaries. We present an analysis of the level of distinction between GR and
two modified gravity theories, the Hu-Sawicki and the symmetron theory.
This study relies on the abundance, linear bias, and density profile of voids
detected in n-body cosmological simulations. We define voids as connected
regions made up of the union of spheres with a {\it \textup{mean}} density
given by , but disconnected from any
other voids. We find that the height of void walls is considerably affected by
the gravitational theory, such that it increases for stronger gravity
modifications. Finally, we show that at the level of dark matter n-body
simulations, our constraints allow us to distinguish between GR and MG models
with and . Differences of best-fit values for
MG parameters that are derived independently from multiple void probes may
indicate an incorrect MG model. This serves as an important consistency check.Comment: 15 pages, 12 figure
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
Disorder effects at low temperatures in La_{0.7-x}Y_{x}Ca_{0.3}MnO_{3} manganites
With the aim of probing the effect of magnetic disorder in the
low-temperature excitations of manganites, specific-heat measurements were
performed in zero field, and in magnetic fields up to 9 T in polycrystalline
samples of La_{0.7-x}Y_{x}Ca_{0.3}MnO_{3}, with Y concentrations x=0, 0.10, and
0.15. Yttrium doping yielded the appearance of a cluster-glass state, giving
rise to unusual low-temperature behavior of the specific-heat. The main feature
observed in the results is a strong enhancement of the specific-heat linear
term, which is interpreted as a direct consequence of magnetic disorder. The
analysis was further corroborated by resistivity measurements in the same
compounds.Comment: 9 pages, 2 figure
- …