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Register a new userCosmic velocity--gravity relation in redshift space
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Added by
Micha{\\l} Chodorowski
Publication date
2008
fields
Physics
and research's language is
English
Authors
Stephane Colombi
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We propose a simple way to estimate the parameter beta = Omega_m^(0.6)/b from three-dimensional galaxy surveys. Our method consists in measuring the relation between the cosmological velocity and gravity fields, and thus requires peculiar velocity measurements. The relation is measured *directly in redshift space*, so there is no need to reconstruct the density field in real space. In linear theory, the radial components of the gravity and velocity fields in redshift space are expected to be tightly correlated, with a slope given, in the distant observer approximation, by g / v = (1 + 6 beta / 5 + 3 beta^2 / 7)^(1/2) / beta. We test extensively this relation using controlled numerical experiments based on a cosmological N-body simulation. To perform the measurements, we propose a new and rather simple adaptive interpolation scheme to estimate the velocity and the gravity field on a grid. One of the most striking results is that nonlinear effects, including `fingers of God, affect mainly the tails of the joint probability distribution function (PDF) of the velocity and gravity field: the 1--1.5 sigma region around the maximum of the PDF is *dominated by the linear theory regime*, both in real and redshift space. This is understood explicitly by using the spherical collapse model as a proxy of nonlinear dynamics. Applications of the method to real galaxy catalogs are discussed, including a preliminary investigation on homogeneous (volume limited) `galaxy samples extracted from the simulation with simple prescriptions based on halo and sub-structure identification, to quantify the effects of the bias between the galaxy and the total matter distibution, and of shot noise (ABRIDGED).
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I propose to compare the redshift-space density field directly to the REAL-SPACE velocity field. Such a comparison possesses all of the advantages of the conventional redshift-space analyses, while at the same time it is free of their disadvantages. In particular, the model-dependent reconstruction of the density field in real space is unnecessary, and so is the reconstruction of the velocity field in redshift space. The redshift-space velocity field can be reconstructed only at the linear order, because only at this order it is irrotational. Unlike the conventional redshift-space density--velocity comparisons, the comparison proposed here does not have to be restricted to the linear regime. Nonlinear effects can then be used to break the Omega-bias degeneracy plaguing the analyses based on the linear theory. I present a degeneracy-breaking method for the case of nonlinear but local bias.
We use large volume N-body simulations to predict the clustering of dark matter in redshift space in f(R) modified gravity cosmologies. This is the first time that the nonlinear matter and velocity fields have been resolved to such a high level of accuracy over a broad range of scales in this class of models. We find significant deviations from the clustering signal in standard gravity, with an enhanced boost in power on large scales and stronger damping on small scales in the f(R) models compared to GR at redshifts z<1. We measure the velocity divergence (P_theta theta) and matter (P_delta delta) power spectra and find a large deviation in the ratios sqrt{P_theta theta/P_delta delta} and P_delta theta/P_deltadelta, between the f(R) models and GR for 0.03<k/(h/Mpc)<0.5. In linear theory these ratios equal the growth rate of structure on large scales. Our results show that the simulated ratios agree with the growth rate for each cosmology (which is scale dependent in the case of modified gravity) only for extremely large scales, k<0.06h/Mpc at z=0. The velocity power spectrum is substantially different in the f(R) models compared to GR, suggesting that this observable is a sensitive probe of modified gravity. We demonstrate how to extract the matter and velocity power spectra from the 2D redshift space power spectrum, P(k,mu), and can recover the nonlinear matter power spectrum to within a few percent for k<0.1h/Mpc. However, the model fails to describe the shape of the 2D power spectrum demonstrating that an improved model is necessary in order to reconstruct the velocity power spectrum accurately. The same model can match the monopole moment to within 3% for GR and 10% for the f(R) cosmology at k<0.2 h/Mpc at z=1. Our results suggest that the extraction of the velocity power spectrum from future galaxy surveys is a promising method to constrain deviations from GR.
We use a range of cosmological data to constrain phenomenological modifications to general relativity on cosmological scales, through modifications to the Poisson and lensing equations. We include cosmic microwave background anisotropy measurements from the Planck satellite, cosmic shear from CFHTLenS and DES-SV, and redshift-space distortions from BOSS data release 12 and the 6dF galaxy survey. We find no evidence of departures from general relativity, with the modified gravity parameters constrained to $Sigma_0 = 0.05^{+0.05}_{-0.07}$ and $mu_0 = -0.10^{+0.20}_{-0.16}$, where $Sigma_0$ and $mu_0$ refer to deviations from general relativity today and are defined to be zero in general relativity. We also forecast the sensitivity to those parameters of the full five-year Dark Energy Survey and of an experiment like the Large Synoptic Survey Telescope, showing a substantial expected improvement in the constraint on $Sigma_0$.
We present measurements of the spatial clustering statistics in redshift space of various scalar field modified gravity simulations. We utilise the two-point and the three-point correlation functions to quantify the spatial distribution of dark matter halos within these simulations and thus discern between the models. We compare $Lambda$CDM simulations to various modified gravity scenarios and find consistency with previous work in terms of 2-point statistics in real and redshift-space. However using higher order statistics such as the three-point correlation function in redshift space we find significant deviations from $Lambda$CDM hinting that higher order statistics may prove to be a useful tool in the hunt for deviations from General Relativity.
We use the EAGLE galaxy formation simulation to study the effects of baryons on the power spectrum of the total matter and dark matter distributions and on the velocity fields of dark matter and galaxies. On scales $k{stackrel{>}{{}_sim}} 4{h,{rm Mpc}^{-1}}$ the effect of baryons on the amplitude of the total matter power spectrum is greater than $1%$. The back-reaction of baryons affects the density field of the dark matter at the level of $sim3%$ on scales of $1leq k/({h,{rm Mpc}^{-1}})leq 5$. The dark matter velocity divergence power spectrum at $k{stackrel{<}{{}_sim}}0.5{h,{rm Mpc}^{-1}}$ is changed by less than $1%$. The 2D redshift-space power spectrum is affected at the level of $sim6%$ at $|vec{k}|{stackrel{>}{{}_sim}} 1{h,{rm Mpc}^{-1}}$ (for $mu>0.5$), but for $|vec{k}|leq 0.4{h,{rm Mpc}^{-1}}$ it differs by less than $1%$. We report vanishingly small baryonic velocity bias for haloes: the peculiar velocities of haloes with $M_{200}>3times10^{11}{{rm M}_{odot}}$ (hosting galaxies with $M_{*}>10^9{{rm M}_{odot}}$) are affected at the level of at most $1~$km/s, which is negligible for $1%$-precision cosmology. We caution that since EAGLE overestimates cluster gas fractions it may also underestimate the impact of baryons, particularly for the total matter power spectrum. Nevertheless, our findings suggest that for theoretical modelling of redshift space distortions and galaxy velocity-based statistics, baryons and their back-reaction can be safely ignored at the current level of observational accuracy. However, we confirm that the modelling of the total matter power spectrum in weak lensing studies needs to include realistic galaxy formation physics in order to achieve the accuracy required in the precision cosmology era.
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