132 research outputs found

    Using the Zeldovich dynamics to test expansion schemes

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    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

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    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 10%10\% of the signal for the shear. For three-point statistics, the source-lens clustering bias is typically of order 10%10\% of the signal, as soon as the three galaxy source redshifts are not identical. The intrinsic-alignment bias is typically about 10%10\% 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 10%10\% accuracy.Comment: 27 page

    Large-scale bias of dark matter halos

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    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 0z2.50\leq z \leq 2.5, 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

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    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 NN-body simulations. This is the lowest-order version of the relations between (+n)(\ell+n)-point and nn-point polyspectra, where one averages over the angles of \ell soft modes. This relation depends on two wave numbers, kk' in the soft domain and kk in the hard domain. We show that it holds up to a good accuracy, when k/k1k'/k\ll 1 and kk' is in the linear regime, while the hard mode kk goes from linear (0.1hMpc10.1\,h\mathrm{Mpc}^{-1}) to nonlinear (1.0hMpc11.0\,h\mathrm{Mpc}^{-1}) scales. On scales k0.4hMpc1k\lesssim 0.4\,h\mathrm{Mpc}^{-1}, 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 30%30\%. 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 z=1z=1, whereas we find deviations as large as 7%\sim 7\% at k1.0hMpc1k \sim 1.0\,h\mathrm{Mpc}^{-1} at z=0.35z=0.35. We show that this can be explained partly by the breakdown of the approximation Ωm/f21\Omega_\mathrm{m}/f^2\simeq1 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

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    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 z2z\sim 2. 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 Λ\Lambda-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 Λ\Lambda-CDM on nonlinear scales where a halo model is utilized. We find that the discrepancy peaks around 1 hMpc11\ h{\rm Mpc}^{-1} with a relative difference which can reach fifty percent. Importantly, these features are still true at larger redshifts, contrary to models of the chameleon-f(R)f(R) and Galileon types.Comment: 24 page
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