23,180 research outputs found
CCDM Model with Spatial Curvature and The Breaking of "Dark Degeneracy"
Creation of Cold Dark Matter (CCDM), in the context of Einstein Field
Equations, leads to a negative creation pressure, which can be used to explain
the accelerated expansion of the Universe. Recently, it has been shown that the
dynamics of expansion of such models can not be distinguished from the
concordance CDM model, even at higher orders in the evolution of
density perturbations, leading at the so called "dark degeneracy". However,
depending on the form of the CDM creation rate, the inclusion of spatial
curvature leads to a different behavior of CCDM when compared to CDM,
even at background level. With a simple form for the creation rate, namely,
, we show that this model can be distinguished from
CDM, provided the Universe has some amount of spatial curvature.
Observationally, however, the current limits on spatial flatness from CMB
indicate that neither of the models are significantly favored against the other
by current data, at least in the background level.Comment: 13 pages, 5 figure
Beyond the unitarity bound in AdS/CFT_(A)dS
In this work we expand on the holographic description of CFTs on de Sitter
(dS) and anti-de Sitter (AdS) spacetimes and examine how violations of the
unitarity bound in the boundary theory are recovered in the bulk physics. To
this end we consider a Klein-Gordon field on AdS_(d+1) conformally compactified
such that the boundary is (A)dS_d, and choose masses and boundary conditions
such that the corresponding boundary operator violates the CFT unitarity bound.
The setup in which the boundary is AdS_d exhibits a particularly interesting
structure, since in this case the boundary itself has a boundary. The bulk
theory turns out to crucially depend on the choice of boundary conditions on
the boundary of the AdS_d slices. Our main result is that violations to the
unitarity bound in CFTs on dS_d and AdS_d are reflected in the bulk through the
presence of ghost excitations. In addition, analyzing the setup with AdS_d on
the boundary allows us to draw conclusions on multi-layered AdS/CFT-type
dualities.Comment: 30 pages, 2 figures; reference adde
Ising model on the Apollonian network with node dependent interactions
This work considers an Ising model on the Apollonian network, where the
exchange constant between two neighboring spins
is a function of the degree of both spins. Using the exact
geometrical construction rule for the network, the thermodynamical and magnetic
properties are evaluated by iterating a system of discrete maps that allows for
very precise results in the thermodynamic limit. The results can be compared to
the predictions of a general framework for spins models on scale-free networks,
where the node distribution , with node dependent
interacting constants. We observe that, by increasing , the critical
behavior of the model changes, from a phase transition at for a
uniform system , to a T=0 phase transition when : in the
thermodynamic limit, the system shows no exactly critical behavior at a finite
temperature. The magnetization and magnetic susceptibility are found to present
non-critical scaling properties.Comment: 6 figures, 12 figure file
Analytical approach to directed sandpile models on the Apollonian network
We investigate a set of directed sandpile models on the Apollonian network,
which are inspired on the work by Dhar and Ramaswamy (PRL \textbf{63}, 1659
(1989)) for Euclidian lattices. They are characterized by a single parameter
, that restricts the number of neighbors receiving grains from a toppling
node. Due to the geometry of the network, two and three point correlation
functions are amenable to exact treatment, leading to analytical results for
the avalanche distributions in the limit of an infinite system, for .
The exact recurrence expressions for the correlation functions are numerically
iterated to obtain results for finite size systems, when larger values of
are considered. Finally, a detailed description of the local flux properties is
provided by a multifractal scaling analysis.Comment: 7 pages in two-column format, 10 illustrations, 5 figure
Bayesian analysis of CCDM Models
Creation of Cold Dark Matter (CCDM), in the context of Einstein Field
Equations, leads to negative creation pressure, which can be used to explain
the accelerated expansion of the Universe. In this work we tested six different
spatially flat models for matter creation using statistical tools, at light of
SN Ia data: Akaike Information Criterion (AIC), Bayesian Information Criterion
(BIC) and Bayesian Evidence (BE). These approaches allow to compare models
considering goodness of fit and number of free parameters, penalizing excess of
complexity. We find that JO model is slightly favoured over LJO/CDM
model, however, neither of these, nor model can be
discarded from the current analysis. Three other scenarios are discarded either
from poor fitting, either from excess of free parameters.Comment: 16 pages, 6 figures, 6 tables. Corrected some text and language in
new versio
Effects of quantum deformation on the spin-1/2 Aharonov-Bohm problem
In this letter we study the Aharonov-Bohm problem for a spin-1/2 particle in
the quantum deformed framework generated by the -Poincar\'{e}-Hopf
algebra. We consider the nonrelativistic limit of the -deformed Dirac
equation and use the spin-dependent term to impose an upper bound on the
magnitude of the deformation parameter . By using the self-adjoint
extension approach, we examine the scattering and bound state scenarios. After
obtaining the scattering phase shift and the -matrix, the bound states
energies are obtained by analyzing the pole structure of the latter. Using a
recently developed general regularization prescription [Phys. Rev. D.
\textbf{85}, 041701(R) (2012)], the self-adjoint extension parameter is
determined in terms of the physics of the problem. For last, we analyze the
problem of helicity conservation.Comment: 12 pages, no figures, submitted for publicatio
Remarks on the Aharonov-Casher dynamics in a CPT-odd Lorentz-violating background
The Aharonov-Casher problem in the presence of a Lorentz-violating background
nonminimally coupled to a spinor and a gauge field is examined. Using an
approach based on the self-adjoint extension method, an expression for the
bound state energies is obtained in terms of the physics of the problem by
determining the self-adjoint extension parameter.Comment: Matches published versio
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