132 research outputs found
Using the Zeldovich dynamics to test expansion schemes
We apply various expansion schemes that may be used to study gravitational
clustering to the simple case of the Zeldovich dynamics. Using the well-known
exact solution of the Zeldovich dynamics we can compare the predictions of
these various perturbative methods with the exact nonlinear result and study
their convergence properties. We find that most systematic expansions fail to
recover the decay of the response function in the highly nonlinear regime.
``Linear methods'' lead to increasingly fast growth in the nonlinear regime for
higher orders, except for Pade approximants that give a bounded response at any
order. ``Nonlinear methods'' manage to obtain some damping at one-loop order
but they fail at higher orders. Although it recovers the exact Gaussian
damping, a resummation in the high-k limit is not justified very well as the
generation of nonlinear power does not originate from a finite range of
wavenumbers (hence there is no simple separation of scales). No method is able
to recover the relaxation of the matter power spectrum on highly nonlinear
scales. It is possible to impose a Gaussian cutoff in a somewhat ad-hoc fashion
to reproduce the behavior of the exact two-point functions for two different
times. However, this cutoff is not directly related to the clustering of matter
and disappears in exact equal-time statistics such as the matter power
spectrum. On a quantitative level, the usual perturbation theory, and the
nonlinear scheme to which one adds an ansatz for the response function with
such a Gaussian cutoff, are the two most efficient methods. These results
should hold for the gravitational dynamics as well (this has been checked at
one-loop order), since the structure of the equations of motion is identical
for both dynamics.Comment: 29 pages, published in A&
Source-lens clustering and intrinsic-alignment bias of weak-lensing estimators
We estimate the amplitude of the source-lens clustering bias and of the
intrinsic-alignment bias of weak lensing estimators of the two-point and
three-point convergence and cosmic-shear correlation functions. We use a linear
galaxy bias model for the galaxy-density correlations, as well as a linear
intrinsic-alignment model. For the three-point and four-point density
correlations, we use analytical or semi-analytical models, based on a
hierarchical ansatz or a combination of one-loop perturbation theory with a
halo model. For two-point statistics, we find that the source-lens clustering
bias is typically several orders of magnitude below the weak lensing signal,
except when we correlate a very low-redshift galaxy (z_2 \la 0.05) with a
higher redshift galaxy (z_1 \ga 0.5), where it can reach of the signal
for the shear. For three-point statistics, the source-lens clustering bias is
typically of order of the signal, as soon as the three galaxy source
redshifts are not identical. The intrinsic-alignment bias is typically about
of the signal for both two-point and three-point statistics. Thus, both
source-lens clustering bias and intrinsic-alignment bias must be taken into
account for three-point estimators aiming at a better than accuracy.Comment: 27 page
Large-scale bias of dark matter halos
We build a simple analytical model for the bias of dark matter halos that
applies to objects defined by an arbitrary density threshold,
200\leq\deltas\leq 1600, and that provides accurate predictions from low-mass
to high-mass halos. We point out that it is possible to build simple and
efficient models, with no free parameter for the halo bias, by using integral
constraints that govern the behavior of low-mass and typical halos, whereas the
properties of rare massive halos are derived through explicit asymptotic
approaches. We also describe how to take into account the impact of halo
motions on their bias, using their linear displacement field. We obtain a good
agreement with numerical simulations for the halo mass functions and
large-scale bias at redshifts , for halos defined by a
nonlinear density threshold 200\leq\deltas\leq 1600. We also evaluate the
impact on the halo bias of two common approximations, i) neglecting halo
motions, and ii) linearizing the halo two-point correlation.Comment: 12 page
Testing the equal-time angular-averaged consistency relation of the gravitational dynamics in N-body simulations
We explicitly test the equal-time consistency relation between the
angular-averaged bispectrum and the power spectrum of the matter density field,
employing a large suite of cosmological -body simulations. This is the
lowest-order version of the relations between -point and -point
polyspectra, where one averages over the angles of soft modes. This
relation depends on two wave numbers, in the soft domain and in the
hard domain. We show that it holds up to a good accuracy, when and
is in the linear regime, while the hard mode goes from linear
() to nonlinear () scales. On
scales , we confirm the relation within the
statistical error of the simulations (typically a few percent depending on the
wave number), even though the bispectrum can already deviate from leading-order
perturbation theory by more than . We further examine the relation on
smaller scales with higher resolution simulations. We find that the relation
holds within the statistical error of the simulations at , whereas we find
deviations as large as at at
. We show that this can be explained partly by the breakdown of the
approximation with supplemental simulations done
in the Einstein-de Sitter background cosmology. We also estimate the impact of
this approximation on the power spectrum and bispectrum.Comment: 14 pages, 15 figures, added Sec. III E and Appendixes, matched to PRD
published versio
K-mouflage Cosmology: Formation of Large-Scale Structures
We study structure formation in K-mouflage cosmology whose main feature is
the absence of screening effect on quasilinear scales. We show that the growth
of structure at the linear level is affected by both a new time dependent
Newton constant and a friction term which depend on the background evolution.
These combine with the modified background evolution to change the growth rate
by up to ten percent since . At the one loop level, we find that the
nonlinearities of the K-mouflage models are mostly due to the matter dynamics
and that the scalar perturbations can be treated at tree level. We also study
the spherical collapse in K-mouflage models and show that the critical density
contrast deviates from its -CDM value and that, as a result, the halo
mass function is modified for large masses by an order one factor. Finally we
consider the deviation of the matter spectrum from -CDM on nonlinear
scales where a halo model is utilized. We find that the discrepancy peaks
around with a relative difference which can reach fifty
percent. Importantly, these features are still true at larger redshifts,
contrary to models of the chameleon- and Galileon types.Comment: 24 page
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