185 research outputs found
The growth index of matter perturbations using the clustering of dark energy
We have put forward a new unified framework which provides a consistent and
rather complete account of the growth index of matter perturbations in the
regime where the dark energy is allowed to have clustering. In particular, we
find that the growth index is not only affected by the cosmological parameters
but rather it depends on the choice of the considered dark energy (homogeneous
or clustered). Using the {\em Planck} priors and performing a standard
-minimization between theoretical expectations and growth data, we
statistically quantify the ability of the growth index to represent the
observations. Finally, based on the growth index analysis we find that the
growth data favour the clustered dark energy scenario.Comment: 6 pages, 2 figures Accepted for publication in MNRAS (minor
corrections: a new title, some new references and the Planck2015 priors are
included
Cosmic expansion and structure formation in running vacuum cosmologies
We investigate the dynamics of the FLRW flat cosmological models in which the
vacuum energy varies with redshift. A particularly well motivated model of this
type is the so-called quantum field vacuum, in which both kind of terms
and constant appear in the effective dark energy density affecting the
evolution of the main cosmological functions at the background and perturbation
levels. Specifically, it turns out that the functional form of the quantum
vacuum endows the vacuum energy of a mild dynamical evolution which could be
observed nowadays and appears as dynamical dark energy. Interestingly, the
low-energy behaviour is very close to the usual CDM model, but it is
by no means identical. Finally, within the framework of the quantum field
vacuum we generalize the large scale structure properties, namely growth of
matter perturbations, cluster number counts and spherical collapse model.Comment: 16 pages, 2 figures Invited paper for the Special Issue: "Fundamental
Constants in Physics and Their Time Variation" (Modern Physics Letters A,
Guest Editor Joan Sol\`a
The Physical Properties of the Cosmic Acceleration
The observed late-time acceleration of the cosmic expansion constitutes a
fundamental problem in modern theoretical physics and cosmology. In an attempt
to weight the validity of a large number of dark energy models, I use the
recent measurements of the expansion rate of the Universe, the clustering of
galaxies the CMB fluctuations as well as the large scale structure formation,
to put tight constraints on the different models.Comment: 6 pages, 1 figure, invited talk in "The 9th Hellenic Astronomical
Conference 20-24 September 2009, Athens", The proceedings will be published
by (PASP
Cosmological implications and structure formation from a time varying vacuum
We study the dynamics of the FLRW flat cosmological models in which the
vacuum energy varies with time, . In this model we find that the
main cosmological functions such as the scale factor of the universe and the
Hubble flow are defined in terms of exponential functions. Applying a joint
likelihood analysis of the recent supernovae type Ia data, the Cosmic Microwave
Background shift parameter and the Baryonic Acoustic Oscillations traced by the
Sloan Digital Sky Survey (SDSS) galaxies, we place tight constraints on the
main cosmological parameters of the scenario. Also, we compare the
model with the traditional cosmology and we find that
the former model provides a Hubble expansion which compares well with that of
the cosmology. However, the scenario predicts stronger
small scale dynamics, which implies a faster growth rate of perturbations with
respect to the usual -cosmology, despite the fact that they share the
same equation of state parameter. In this framework, we find that galaxy
clusters in the model appear to form earlier than in the
model.Comment: Accepted for publication, MNRAS, 10 pages, 5 figure
Linear growth in power law gravity
We provide for the first time the growth index of linear matter fluctuations
of the power law gravity model. We find that the
asymptotic form of this particular model is which obviously extends that of the CDM model,
. Finally, we generalize the growth index
analysis of gravity in the case where is allowed to vary with
redshift.Comment: 8 pages, 3 figures, accepted for publication by Phys. Rev. D (typos
corrected: this is the published version
Testing dynamical vacuum models with CMB power spectrum from Planck
The cosmic expansion is computed for various dynamical vacuum models
and confronted to the Cosmic Microwave Background (CMB) power
spectrum from Planck. We also combined CMB in a joint analysis with other
probes in order to place constraints on the cosmological parameters of the
dynamical vacuum models. We find that all models are very
efficient and in very good agreement with the data. Considering that the
interaction term of the dark sector is given in terms of matter and radiation
densities, we find that the corresponding model shows a small but
non-zero deviation from cosmology, nevertheless the confidence level
is close to .Comment: 7 pages, 6 figures, accepted by MNRA
