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Register a new userThe skewness of z=0.5 redshift-space galaxy distribution in Modified Gravity
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Wojciech A. Hellwing
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We study the reduced skewness, $S_{3,g}equivbar{xi}_{3,g}/bar{xi}_{2,g}^2$ of galaxy distribution at $z=0.5$ in two families of modfied gravity models: the Hu-Sawicki $f(R)$-gravity and normal-branch of Dvali-Gabadadze-Porrati (nDGP) models. We use a set of mock galaxy catalogues specifally designed to match CMASS spectroscopic galaxy sample. For the first time we investigate the third reduced moment of such galaxy distributions both in the redshift space. Our analysis confirms that the signal previously indicated only for dark matter halo catalogues persists also in realistic mock galaxy samples. This result offers a possibility to extract a potential modified gravity signal in $S_3$ from spectroscopic galaxy data without a need for a very precise and self-consistent RSD models constructed for each and every modified gravity scenario separately. We show that the relative deviations from $Lambda$CDM~ $S_{3,g}$ of various modified gravity models can vary from $7$ down to $sim 2-3%$ effects. Albeit, the effect looks small, we show that for considered models it can foster a $2-3sigma$ falsification. Finally we argue that galaxy sample of a significantly higher number density should provide even stronger constraints by limiting shot-noise effects affecting the $S_{3,g}$ estimates at small comoving separations.
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We carry out a joint analysis of redshift-space distortions and galaxy-galaxy lensing, with the aim of measuring the growth rate of structure; this is a key quantity for understanding the nature of gravity on cosmological scales and late-time cosmic acceleration. We make use of the final VIPERS redshift survey dataset, which maps a portion of the Universe at a redshift of $z simeq 0.8$, and the lensing data from the CFHTLenS survey over the same area of the sky. We build a consistent theoretical model that combines non-linear galaxy biasing and redshift-space distortion models, and confront it with observations. The two probes are combined in a Bayesian maximum likelihood analysis to determine the growth rate of structure at two redshifts $z=0.6$ and $z=0.86$. We obtain measurements of $fsigma_8(0.6) = 0.48 pm 0.12$ and $fsigma_8(0.86) = 0.48 pm 0.10$. The additional galaxy-galaxylensing constraint alleviates galaxy bias and $sigma_8$ degeneracies, providing direct measurements of $[f(0.6),sigma_8(0.6)] = [0.93 pm 0.22, 0.52 pm 0.06]$ and $f(0.86),sigma_8(0.86)] = [0.99 pm 0.19, 0.48 pm 0.04]$. These measurements are statistically consistent with a Universe where the gravitational interactions can be described by General Relativity, although they are not yet accurate enough to rule out some commonly considered alternatives. Finally, as a complementary test we measure the gravitational slip parameter, $E_G$ , for the first time at $z>0.6$. We find values of $smash{overline{E}_G}(0.6) = 0.16 pm 0.09$ and $smash{overline{E}_G}(0.86) = 0.09 pm 0.07$, when $E_G$ is averaged over scales above $3 h^{-1} rm{Mpc}$. We find that our $E_G$ measurements exhibit slightly lower values than expected for standard relativistic gravity in a {Lambda}CDM background, although the results are consistent within $1-2sigma$.
We study the evolution of the low-order moments of the galaxy overdensity distribution over the redshift interval 0.7<z<1.5. We find that the variance and the normalized skewness evolve over this redshift interval in a way that is remarkably consistent with predictions of first- and second-order perturbation theory. This finding confirms the standard gravitational instability paradigm over nearly 9 Gyrs of cosmic time and demonstrates the importance of accounting for the non-linear component of galaxy biasing to avoid disagreement between theory and observations.
Modified gravity and massive neutrino cosmologies are two of the most interesting scenarios that have been recently explored to account for possible observational deviations from the concordance $Lambda$-cold dark matter ($Lambda$CDM) model. In this context, we investigated the large-scale structure of the Universe by exploiting the dustp simulations that implement, simultaneously, the effects of $f(R)$ gravity and massive neutrinos. To study the possibility of breaking the degeneracy between these two effects, we analysed the redshift-space distortions in the clustering of dark matter haloes at different redshifts. Specifically, we focused on the monopole and quadrupole of the two-point correlation function, both in real and redshift space. The deviations with respect to $Lambda$CDM model have been quantified in terms of the linear growth rate parameter. We found that redshift-space distortions provide a powerful probe to discriminate between $Lambda$CDM and modified gravity models, especially at high redshifts ($z gtrsim 1$), even in the presence of massive neutrinos.
We extend the scale-dependent Gaussian Streaming Model (GSM) to produce analytical predictions for the anisotropic redshift-space correlation function for biased tracers in modified gravity models. Employing the Convolution Lagrangian Perturbation Theory (CLPT) re-summation scheme, with a local Lagrangian bias schema provided by the peak-background split formalism, we predict the necessary ingredients that enter the GSM, the real-space halo pairwise velocity and the pairwise velocity dispersion. We further consider effective field theory contributions to the pairwise velocity dispersion in order to model correctly its large scale behavior. We apply our method on two widely-considered modified gravity models, the chameleon-screened f(R) Hu-Sawicki model and the nDGP Vainshtein model and compare our predictions against state-of-the-art N-body simulations for these models. We demonstrate that the GSM approach to predict the monopole and the quadrupole of the redshift-space correlation function for halos, gives very good agreement with the simulation data, for a wide range of screening mechanisms, levels of screening and halo masses at z=0.5 and z=1. Our work shows the applicability of the GSM, for cosmologies beyond GR, demonstrating that it can serve as a powerful predictive tool for the next stage of cosmological surveys like DESI, Euclid, LSST and WFIRST.
Evidence is presented that the galaxy distribution can be described as a fractal system in the redshift range of the FDF galaxy survey. The fractal dimension $D$ was derived using the FDF galaxy volume number densities in the spatially homogeneous standard cosmological model with $Omega_{m_0}=0.3$, $Omega_{Lambda_0}=0.7$ and $H_0=70 ; mbox{km} ; {mbox{s}}^{-1} ; {mbox{Mpc}}^{-1}$. The ratio between the differential and integral number densities $gamma$ and $gamma^ast$ obtained from the red and blue FDF galaxies provides a direct method to estimate $D$, implying that $gamma$ and $gamma^ast$ vary as power-laws with the cosmological distances. The luminosity distance $d_{scriptscriptstyle L}$, galaxy area distance $d_{scriptscriptstyle G}$ and redshift distance $d_z$ were plotted against their respective number densities to calculate $D$ by linear fitting. It was found that the FDF galaxy distribution is characterized by two single fractal dimensions at successive distance ranges. Two straight lines were fitted to the data, whose slopes change at $z approx 1.3$ or $z approx 1.9$ depending on the chosen cosmological distance. The average fractal dimension calculated using $gamma^ast$ changes from $langle D rangle=1.4^{scriptscriptstyle +0.7}_{scriptscriptstyle -0.6}$ to $langle D rangle=0.5^{scriptscriptstyle +1.2}_{scriptscriptstyle -0.4}$ for all galaxies, and $D$ decreases as $z$ increases. Small values of $D$ at high $z$ mean that in the past galaxies were distributed much more sparsely and the large-scale galaxy structure was then possibly dominated by voids. Results of Iribarrem et al. (2014, arXiv:1401.6572) indicating similar fractal features with $langle D rangle =0.6 pm 0.1$ in the far-infrared sources of the Herschel/PACS evolutionary probe (PEP) at $1.5 lesssim z lesssim 3.2$ are also mentioned.
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