28,989 research outputs found
Influence of Small-Scale Inhomogeneities on the Cosmological Consistency Tests
The current cosmological dark sector (dark matter plus dark energy) is
challenging our comprehension about the physical processes taking place in the
Universe. Recently, some authors tried to falsify the basic underlying
assumptions of such dark matter-dark energy paradigm. In this Letter, we show
that oversimplifications of the measurement process may produce false positives
to any consistency test based on the globally homogeneous and isotropic LCDM
model and its expansion history based on distance measurements. In particular,
when local inhomogeneity effects due to clumped matter or voids are taken into
account, an apparent violation of the basic assumptions ("Copernican
Principle") seems to be present. Conversely, the amplitude of the deviations
also probes the degree of reliability underlying the phenomenological
Dyer-Roeder procedure by confronting its predictions with the accuracy of the
weak lensing approach. Finally, a new method is devised to reconstruct the
effects of the inhomogeneities in a LCDM model, and some suggestions of how to
distinguish between clumpiness (or void) effects from different cosmologies are
discussed.Comment: 18 pages, 2 figures. Improved version accepted for publication as a
Letter in MNRA
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.
Hamilton-Jacobi Approach for Power-Law Potentials
The classical and relativistic Hamilton-Jacobi approach is applied to the
one-dimensional homogeneous potential, , where and
are continuously varying parameters. In the non-relativistic case, the
exact analytical solution is determined in terms of , and the total
energy . It is also shown that the non-linear equation of motion can be
linearized by constructing a hypergeometric differential equation for the
inverse problem . A variable transformation reducing the general problem
to that one of a particle subjected to a linear force is also established. For
any value of , it leads to a simple harmonic oscillator if , an
"anti-oscillator" if , or a free particle if E=0. However, such a
reduction is not possible in the relativistic case. For a bounded relativistic
motion, the first order correction to the period is determined for any value of
. For , it is found that the correction is just twice that one
deduced for the simple harmonic oscillator (), and does not depend on the
specific value of .Comment: 12 pages, Late
Is CDM an effective CCDM cosmology?
We show that a cosmology driven by gravitationally induced particle
production of all non-relativistic species existing in the present Universe
mimics exactly the observed flat accelerating CDM cosmology with just
one dynamical free parameter. This kind of scenario includes the creation cold
dark matter (CCDM) model [Lima, Jesus & Oliveira, JCAP 011(2010)027] as a
particular case and also provides a natural reduction of the dark sector since
the vacuum component is not needed to accelerate the Universe. The new cosmic
scenario is equivalent to CDM both at the background and perturbative
levels and the associated creation process is also in agreement with the
universality of the gravitational interaction and equivalence principle.
Implicitly, it also suggests that the present day astronomical observations
cannot be considered the ultimate proof of cosmic vacuum effects in the evolved
Universe because CDM may be only an effective cosmology.Comment: 6 pages, 2 figures, changes in the abstract, introduction, new
references and typo correction
Studying light propagation in a locally homogeneous universe through an extended Dyer-Roeder approach
Light is affected by local inhomogeneities in its propagation, which may
alter distances and so cosmological parameter estimation. In the era of
precision cosmology, the presence of inhomogeneities may induce systematic
errors if not properly accounted. In this vein, a new interpretation of the
conventional Dyer-Roeder (DR) approach by allowing light received from distant
sources to travel in regions denser than average is proposed. It is argued that
the existence of a distribution of small and moderate cosmic voids (or "black
regions") implies that its matter content was redistributed to the homogeneous
and clustered matter components with the former becoming denser than the cosmic
average in the absence of voids. Phenomenologically, this means that the DR
smoothness parameter (denoted here by ) can be greater than unity,
and, therefore, all previous analyses constraining it should be rediscussed
with a free upper limit. Accordingly, by performing a statistical analysis
involving 557 type Ia supernovae (SNe Ia) from Union2 compilation data in a
flat CDM model we obtain for the extended parameter,
(). The effects of are also
analyzed for generic CDM models and flat XCDM cosmologies. For both
models, we find that a value of greater than unity is able to
harmonize SNe Ia and cosmic microwave background observations thereby
alleviating the well-known tension between low and high redshift data. Finally,
a simple toy model based on the existence of cosmic voids is proposed in order
to justify why can be greater than unity as required by supernovae
data.Comment: 5 pages, 2 figures. Title modified, results unchanged. It matches
version published as a Brief Report in Phys. Rev.
From de Sitter to de Sitter: decaying vacuum models as a possible solution to the main cosmological problems
Decaying vacuum cosmological models evolving smoothly between two extreme
(very early and late time) de Sitter phases are capable to solve or at least to
alleviate some cosmological puzzles, among them: (i) the singularity, (ii)
horizon, (iii) graceful-exit from inflation, and (iv) the baryogenesis problem.
Our basic aim here is to discuss how the coincidence problem based on a large
class of running vacuum cosmologies evolving from de Sitter to de Sitter can
also be mollified. It is also argued that even the cosmological constant
problem become less severe provided that the characteristic scales of the two
limiting de Sitter manifolds are predicted from first principles.Comment: 7 pages, 2 figures, title changed, typos corrected, text and new
references adde
New coupled quintessence cosmology
A component of dark energy has been recently proposed to explain the current
acceleration of the Universe. Unless some unknown symmetry in Nature prevents
or suppresses it, such a field may interact with the pressureless component of
dark matter, giving rise to the so-called models of coupled quintessence. In
this paper we propose a new cosmological scenario where radiation and baryons
are conserved, while the dark energy component is decaying into cold dark
matter (CDM). The dilution of CDM particles, attenuated with respect to the
usual scaling due to the interacting process, is characterized by a
positive parameter , whereas the dark energy satisfies the equation
of state (). We carry out a joint statistical
analysis involving recent observations from type Ia supernovae, baryon acoustic
oscillation peak, and Cosmic Microwave Background shift parameter to check the
observational viability of the coupled quintessence scenario here proposed.Comment: 7 pages, 7 figures. Minor corrections to match published versio
Experience with the Open Source based implementation for ATLAS Conditions Data Management System
Conditions Data in high energy physics experiments is frequently seen as
every data needed for reconstruction besides the event data itself. This
includes all sorts of slowly evolving data like detector alignment, calibration
and robustness, and data from detector control system. Also, every Conditions
Data Object is associated with a time interval of validity and a version.
Besides that, quite often is useful to tag collections of Conditions Data
Objects altogether. These issues have already been investigated and a data
model has been proposed and used for different implementations based in
commercial DBMSs, both at CERN and for the BaBar experiment. The special case
of the ATLAS complex trigger that requires online access to calibration and
alignment data poses new challenges that have to be met using a flexible and
customizable solution more in the line of Open Source components. Motivated by
the ATLAS challenges we have developed an alternative implementation, based in
an Open Source RDBMS. Several issues were investigated land will be described
in this paper:
-The best way to map the conditions data model into the relational database
concept considering what are foreseen as the most frequent queries.
-The clustering model best suited to address the scalability problem.
-Extensive tests were performed and will be described.
The very promising results from these tests are attracting the attention from
the HEP community and driving further developments.Comment: 8 pages, 4 figures, 3 tables, conferenc
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