109 research outputs found
Wikipedias: Collaborative web-based encyclopedias as complex networks
Wikipedia is a popular web-based encyclopedia edited freely and
collaboratively by its users. In this paper we present an analysis of
Wikipedias in several languages as complex networks. The hyperlinks pointing
from one Wikipedia article to another are treated as directed links while the
articles represent the nodes of the network. We show that many network
characteristics are common to different language versions of Wikipedia, such as
their degree distributions, growth, topology, reciprocity, clustering,
assortativity, path lengths and triad significance profiles. These
regularities, found in the ensemble of Wikipedias in different languages and of
different sizes, point to the existence of a unique growth process. We also
compare Wikipedias to other previously studied networks.Comment: v3: 9 pages, 12 figures, Change of title, few paragraphs and two
figures. Accepted for publication in Phys. Rev.
Asymptotic behavior of Cardassian cosmologies with exponential potentials
In this paper we analyze the asymptotic behavior of Cardassian cosmological
models filled with a perfect fluid and a scalar field with an exponential
potential. Cardassian cosmologies arise from modifications of the Friedmann
equation, and among the different proposals within that framework we will
choose those of the form with . We construct two
three dimensional dynamical systems arising from the evolution equations,
respectively adapted for studying the high and low energy limits. Using
standard dynamical systems techniques we find the fixed points and characterize
the solutions they represent. We pay especial attention to the properties
inherent to the modifications and compare with the (standard) unmodified
scenario. Among other interesting results, we find there are no late-time
tracking attractors.Comment: 9 pages, 8 figures, revtex
Equation of state description of the dark energy transition between quintessence and phantom regimes
The dark energy crossing of the cosmological constant boundary (the
transition between the quintessence and phantom regimes) is described in terms
of the implicitly defined dark energy equation of state. The generalizations of
the models explicitly constructed to exhibit the crossing provide the insight
into the cancellation mechanism which makes the transition possible.Comment: 3 pages, talk given at TAUP200
Can we be tricked into thinking that w is less than -1?
Dark energy candidates for which the equation-of-state parameter w is less
than -1 violate the dominant energy condition, and are typically unstable. In
scalar-tensor theories of gravity, however, the expansion of the universe can
mimic the behavior of general relativity with w<-1 dark energy, without
violating any energy conditions. We examine whether this possibility is
phenomenologically viable by studying Brans-Dicke models and characterizing
both the naturalness of the models themselves, and additional observational
constraints from limits on the time-dependence of Newton's constant. We find
that only highly contrived models would lead observers to measure w<-1.Comment: 20 pages, 4 figures, uses RevTe
Can a matter-dominated model with constant bulk viscosity drive the accelerated expansion of the universe?
We test a cosmological model which the only component is a pressureless fluid
with a constant bulk viscosity as an explanation for the present accelerated
expansion of the universe. We classify all the possible scenarios for the
universe predicted by the model according to their past, present and future
evolution and we test its viability performing a Bayesian statistical analysis
using the SCP ``Union'' data set (307 SNe Ia), imposing the second law of
thermodynamics on the dimensionless constant bulk viscous coefficient \zeta and
comparing the predicted age of the universe by the model with the constraints
coming from the oldest globular clusters.
The best estimated values found for \zeta and the Hubble constant Ho are:
\zeta=1.922 \pm 0.089 and Ho=69.62 \pm 0.59 km/s/Mpc with a \chi^2=314. The age
of the universe is found to be 14.95 \pm 0.42 Gyr. We see that the estimated
value of Ho as well as of \chi^2 are very similar to those obtained from LCDM
model using the same SNe Ia data set. The estimated age of the universe is in
agreement with the constraints coming from the oldest globular clusters.
Moreover, the estimated value of \zeta is positive in agreement with the second
law of thermodynamics (SLT).
