Guilhem Lavaux

Cosmic flow from 2MASS redshift survey: The origin of CMB dipole and implications for LCDM cosmology

We generate the peculiar velocity field for the Two Micron All-Sky Redshift Survey (2MRS) catalog using an orbit-reconstruction algorithm. The reconstructed velocities of individual objects in 2MRS are well correlated with the peculiar velocities obtained from high-precision observed distances within 3000?km?s?1. We estimate the mean matter density to be ?m = 0.31 ? 0.05 by comparing observed to reconstructed velocities in this volume. The reconstructed motion of the Local Group in the rest frame established by distances within 3000?km?s?1 agrees with the observed motion and is generated by fluctuations within this volume, in agreement with observations. Having tested our method against observed distances, we reconstruct the velocity field of 2MRS in successively larger radii, to study the problem of convergence toward the cosmic microwave background (CMB) dipole. We find that less than half of the amplitude of the CMB dipole is generated within a volume enclosing the Hydra-Centaurus-Norma supercluster at around 40 h ?1 Mpc. Although most of the amplitude of the CMB dipole seems to be recovered by 120 h ?1 Mpc, the direction does not agree and hence we observe no convergence up to this scale. Due to dominant superclusters such as Shapley or Horologium-Reticulum in the southern hemisphere at scales above 120 h ?1 Mpc, one might need to go well beyond 200 h ?1 Mpc to fully recover the dipole vector. We develop a statistical model which allows us to estimate cosmological parameters from the reconstructed growth of convergence of the velocity of the Local Group toward the CMB dipole motion. For scales up to 60 h ?1 Mpc, assuming a Local Group velocity of 627 km s?1, we estimate ?m h 2 = 0.11 ? 0.06 and ?8 = 0.9 ? 0.4, in agreement with WMAP5 measurements at the 1? level. However, for scales up to 100 h ?1?Mpc, we obtain ?m h 2 = 0.08 ? 0.03 and ?8 = 1.0 ? 0.4, which agrees at the 1? to 2? level with WMAP5 results.

PRECISION COSMOGRAPHY WITH STACKED VOIDS

We present a purely geometrical method for probing the expansion history of the universe from the observation of the shape of stacked voids in spectroscopic redshift surveys. Our method is an Alcock-Paczynski (AP) test based on the average sphericity of voids posited on the local isotropy of the universe. It works by comparing the temporal extent of cosmic voids along the line of sight with their angular, spatial extent. We describe the algorithm that we use to detect and stack voids in redshift shells on the light cone and test it on mock light cones produced from N-body simulations. We establish a robust statistical model for estimating the average stretching of voids in redshift space and quantify the contamination by peculiar velocities. Finally, assuming that the void statistics that we derive from N-body simulations is preserved when considering galaxy surveys, we assess the capability of this approach to constrain dark energy parameters. We report this assessment in terms of the figure of merit (FoM) of the dark energy task force and in particular of the proposed Euclid mission which is particularly suited for this technique since it is a spectroscopic survey. The FoM due to stacked voids from the Euclid wide survey may double that of all other dark energy probes derived from Euclid data alone (combined with Planck priors). In particular, voids seem to outperform baryon acoustic oscillations by an order of magnitude. This result is consistent with simple estimates based on mode counting. The AP test based on stacked voids may be a significant addition to the portfolio of major dark energy probes and its potentialities must be studied in detail.

Cosmology with void-galaxy correlations.

Galaxy bias, the unknown relationship between the clustering of galaxies and the underlying dark matter density field is a major hurdle for cosmological inference from large-scale structure. While traditional analyses focus on the absolute clustering amplitude of high-density regions mapped out by galaxy surveys, we propose a relative measurement that compares those to the underdense regions, cosmic voids. On the basis of realistic mock catalogs we demonstrate that cross correlating galaxies and voids opens up the possibility to calibrate galaxy bias and to define a static ruler thanks to the observable geometric nature of voids. We illustrate how the clustering of voids is related to mass compensation and show that volume-exclusion significantly reduces the degree of stochasticity in their spatial distribution. Extracting the spherically averaged distribution of galaxies inside voids from their cross correlations reveals a remarkable concordance with the mass-density profile of voids.

Sparse sampling, galaxy bias, and voids

To study the impact of sparsity and galaxy bias on void statistics, we use a single large-volume, high-resolution N-body simulation to compare voids in multiple levels of subsampled dark matter, halo populations, and mock galaxies from a Halo Occupation Distribution model tuned to dierent galaxy survey densities. We focus our comparison on three key observational statistics: number functions, ellipticity distributions, and radial density proles. We use the hierarchical tree structure of voids to

Cosmological parameters from the comparison of peculiar velocities with predictions from the 2M++ density field

Peculiar velocity measurements are the only tool available in the low-redshift Universe for mapping the large-scale distribution of matter and can thus be used to constrain cosmology. Using redshifts from the 2M++ redshift compilation, we reconstruct the density of galaxies within 200 Mpc/h, allowing for the first time good sampling of important superclusters such as the Shapley Concentration. We compare the predicted peculiar velocities from 2M++ to Tully-Fisher and SNe peculiar velocities. We find a value of

Voids in the SDSS DR9: observations, simulations, and the impact of the survey mask

\beta^* \equiv \Omega_{\rm{m}}^{0.55}/b^* = 0.431 \pm 0.021

The dark matter of galaxy voids

, suggesting

First measurement of the bulk flow of nearby galaxies using the cosmic microwave background

\Omega_{\rm{m}}^{0.55}\sigma_{\rm{8,lin}} = 0.401 \pm 0.024

Observational biases in Lagrangian reconstructions of cosmic velocity fields

, in good agreement with other probes. The predicted peculiar velocity of the Local Group arising from the 2M++ volume alone is

Precision constrained simulation of the local Universe

540 \pm 40