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 $\chi^2$-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 $H^{2}$ 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 $\Lambda$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, $\Lambda(t)$. 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 $\Lambda(t)$ scenario. Also, we compare the $\Lambda(t)$ model with the traditional $\Lambda$ cosmology and we find that the former model provides a Hubble expansion which compares well with that of the $\Lambda$ cosmology. However, the $\Lambda(t)$ scenario predicts stronger small scale dynamics, which implies a faster growth rate of perturbations with respect to the usual $\Lambda$-cosmology, despite the fact that they share the same equation of state parameter. In this framework, we find that galaxy clusters in the $\Lambda(t)$ model appear to form earlier than in the $\Lambda$ model.Comment: Accepted for publication, MNRAS, 10 pages, 5 figure

Linear growth in power law f(T)f(T) gravity

We provide for the first time the growth index of linear matter fluctuations of the power law $f(T) \propto (-T)^{b}$ gravity model. We find that the asymptotic form of this particular $f(T)$ model is $\gamma \approx \frac{6}{11-6b}$ which obviously extends that of the $\Lambda$CDM model, $\gamma_{\Lambda}\approx 6/11$. Finally, we generalize the growth index analysis of $f(T)$ gravity in the case where $\gamma$ 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 $\Lambda(H)$ 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 $\Lambda(H)$ 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 $\Lambda(H)$ model shows a small but non-zero deviation from $\Lambda$ cosmology, nevertheless the confidence level is close to $\sim 2.5\sigma$.Comment: 7 pages, 6 figures, accepted by MNRA

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 $H(z)$ and cosmic growth $f\sigma_8(z)$. The models we study include the CPL $w_0w_a$ parametrization, the Holographic Dark Energy (HDE) model, the Time varying vacuum ($\Lambda_t$CDM) model, the Dvali, Gabadadze and Porrati (DGP) and Finsler-Randers (FRDE) models, a power law $f(T)$ model and finally the Hu-Sawicki $f(R)$ model. In all cases we perform a simultaneous fit to the SnIa, CMB, BAO, $H(z)$ and growth data, while also following the conjoined visualization of $H(z)$ and $f\sigma_8(z)$ as in Linder (2017). Furthermore, we introduce the Figure of Merit (FoM) in the $H(z)-f\sigma_8(z)$ parameter space as a way to constrain models that jointly fit both probes well. We use both the latest $H(z)$ and $f\sigma_8(z)$ data, but also LSST-like mocks with $1\%$ 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 $H(z)-f\sigma_8(z)$ can be used to discriminate between the $\Lambda$CDM model and certain classes of modified gravity models, namely the DGP and $f(T)$.Comment: 11 pages, 3 figures, 5 tables, comments welcome. Changes match published version in PR

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 H0H_{0}

We study the performance of the latest $H(z)$ data in constraining the cosmological parameters of different cosmological models, including that of Chevalier-Polarski-Linder $w_{0}w_{1}$ 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 $H(z)$ 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 $w(z)$ is not constrained by the current expansion data. Combining the $H(z)$ and the Type Ia supernova data we find that the $H(z)$/SNIa overall statistical analysis provides a substantial improvement of the cosmological constraints with respect to those of the $H(z)$ analysis. Moreover, the $w_{0}-w_{1}$ parameter space provided by the $H(z)$/SNIa joint analysis is in a very good agreement with that of Planck 2015, which confirms that the present analysis with the $H(z)$ 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 $H(z)$ 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 $H(z)$ 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.

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