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 h • , only 10• out of alignment with the Cosmic Microwave Background dipole. To account for velocity contributions arising from sources outside the 2M++ volume, we fit simultaneously for β * and an external bulk flow in our analysis. We find that an external bulk flow is preferred at the 5.1σ level, and the best fit has a velocity of 159 ± 23 km s −1 towards l = 304• . Finally, the predicted bulk flow of a 50 h −1 Mpc Gaussian-weighted volume centred on the Local Group is 230 ± 30 km s −1 , in the direction l = 293• , in agreement with predictions from ΛCDM.

We report on the first application of the Alcock-Paczynski test to stacked voids in spectroscopic galaxy redshift surveys. We use voids from the Sutter et al. (2012) void catalog, which was derived from the Sloan Digital Sky Survey Data Release 7 main sample and luminous red galaxy catalogs. The construction of that void catalog removes potential shape measurement bias by using a modified version of the ZOBOV algorithm and by removing voids near survey boundaries and masks. We apply the shape-fitting procedure presented in to ten void stacks out to redshift z = 0.36. Combining these measurements, we determine the mean cosmologically induced "stretch" of voids in three redshift bins, with 1σ errors of 5-15%. The mean stretch is consistent with unity, providing no indication of a distortion induced by peculiar velocities. While the statistical errors are too large to detect the Alcock-Paczynski effect over our limited redshift range, this proof-ofconcept analysis defines procedures that can be applied to larger spectroscopic galaxy surveys at higher redshifts to constrain dark energy using the expected statistical isotropy of structures that are minimally affected by uncertainties in galaxy velocity bias.

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-Paczyński 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 Alcock-Paczyński 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.

Peculiar velocities arise from gravitational instability, and thus are linked to the surrounding distribution of matter. In order to understand the motion of the Local Group with respect to the Cosmic Microwave Background, a deep all-sky map of the galaxy distribution is required. Here we present a new redshift compilation of 69~160 galaxies, dubbed 2M++, to map large-scale structures of the Local Universe over nearly the whole sky, and reaching depths of K <= 12.5, or 200 Mpc/h. The target catalogue is based on the Two-Micron-All-Sky Extended Source Catalog (2MASS-XSC). The primary sources of redshifts are the 2MASS Redshift Survey, the 6dF galaxy redshift survey and the Sloan Digital Sky Survey (DR7). We assess redshift completeness in each region and compute the weights required to correct for redshift incompleteness and apparent magnitude limits, and discuss corrections for incompleteness in the Zone of Avoidance. We present the density field for this survey, and discuss the importance of large-scale structures such as the Shapley Concentration.Comment: 19 pages, 15 figures, 6 tables, submitted to MNRA

We generate the peculiar velocity field for the 2MASS Redshift Survey (2MRS) catalog (Huchra et al. 2005) 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 3,000 km s −1 . We estimate the mean matter density to be Ω 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 3,000 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 towards the 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 40h −1 Mpc. Although most of the amplitude of the CMB dipole seems to be recovered by 120h −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 120h −1 Mpc, one might need to go well beyond 200h −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 towards the CMB dipole motion. For scales up to 60h −1 Mpc, assuming a Local Group velocity of 627 km s −1 , we estimate Ω m h 2 = 0.11 ± 0.06 and σ 8 = 0.9 ± 0.4, in agreement with WMAP5 measurements at the 1-σ level. However, for scales up to 100h −1 Mpc, we obtain Ω m h 2 = 0.08 ± 0.03 and σ 8 = 1.0 ± 0.4, which agrees at the 1 to 2-σ level with WMAP5 results.

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.

The universe is mostly composed of large and relatively empty domains known as cosmic voids, whereas its matter content is predominantly distributed along their boundaries. The remaining material inside them, either dark or luminous matter, is attracted to these boundaries and causes voids to expand faster and to grow emptier over time. Using the distribution of galaxies centered on voids identified in the Sloan Digital Sky Survey (SDSS) and adopting minimal assumptions on the statistical motion of these galaxies, we constrain the average matter content Ωm = 0.281 ± 0.031 in the universe today, as well as the linear growth rate of structure f /b = 0.417 ± 0.089 at median redshiftz = 0.57, where b is the galaxy bias (68% c.l.). These values originate from a percent-level measurement of the anisotropic distortion in the void-galaxy cross-correlation function, ε = 1.003 ± 0.012, and are robust to consistency tests with bootstraps of the data and simulated mock catalogs within an additional systematic uncertainty of half that size. They surpass (and are complementary to) existing constraints by unlocking cosmological information on smaller scales through an accurate model of nonlinear clustering and dynamics in void environments. As such, our analysis furnishes a powerful probe of deviations from Einstein's general relativity in the low density regime which has largely remained untested so far. We find no evidence for such deviations in the data at hand.PACS numbers: 98.80. Es, 98.65.Dx, 95.36.+x, 04.80.Cc Introduction.-After the epoch of recombination the initially tiny Gaussian density perturbations in the early universe have grown increasingly nonlinear under the influence of gravity, generating what is known as the cosmic web. Because the gravitational force is attractive, structures with densities above the mean always contract in comoving coordinates, while under-dense ones expand. The latter are referred to as cosmic voids and have progressively occupied most of the available space in the universe. Traditionally the formation of structure is viewed as hierarchical build-up of smaller dense clumps of matter into ever larger objects. We take the dual perspective where structure formation is seen as the emptying out of void regions onto the walls, filaments and clusters that surround them.

Conventional approaches to cosmology inference from galaxy redshift surveys are based on n-point functions, which are under rigorous perturbative control on sufficiently large scales. Here, we present an alternative approach, which employs a likelihood at the level of the galaxy density field. By integrating out small-scale modes based on effective-field theory arguments, we prove that this likelihood is under perturbative control if certain specific conditions are met. We further show that the information captured by this likelihood is equivalent to the combination of the next-to-leading order galaxy power spectrum, leadingorder bispectrum, and BAO reconstruction. Combined with MCMC sampling and MAP optimization techniques, our results allow for fully Bayesian cosmology inference from largescale structure that is under perturbative control. We illustrate this via a first demonstration of unbiased cosmology inference from nonlinear large-scale structure using this likelihood. In particular, we show unbiased estimates of the power spectrum normalization σ 8 from a catalog of simulated dark matter halos, where nonlinear information is crucial in breaking the b 1 − σ 8 degeneracy.