The Substructure-Alignment Connection
Using a sample of 903 APM clusters we investigate whether their dynamical
status, as evidenced by the presence of significant substructures, is related
to the large-scale structure of the Universe. We find that the cluster
dynamical activity is strongly correlated with the tendency of clusters to be
aligned with their nearest neighbour and in general with the nearby clusters
that belong to the same supercluster. Furthermore, dynamically active clusters
are more clustered than the overall cluster population. These are strong
indications that clusters develop in a hierarchical fashion by anisotropic
merging along the large-scale filaments within which they are embedded.Comment: 5 pages, accepted for publication in MNRAS pink page
Constraining the dark energy models with H(z) data: an approach independent of
We study the performance of the latest data in constraining the
cosmological parameters of different cosmological models, including that of
Chevalier-Polarski-Linder parametrization. First, we introduce a
statistical procedure in which the chi-square estimator is not affected by the
value of the Hubble constant. As a result, we find that the data do not
rule out the possibility of either non-flat models or dynamical dark energy
cosmological models. However, we verify that the time varying equation of state
parameter is not constrained by the current expansion data. Combining
the and the Type Ia supernova data we find that the /SNIa overall
statistical analysis provides a substantial improvement of the cosmological
constraints with respect to those of the analysis. Moreover, the
parameter space provided by the /SNIa joint analysis is in
a very good agreement with that of Planck 2015, which confirms that the present
analysis with the and SNIa probes correctly reveals the expansion of the
Universe as found by the team of Planck. Finally, we generate sets of Monte
Carlo realizations in order to quantify the ability of the data to
provide strong constraints on the dark energy model parameters. The Monte Carlo
approach shows significant improvement of the constraints, when increasing the
sample to 100 measurements. Such a goal can be achieved in the future,
especially in the light of the next generation of surveys.Comment: 11 pages, 9 figures, accepted for publication by Phys. Rev.
Conjoined constraints on modified gravity from the expansion history and cosmic growth
In this paper we present conjoined constraints on several cosmological models
from the expansion history and cosmic growth . The models
we study include the CPL parametrization, the Holographic Dark Energy
(HDE) model, the Time varying vacuum (CDM) model, the Dvali,
Gabadadze and Porrati (DGP) and Finsler-Randers (FRDE) models, a power law
model and finally the Hu-Sawicki model. In all cases we perform a
simultaneous fit to the SnIa, CMB, BAO, and growth data, while also
following the conjoined visualization of and as in Linder
(2017). Furthermore, we introduce the Figure of Merit (FoM) in the
parameter space as a way to constrain models that jointly
fit both probes well. We use both the latest and data,
but also LSST-like mocks with measurements and we find that the conjoined
method of constraining the expansion history and cosmic growth simultaneously
is able not only to place stringent constraints on these parameters but also to
provide an easy visual way to discriminate cosmological models. Finally, we
confirm the existence of a tension between the growth rate and Planck CMB data
and we find that the FoM in the conjoined parameter space of
can be used to discriminate between the CDM model
and certain classes of modified gravity models, namely the DGP and .Comment: 11 pages, 3 figures, 5 tables, comments welcome. Changes match
published version in PR
Effective equation of state for running vacuum: "mirage" quintessence and phantom dark energy
Past analyses of the equation of state (EoS) of the Dark Energy (DE) were not
incompatible with a phantom phase near our time. This has been the case in the
years of WMAP observations, in combination with the remaining cosmological
observables. Such situation did not completely disappear from the data
collected from the Planck satellite mission. In it the EoS analysis may still
be interpreted as suggesting w<-1, and so a mildly evolving DE cannot be
discarded. In our opinion the usual ansatzs made on the structure of the EoS
for dynamical DE models (e.g. quintessence and the like) are too simplified. In
this work we examine in detail some of these issues and suggest that a general
class of models with a dynamical vacuum energy density could explain the
persistent phantom anomaly, despite there is no trace of real phantom behavior
in them. The spurious or "mirage" effect is caused by an attempt to describe
them as if the DE would be caused by fundamental phantom scalar fields.
Remarkably, the effective DE behavior can also appear as quintessence in
transit to phantom, or vice versa.Comment: Version accepted in Mon. Not. Roy. Astron. Soc. (in press), extended
discussion and references adde
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