Spectra of dynamical Dark Energy cosmologies from constant-w models

WMAP5 and related data have greatly restricted the range of acceptable cosmologies, by providing precise likelihood ellypses on the the w_0-w_a plane. We discuss first how such ellypses can be numerically rebuilt, and present then a map of constant-w models whose spectra, at various redshift, are expected to coincide with acceptable models within ~1%

The Observed Growth of Massive Galaxy Clusters III: Testing General Relativity on Cosmological Scales

This is the third of a series of papers in which we derive simultaneous constraints on cosmological parameters and X-ray scaling relations using observations of the growth of massive, X-ray flux-selected galaxy clusters. Our data set consists of 238 clusters drawn from the ROSAT All-Sky Survey, and incorporates extensive follow-up observations using the Chandra X-ray Observatory. Here we present improved constraints on departures from General Relativity (GR) on cosmological scales, using the growth index, gamma, to parameterize the linear growth rate of cosmic structure. Using the method of Mantz et al. (2009a), we simultaneously and self-consistently model the growth of X-ray luminous clusters and their observable-mass scaling relations, accounting for survey biases, parameter degeneracies and systematic uncertainties. We combine the cluster growth data with gas mass fraction, SNIa, BAO and CMB data. This combination leads to a tight correlation between gamma and sigma_8. Consistency with GR requires gamma~0.55. Under the assumption of self-similar evolution and constant scatter in the scaling relations, and for a flat LCDM model, we measure gamma(sigma_8/0.8)^6.8=0.55+0.13-0.10, with 0.79<sigma_8<0.89. Relaxing the assumptions on the scaling relations by introducing two additional parameters to model possible evolution in the normalization and scatter of the luminosity-mass relation, we obtain consistent constraints on gamma that are only ~20% weaker than those above. Allowing the dark energy equation of state, w, to take any constant value, we simultaneously constrain the growth and expansion histories, and find no evidence for departures from either GR or LCDM. Our results represent the most robust consistency test of GR on cosmological scales to date. (Abridged)Comment: Accepted for publication in MNRAS. 11 pages, 5 figures, 1 table. New figure added: Fig. 4 shows the tight constraints on gamma from the cluster growth data alone compared with those from the other data sets combined

Clarifying the effects of interacting dark energy on linear and nonlinear structure formation processes

We present a detailed numerical study of the impact that cosmological models featuring a direct interaction between the Dark Energy component that drives the accelerated expansion of the Universe and Cold Dark Matter can have on the linear and nonlinear stages of structure formation. By means of a series of collisionless N-body simulations we study the influence that each of the different effects characterizing these cosmological models - which include among others a fifth force, a time variation of particle masses, and a velocity-dependent acceleration - separately have on the growth of density perturbations and on a series of observable quantities related to linear and nonlinear cosmic structures, as the matter power spectrum, the gravitational bias between baryons and Cold Dark Matter, the halo mass function and the halo density profiles. We perform our analysis applying and comparing different numerical approaches previously adopted in the literature, and we address the partial discrepancies recently claimed in a similar study by Li & Barrow (2010b) with respect to the first outcomes of Baldi et al. (2010), which are found to be related to the specific numerical approach adopted in the former work. Our results fully confirm the conclusions of Baldi et al. (2010) and show that when linear and nonlinear effects of the interaction between Dark Energy and Cold Dark Matter are properly disentangled, the velocity-dependent acceleration is the leading effect acting at nonlinear scales, and in particular is the most important mechanism in lowering the concentration of Cold Dark Matter halos.Comment: 14 pages, 1 Table, 6 Figures. MNRAS accepte

Hydrodynamical N-body simulations of coupled dark energy cosmologies

If the accelerated expansion of the Universe at the present epoch is driven by a dark energy scalar field, there may well be a non-trivial coupling between the dark energy and the cold dark matter (CDM) fluid. Such interactions give rise to new features in cosmological structure growth, like an additional long-range attractive force between CDM particles, or variations of the dark matter particle mass with time. We have implemented these effects in the N-body code GADGET-2 and present results of a series of high-resolution N-body simulations where the dark energy component is directly interacting with the cold dark matter. As a consequence of the new physics, CDM and baryon distributions evolve differently both in the linear and in the nonlinear regime of structure formation. Already on large scales a linear bias develops between these two components, which is further enhanced by the nonlinear evolution. We also find, in contrast with previous work, that the density profiles of CDM halos are less concentrated in coupled dark energy cosmologies compared with LCDM, and that this feature does not depend on the initial conditions setup, but is a specific consequence of the extra physics induced by the coupling. Also, the baryon fraction in halos in the coupled models is significantly reduced below the universal baryon fraction. These features alleviate tensions between observations and the LCDM model on small scales. Our methodology is ideally suited to explore the predictions of coupled dark energy models in the fully non-linear regime, which can provide powerful constraints for the viable parameter space of such scenarios.Comment: 21 pages, 18 figures, 4 tables, title changed, several references added. Revised version accepted for publication in MNRAS. Main conclusions unchange

