22,042 research outputs found
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
Tax evasion dynamics and Zaklan model on Opinion-dependent Network
Within the context of agent-based Monte-Carlo simulations, we study the
well-known majority-vote model (MVM) with noise applied to tax evasion on
Stauffer-Hohnisch-Pittnauer (SHP) networks. To control the fluctuations for tax
evasion in the economics model proposed by Zaklan, MVM is applied in the
neighborhood of the critical noise to evolve the Zaklan model. The
Zaklan model had been studied recently using the equilibrium Ising model. Here
we show that the Zaklan model is robust because this can be studied besides
using equilibrium dynamics of Ising model also through the nonequilibrium MVM
and on various topologies giving the same behavior regardless of dynamic or
topology used here.Comment: 14 page, 4 figure
Chemical Potential and the Nature of the Dark Energy: The case of phantom
The influence of a possible non zero chemical potential on the nature
of dark energy is investigated by assuming that the dark energy is a
relativistic perfect simple fluid obeying the equation of state (EoS),
(). The entropy condition, ,
implies that the possible values of are heavily dependent on the
magnitude, as well as on the sign of the chemical potential. For , the
-parameter must be greater than -1 (vacuum is forbidden) while for not only the vacuum but even a phantomlike behavior () is
allowed. In any case, the ratio between the chemical potential and temperature
remains constant, that is, . Assuming that the dark energy
constituents have either a bosonic or fermionic nature, the general form of the
spectrum is also proposed. For bosons is always negative and the extended
Wien's law allows only a dark component with which includes
vacuum and the phantomlike cases. The same happens in the fermionic branch for
are permmited only if . The thermodynamics and statistical arguments constrain the
EoS parameter to be , a result surprisingly close to the maximal
value required to accelerate a FRW type universe dominated by matter and dark
energy ().Comment: 7 pages, 5 figure
Accelerating Cold Dark Matter Cosmology ()
A new kind of accelerating flat model with no dark energy that is fully
dominated by cold dark matter (CDM) is investigated. The number of CDM
particles is not conserved and the present accelerating stage is a consequence
of the negative pressure describing the irreversible process of gravitational
particle creation. A related work involving accelerating CDM cosmology has been
discussed before the SNe observations [Lima, Abramo & Germano, Phys. Rev. D53,
4287 (1996)]. However, in order to have a transition from a decelerating to an
accelerating regime at low redshifts, the matter creation rate proposed here
includes a constant term of the order of the Hubble parameter. In this case,
does not need to be small in order to solve the age problem and the
transition happens even if the matter creation is negligible during the
radiation and part of the matter dominated phase. Therefore, instead of the
vacuum dominance at redshifts of the order of a few, the present accelerating
stage in this sort of Einstein-de Sitter CDM cosmology is a consequence of the
gravitational particle creation process. As an extra bonus, in the present
scenario does not exist the coincidence problem that plagues models with
dominance of dark energy. The model is able to harmonize a CDM picture with the
present age of the universe, the latest measurements of the Hubble parameter
and the Supernovae observations.Comment: 9 pages, 6 figures, typos corrected, references added, discussion in
Appendix B extende
Constraints on Cold Dark Matter Accelerating Cosmologies and Cluster Formation
We discuss the properties of homogeneous and isotropic flat cosmologies in
which the present accelerating stage is powered only by the gravitationally
induced creation of cold dark matter (CCDM) particles (). For
some matter creation rates proposed in the literature, we show that the main
cosmological functions such as the scale factor of the universe, the Hubble
expansion rate, the growth factor and the cluster formation rate are
analytically defined. The best CCDM scenario has only one free parameter and
our joint analysis involving BAO + CMB + SNe Ia data yields
() where
is the observed matter density parameter. In particular, this implies that the
model has no dark energy but the part of the matter that is effectively
clustering is in good agreement with the latest determinations from large scale
structure. The growth of perturbation and the formation of galaxy clusters in
such scenarios are also investigated. Despite the fact that both scenarios may
share the same Hubble expansion, we find that matter creation cosmologies
predict stronger small scale dynamics which implies a faster growth rate of
perturbations with respect to the usual CDM cosmology. Such results
point to the possibility of a crucial observational test confronting CCDM with
CDM scenarios trough a more detailed analysis involving CMB, weak
lensing, as well as the large scale structure.Comment: 12 pages, 3 figures, Accepted for publication by Physical Rev.
Particle creation due to tachyonic instability in relativistic stars
Dense enough compact objects were recently shown to lead to an exponentially
fast increase of the vacuum energy density for some free scalar fields properly
coupled to the spacetime curvature as a consequence of a tachyonic-like
instability. Once the effect is triggered, the star energy density would be
overwhelmed by the vacuum energy density in a few milliseconds. This demands
that eventually geometry and field evolve to a new configuration to bring the
vacuum back to a stationary regime. Here, we show that the vacuum fluctuations
built up during the unstable epoch lead to particle creation in the final
stationary state when the tachyonic instability ceases. The amount of created
particles depends mostly on the duration of the unstable epoch and final
stationary configuration, which are open issues at this point. We emphasize
that the particle creation coming from the tachyonic instability will occur
even in the adiabatic limit, where the spacetime geometry changes arbitrarily
slowly, and therefore is quite distinct from the usual particle creation due to
the change in the background geometry.Comment: 12 pages, 2 figures, discussion improved: paragraph added at the end
of Sec. V B (published version
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