Imprints of Dark Energy on Cosmic Structure Formation: III. Sparsity of Dark Matter Halo Profiles

We study the imprint of Dark Energy on the density profile of Dark Matter halos using a set of high-resolution large volume cosmological N-body simulations from the Dark Energy Universe Simulation Series (DEUSS). We first focus on the analysis of the goodness-of-fit of the Navarro-Frenk-White (NFW) profile which we find to vary with halo mass and redshift. We also find that the fraction of halos ill-fitted by NFW varies with cosmology, thus indicating that the mass assembly of halos with perturbed density profiles carries a characteristic signature of Dark Energy. To access this information independently of any parametric profile, we introduce a new observable quantity: the halo sparsity $s_\Delta$. This is defined as the mass ratio $M_{200}/M_\Delta$, i.e. the ratio of mass inside a sphere of radius $r_{200}$ to that contained within a radius $r_\Delta$, enclosing 200 and $\Delta$ times the mean matter density respectively. We find the average sparsity to be nearly independent of the total halo mass, while its value can be inferred to better than a few percent from the ratio of the integrated halo mass functions at overdensities $\Delta$ and 200 respectively. This provides a consistency relation that can validate observational measurements of the halo sparsity. Most importantly, the sparsity significantly varies with the underlying Dark Energy model, thus providing an alternative cosmological probe.Comment: 12 pages, 16 figures. accepted by MNRA

Probing dark energy models with extreme pairwise velocities of galaxy clusters from the DEUS-FUR simulations

Observations of colliding galaxy clusters with high relative velocity probe the tail of the halo pairwise velocity distribution with the potential of providing a powerful test of cosmology. As an example it has been argued that the discovery of the Bullet Cluster challenges standard $\Lambda$CDM model predictions. Halo catalogs from N-body simulations have been used to estimate the probability of Bullet-like clusters. However, due to simulation volume effects previous studies had to rely on a Gaussian extrapolation of the pairwise velocity distribution to high velocities. Here, we perform a detail analysis using the halo catalogs from the Dark Energy Universe Simulation Full Universe Runs (DEUS-FUR), which enables us to resolve the high-velocity tail of the distribution and study its dependence on the halo mass definition, redshift and cosmology. Building upon these results we estimate the probability of Bullet-like systems in the framework of Extreme Value Statistics. We show that the tail of extreme pairwise velocities significantly deviates from that of a Gaussian, moreover it carries an imprint of the underlying cosmology. We find the Bullet Cluster probability to be two orders of magnitude larger than previous estimates, thus easing the tension with the $\Lambda$CDM model. Finally, the comparison of the inferred probabilities for the different DEUS-FUR cosmologies suggests that observations of extreme interacting clusters can provide constraints on dark energy models complementary to standard cosmological tests.Comment: Submitted to MNRAS, 15 pages, 12 figures, 3 table

Adaptative Smooth Particle Hydrodynamics and Particle-Particle coupled codes: Energy and Entropy Conservation

We present and test a general-purpose code, called PPASPH, for evolving self-gravitating fluids in astrophysics, both with and without a collisionless component. In PPASPH, hydrodynamical properties are computed by using the SPH (Smoothed Particle Hydrodynamics) method while, unlike most previous implementations of SPH, gravitational forces are computed by a PP (Particle-Particle) approach. another important feature of this code is that hydrodynamics equations optionally include the correction terms appearing when h(t,\br) is not constant. Our code has been implemented by using the data parallel programming model on CM5(CM). PPASPH has been applied to study the importance of adaptative smoothing correction terms on the entropy conservation. We confirm Hernquist's (1993) interpretation of the entropy violation observed in previous SPH simulations as a result of having neglected these terms. An improvement on the entropy conservation is not found by merely considering larger numbers of particles or different $N_S$ choices. The correct continuum description is only obtained if the \bn h correction terms are included. Otherwise, the entropy conservation is always rather poor as compared to that found for the total energy.Comment: uuencoded gzip postscript containing 16 pages (incLuding figures) accepted for publication in Astrophysical Journa

Upper limit to ΩB\Omega_B in scalar-tensor gravity theories

In a previous paper (Serna & Alimi 1996), we have pointed out the existence of some particular scalar-tensor gravity theories able to relax the nucleosynthesis constraint on the cosmic baryonic density. In this paper, we present an exhaustive study of primordial nucleosynthesis in the framework of such theories taking into account the currently adopted observational constraints. We show that a wide class of them allows for a baryonic density very close to that needed for the universe closure. This class of theories converges soon enough towards General Relativity and, hence, is compatible with all solar-system and binary pulsar gravitational tests. In other words, we show that primordial nucleosynthesis does not always impose a very stringent bound on the baryon contribution to the density parameter.Comment: uuencoded tar-file containing 16 pages, latex with 5 figures, accepted for publication in Astrophysical Journal (Part 1

Big Bang nucleosynthesis in scalar tensor gravity: the key problem of the 7^7Li abundance

Combined with other CMB experiments, the WMAP survey provides an accurate estimate of the baryon density of the Universe. In the framework of the standard Big Bang Nucleosynthesis (BBN), such a baryon density leads to predictions for the primordial abundances of $^{4}$He and D in good agreement with observations. However, it also leads to a significant discrepancy between the predicted and observed primordial abundance of $^{7}$Li. Such a discrepancy is often termed as 'the lithium problem'. In this paper, we analyze this problem in the framework of scalar-tensor theories of gravity. It is shown that an expansion of the Universe slightly slower than in General Relativity before BBN, but faster during BBN, solves the lithium problem and leads to $^4$He and D primordial abundances consistent with the observational constraints. This kind of behavior is obtained in numerous scalar-tensor models, both with and without a self-interaction potential for the scalar field. In models with a self-interacting scalar field, the convergence towards General Relativity is ensured without any condition, thanks to an attraction mechanism which starts to work during the radiation-dominated epoch.Comment: Revised version. CMB and matter power spectrum constraints added. Accepted for publication in Ap