Smith, Robert E.
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Publication Motion of the Acoustic Peak in the Correlation Function(2008-02-25) Smith, Robert E.; Sheth, Ravi K.; Scoccimarro, RománThe baryonic acoustic signature in the large-scale clustering pattern of galaxies has been detected in the two-point correlation function. Its precise spatial scale has been forwarded as a rigid-rod ruler test for the space-time geometry, and hence as a probe for tracking the evolution of dark energy. Percent-level shifts in the measured position can bias such a test and erode its power to constrain cosmology. This paper addresses some of the systematic effects that might induce shifts; namely, nonlinear corrections from matter evolution, redshift space distortions, and biasing. We tackle these questions through analytic methods and through a large battery of numerical simulations, with total volume of the order ∼ 100 [Gpc3h-3]. A toy-model calculation shows that if the nonlinear corrections simply smooth the acoustic peak, then this gives rise to an ‘‘apparent’’ shifting to smaller scales. However if tilts in the broadband power spectrum are induced then this gives rise to more pernicious ‘‘physical’’ shifts. Our numerical simulations show evidence of both: in real space and at z = 0, for the dark matter we find percent-level shifts; for haloes the shifts depend on halo mass, with larger shifts being found for the most biased samples, up to 3%. From our analysis we find that physical shifts are greater than ∼0.4% at z = 0 for a LCDM model with σ8 = 0.9. In redshift space these effects are exacerbated, but at higher redshifts are alleviated. We develop an analytical model to understand this, based on solutions to the pair conservation equation using characteristic curves. When combined with modeling of pairwise velocities the model reproduces the main trends found in the data. The model may also help to unbias the acoustic peak.Publication Analytic Model for the Bispectrum of Galaxies in Redshift Space(2008-07-18) Smith, Robert E.; Sheth, Ravi K.; Scoccimarro, RománWe develop an analytic theory for the redshift space bispectrum of dark matter, haloes, and galaxies. This is done within the context of the halo model of structure formation, as this allows for the self consistent inclusion of linear and nonlinear redshift-space distortions and also for the nonlinearity of the halo bias. The model is applicable over a wide range of scales: on the largest scales the predictions reduce to those of the standard perturbation theory (PT); on smaller scales they are determined primarily by the nonlinear virial velocities of galaxies within haloes, and this gives rise to the U-shaped anisotropy in the reduced bispectrum—a finger print of the Finger-Of-God distortions. We then confront the predictions with measurements of the redshift-space bispectrum of dark matter from an ensemble of numerical simulations. On very large scales, k = 0.05h Mpc-1, we find reasonably good agreement between our halo model, PT and the data, to within the errors. On smaller scales, k = 0.1h Mpc-1, the measured bispectra differ from the PT at the level of ∼10%–20%, especially for colinear triangle configurations. The halo model predictions improve over PT, but are accurate to no better than 10%. On smaller scales k = 0.5–1.0h Mpc-1, our model provides a significant improvement over PT, which breaks down. This implies that studies which use the lowest order PT to extract galaxy bias information are not robust on scales k ≳ 0.1h Mpc-1. The analytic and simulation results also indicate that there is no observable scale for which the configuration dependence of the reduced bispectrum is constant—hierarchical models for the higher-order correlation functions in redshift space are unlikely to be useful. It is hoped that our model will facilitate extraction of information from large-scale structure surveys of the Universe, because different galaxy populations are naturally included into our description.