Imprints of Dark Energy on Cosmic Structure Formation I) Realistic Quintessence Models and the Non-Linear Matter Power Spectrum

Dark energy as a quintessence component causes a typical modification of the background cosmic expansion, which in addition to its clustering properties, can leave a potentially distinctive signature on large scale structures. Many previous studies have investigated this topic, particularly in relation to the non-linear regime of structure formation. However, no careful pre-selection of viable quintessence models with high precision cosmological data was performed. Here we show that this has led to a misinterpretation (and underestimation) of the imprint of quintessence on the distribution of large scale structures. To this purpose we perform a likelihood analysis of the combined Supernova Ia UNION dataset and WMAP5-years data to identify realistic quintessence models. Differences from the vanilla LambdaCDM are especially manifest in the predicted amplitude and shape of the linear matter power spectrum, though these remain within the uncertainties of the SDSS data. We use these models as benchmark for studying the clustering properties of dark matter halos by performing a series of high resolution N-body simulations. We find that realistic quintessence models allow for relevant differences of the dark matter distribution with the respect to the LambdaCDM scenario well into the non-linear regime, with deviations up to 40% in the non-linear power spectrum. Such differences are shown to depend on the nature of DE, as well as the scale and epoch considered. At small scales (k~1-5 h Mpc^{-1}, depending on the redshift) the structure formation process is about 20% more efficient than in LambdaCDM. We show that these imprints are a specific record of the cosmic structure formation history in DE cosmologies and therefore cannot be accounted in standard fitting functions of the non-linear matter power spectrum.Comment: 24 pages, 11 figures. Higher resolution paper available at http://cp3.phys.ucl.ac.be/upload/papers/astro-ph-0903.5490.ps (ps) and http://cp3.phys.ucl.ac.be/upload/papers/astro-ph-0903.5490.pdf (pdf). v2: New discussion on the non-linear power spectrum at small scales. v3: same as v2 with corrected references. Matches version to appear in MNRA

The study of Type Ia supernovae spectral diversity using principal component analysis

In order to use supernovae (SNe) as cosmological probes, a good understanding of their properties and diversity is necessary. Here we investigate whether principal component analysis (PCA) can be used to improve the calibration of Type Ia SNe. We apply PCA to two different cases: a small data set of supernova spectra taken at maximum light and a larger data set with more spectra taken at various epochs. On the SN Ia luminosity scale, the supernova SN 1991T appears at the upper end and SN 1991bg at the lower end. While 91bg-like SNe seem to form a distinct group, 91T-like SNe show a continuum of properties with normal SNe. The differences are mainly explained by line shifts in the spectra. However, we do not find that PCA is able to distinguish trends or subsets in the supernova data beyond what has already been found using specific spectral features

Near-optimal distributed detection in balanced binary relay trees

We study the distributed detection problem in a balanced binary relay tree, where the leaves of the tree are sensors generating binary messages. The root of the tree is a fusion center that makes an overall decision. Every other node in the tree is a relay node that fuses binary messages from its two child nodes into a new binary message and sends it to the parent node at the next level. We assume that the relay nodes at the same level use identical fusion rule. The goal is to find a string of fusion rules used at all the levels in the tree that maximizes the reduction in the total error probability between the leaf nodes and the fusion center. We formulate this problem as a deterministic dynamic program and express the optimal strategy in terms of Bellman&#039;s equation. Moreover, we use the notion of string-submodularity to show that the reduction in the total error probability is a string-submodular function. Consequentially, we show that the greedy strategy, which only maximizes the level-wise reduction in the total error probability, performs at least within a factor (1 - 1/e) of the optimal strategy in terms of reduction in the total error probability, even if the nodes and links in the trees are subject to random failures