1,117 research outputs found
Non-extensive statistics and the stellar polytrope index
We use physical constrains imposed from the H-Theorem and from the negative
nature of the heat capacity of self-gravitating thermodynamically isolated
systems to investigate some possible limits on the stellar polytrope index
within the domain of a classical non-extensive kinetic theory.Comment: 4 pages, 2 figures, LaTe
Constraining the dark energy with galaxy clusters X-ray data
The equation of state characterizing the dark energy component is constrained
by combining Chandra observations of the X-ray luminosity of galaxy clusters
with independent measurements of the baryonic matter density and the latest
measurements of the Hubble parameter as given by the HST key project. By
assuming a spatially flat scenario driven by a "quintessence" component with an
equation of state we place the following limits on the
cosmological parameters and : (i) and (1) if the
equation of state of the dark energy is restricted to the interval (\emph{usual} quintessence) and (ii) and
() if violates the null energy condition and assume values (\emph{extended} quintessence or ``phantom'' energy). These results are in
good agreement with independent studies based on supernovae observations,
large-scale structure and the anisotropies of the cosmic background radiation.Comment: 6 pages, 4 figures, LaTe
Negative heat capacity and non-extensive kinetic theory
The negative nature of the heat capacity of thermodynamically isolated
self-gravitating systems is rediscussed in the framework of a non-extensive
kinetic theory. It is found that the dependence of on the non-extensive
parameter gives rise to a negative branch with the critical value
corresponding to ().Comment: 7 pages, 1 figure, revised version to appear in Phys. Lett.
CDM Accelerating Cosmology as an Alternative to LCDM model
A new accelerating cosmology driven only by baryons plus cold dark matter
(CDM) is proposed in the framework of general relativity. In this model the
present accelerating stage of the Universe is powered by the negative pressure
describing the gravitationally-induced particle production of cold dark matter
particles. This kind of scenario has only one free parameter and the
differential equation governing the evolution of the scale factor is exactly
the same of the CDM model. For a spatially flat Universe, as predicted
by inflation (), it is found that the
effectively observed matter density parameter is ,
where is the constant parameter specifying the CDM particle creation
rate. The supernovae test based on the Union data (2008) requires so that as independently derived from weak
gravitational lensing, the large scale structure and other complementary
observations.Comment: 6 pages, 3 figure
Fragmentation Experiment and Model for Falling Mercury Drops
The experiment consists of counting and measuring the size of the many
fragments observed after the fall of a mercury drop on the floor. The size
distribution follows a power-law for large enough fragments. We address the
question of a possible crossover to a second, different power-law for small
enough fragments. Two series of experiments were performed. The first uses a
traditional film photographic camera, and the picture is later treated on a
computer in order to count the fragments and classify them according to their
sizes. The second uses a modern digital camera. The first approach has the
advantage of a better resolution for small fragment sizes. The second, although
with a poorer size resolution, is more reliable concerning the counting of all
fragments up to its resolution limit. Both together clearly indicate the real
existence of the quoted crossover.
The model treats the system microscopically during the tiny time interval
when the initial drop collides with the floor. The drop is modelled by a
connected cluster of Ising spins pointing up (mercury) surrounded by Ising
spins pointing down (air). The Ising coupling which tends to keep the spins
segregated represents the surface tension. Initially the cluster carries an
extra energy equally shared among all its spins, corresponding to the coherent
kinetic energy due to the fall. Each spin which touches the floor loses its
extra energy transformed into a thermal, incoherent energy represented by a
temperature used then to follow the dynamics through Monte Carlo simulations.
Whenever a small piece becomes disconnected from the big cluster, it is
considered a fragment, and counted. The results also indicate the existence of
the quoted crossover in the fragment-size distribution.Comment: 6 pages, 3 figure
Hole spin polarization in GaAlAs:Mn structures
A self-consistent calculation of the electronic properties of GaAlAs:Mn
magnetic semiconductor quantum well structures is performed including the
Hartree term and the sp-d exchange interaction with the Mn magnetic moments.
The spin polarization density is obtained for several structure configurations.
Available experimental results are compared with theory.Comment: 4 page
Nonextensivity in Geological Faults?
Geological fault systems, as the San Andreas fault (SAF) in USA, constitute
typical examples of self-organizing systems in nature. In this paper, we have
considered some geophysical properties of the SAF system to test the viability
of the nonextensive models for earthquakes developed in [Phys. Rev. E {\bf 73},
026102, 2006]. To this end, we have used 6188 earthquakes events ranging in the
magnitude interval that were taken from the Network Earthquake
International Center catalogs (NEIC, 2004-2006) and the Bulletin of the
International Seismological Centre (ISC, 1964-2003). For values of the Tsallis
nonextensive parameter , it is shown that the energy
distribution function deduced in above reference provides an excellent fit to
the NEIC and ISC SAF data.Comment: 9 pages, 1 figure, standard LaTeX fil
Thermodynamics, Spectral Distribution and the Nature of Dark Energy
Recent astronomical observations suggest that the bulk of energy in the
Universe is repulsive and appears like a dark component with negative pressure
(). In this work we investigate thermodynamic and
statistical properties of such a component. It is found that its energy and
temperature grow during the evolution of the Universe since work is done on the
system. Under the hypothesis of a null chemical potential, the case of phantom
energy () seems to be physically meaningless because its entropy
is negative. It is also proved that the wavelengths of the -quanta
decrease in an expanding Universe. This unexpected behavior explains how their
energy may be continuously stored in the course of expansion. The spectrum and
the associated Wien-type law favors a fermionic nature with naturally
restricted to the interval . Our analysis also
implies that the ultimate fate of the Universe may be considerably modified. If
a dark energy dominated Universe expands forever, it will become increasingly
hot.Comment: 5 pages, 2 figures, to appear in Physics Letters
Critical Exponents for Nuclear Multifragmentation: dynamical lattice model
We present a dynamical and dissipative lattice model, designed to mimic
nuclear multifragmentation. Monte-Carlo simulations with this model show clear
signature of critical behaviour and reproduce experimentally observed
correlations. In particular, using techniques devised for finite systems, we
could obtain two of its critical exponents, whose values are in agreement with
those of the universality class to which nuclear multifragmentation is supposed
to belong.Comment: 10 pages, 3 figures, to be published in Nuclear Physics
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