On the other hand, we perform different forms of marginalization over the
parameter Ho in order to study the sensibility of the results to the way how Ho
is marginalized. We found that it is almost negligible the dependence between
the best estimated values of the free parameters of this model and the way how
Ho is marginalized in the present work. Therefore, this simple model might be a
viable candidate to explain the present acceleration in the expansion of the
universe.Comment: 31 pages, 12 figures and 2 tables. Accepted to be published in the
Journal of Cosmology and Astroparticle Physics. Analysis using the new SCP
"Union" SNe Ia dataset instead of the Gold 2006 and ESSENCE datasets and
without changes in the conclusions. Added references. Related works:
arXiv:0801.1686 and arXiv:0810.030
Renormalization group scale-setting from the action - a road to modified gravity theories
The renormalization group (RG) corrected gravitational action in
Einstein-Hilbert and other truncations is considered. The running scale of the
renormalization group is treated as a scalar field at the level of the action
and determined in a scale-setting procedure recently introduced by Koch and
Ramirez for the Einstein-Hilbert truncation. The scale-setting procedure is
elaborated for other truncations of the gravitational action and applied to
several phenomenologically interesting cases. It is shown how the logarithmic
dependence of the Newton's coupling on the RG scale leads to exponentially
suppressed effective cosmological constant and how the scale-setting in
particular RG corrected gravitational theories yields the effective
modified gravity theories with negative powers of the Ricci scalar . The
scale-setting at the level of the action at the non-gaussian fixed point in
Einstein-Hilbert and more general truncations is shown to lead to universal
effective action quadratic in Ricci tensor.Comment: v1: 15 pages; v2: shortened to 10 pages, main results unchanged,
published in Class. Quant. Gra
Dilatonic ghost condensate as dark energy
We explore a dark energy model with a ghost scalar field in the context of
the runaway dilaton scenario in low-energy effective string theory. We address
the problem of vacuum stability by implementing higher-order derivative terms
and show that a cosmologically viable model of ``phantomized'' dark energy can
be constructed without violating the stability of quantum fluctuations. We also
analytically derive the condition under which cosmological scaling solutions
exist starting from a general Lagrangian including the phantom type scalar
field. We apply this method to the case where the dilaton is coupled to
non-relativistic dark matter and find that the system tends to become quantum
mechanically unstable when a constant coupling is always present. Nevertheless,
it is possible to obtain a viable cosmological solution in which the energy
density of the dilaton eventually approaches the present value of dark energy
provided that the coupling rapidly grows during the transition to the scalar
field dominated era.Comment: 26 pages, 6 figure
K-essential Phantom Energy: Doomsday around the Corner? Revisited
We generalize some of those results reported by Gonz\'{a}lez-D\'{i}az by
further tuning the parameter () which is closely related to the
canonical kinetic term in -essence formalism. The scale factor could
be negative and decreasing within a specific range of (, : the equation-of-state parameter) during the initial
evolutional period.Comment: 1 Figure, 6 page
Unifying phantom inflation with late-time acceleration: scalar phantom-non-phantom transition model and generalized holographic dark energy
The unifying approach to early-time and late-time universe based on phantom
cosmology is proposed. We consider gravity-scalar system which contains usual
potential and scalar coupling function in front of kinetic term. As a result,
the possibility of phantom-non-phantom transition appears in such a way that
universe could have effectively phantom equation of state at early time as well
as at late time. In fact, the oscillating universe may have several phantom and
non-phantom phases. As a second model we suggest generalized holographic dark
energy where infrared cutoff is identified with combination of FRW parameters:
Hubble constant, particle and future horizons, cosmological constant and
universe life-time (if finite). Depending on the specific choice of the model
the number of interesting effects occur: the possibility to solve the
coincidence problem, crossing of phantom divide and unification of early-time
inflationary and late-time accelerating phantom universe. The bound for
holographic entropy which decreases in phantom era is also discussed.Comment: 13 pages, clarifications/refs added, to match with published versio
Crossing w=-1 in Gauss-Bonnet Brane World with Induced Gravity
Recent type Ia supernovas data seemingly favor a dark energy model whose
equation of state crosses -1 very recently, which is a much more amazing
problem than the acceleration of the universe. In this paper we show that it is
possible to realize such a crossing without introducing any phantom component
in a Gauss-Bonnet brane world with induced gravity, where a four dimensional
curvature scalar on the brane and a five dimensional Gauss-Bonnet term in the
bulk are present. In this realization, the Gauss-Bonnet term and the mass
parameter in the bulk play a crucial role.Comment: Revtex 16 pages including 10 eps files, references added, to appear
in Comm. Theor. Phy
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