88 research outputs found
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 . This is defined as the mass ratio
, i.e. the ratio of mass inside a sphere of radius
to that contained within a radius , enclosing 200 and 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 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 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 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 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 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 Li 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 He and D in good agreement
with observations. However, it also leads to a significant discrepancy between
the predicted and observed primordial abundance of 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 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
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