Article

# Statistical Determination of Bulk Flow Motions

Journal of Cosmology and Astroparticle Physics (Impact Factor: 5.81). 01/2010; 1(01). DOI: 10.1088/1475-7516/2010/01/025

Source: arXiv

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**ABSTRACT:**We develop an optimized technique to extract density--density and velocity--velocity spectra out of observed spectra in redshift space. The measured spectra of the distribution of halos from redshift distorted mock map are binned into 2--dimensional coordinates in Fourier space so as to be decomposed into both spectra using angular projection dependence. With the threshold limit introduced to minimize nonlinear suppression, the decomposed velocity--velocity spectra are reasonably well measured up to scale k=0.07 h/Mpc, and the measured variances using our method are consistent with errors predicted from a Fisher matrix analysis. The detectability is extendable to k\sim 0.1 h/Mpc with more conservative bounds at the cost of weakened constraint. - [Show abstract] [Hide abstract]

**ABSTRACT:**The mounting evidence for anomalously large peculiar velocities in our Universe presents a challenge for the LCDM paradigm. The recent estimates of the large scale bulk flow by Watkins et al. are inconsistent at the nearly 3 sigma level with LCDM predictions. Meanwhile, Lee and Komatsu have recently estimated that the occurrence of high-velocity merging systems such as the Bullet Cluster (1E0657-57) is unlikely at a 6.5-5.8 sigma level, with an estimated probability between 3.3x10^{-11} and 3.6x10^{-9} in LCDM cosmology. We show that these anomalies are alleviated in a broad class of infrared-modifed gravity theories, called brane-induced gravity, in which gravity becomes higher-dimensional at ultra large distances. These theories include additional scalar forces that enhance gravitational attraction and therefore speed up structure formation at late times and on sufficiently large scales. The peculiar velocities are enhanced by 24-34% compared to standard gravity, with the maximal enhancement nearly consistent at the 2 sigma level with bulk flow observations. The occurrence of the Bullet Cluster in these theories is 10^4 times more probable than in LCDM cosmology. Comment: 15 pages, 6 figures. v2: added references - [Show abstract] [Hide abstract]

**ABSTRACT:**We present new measurements of the coherent motion of galaxies based on observations of the large-scale redshift-space distortions seen in the two-dimensional two-point correlation function of Luminous Red Galaxies in Data Release Seven of the Sloan Digital Sky Survey. We have developed a new methodology for estimating these coherent motions, which is less dependent on the details of galaxy bias and of the cosmological model to explain the late-time acceleration of the expansion of the Universe. We measure a one-dimensional velocity dispersion of galaxies on large-scales of \sigma_v=3.01^{+0.45}_{-0.46} Mpc/h and \sigma_v=3.69^{+0.47}_{-0.47} \mpcoh$ at a mean redshift of z=0.25 and 0.38 respectively. These values are fully consistent with predictions for a WMAP7-normalised LCDM Universe and inconsistent at confidence of 3.8\sigma with a Dvali-Gabadadze-Porrati (DGP) model for the Universe. We can convert the units of these $\sigma_v$ measurements to 270^{+40}_{-41} km/s and 320^{+41}_{-41} km/s respectively (assuming a $\Lambda$CDM universe), which are lower that expected based on recent low redshift (z<0.2) measurements of the peculiar velocity field (or "bulk flows"). It is difficult to directly compare these measurements as they cover different redshift ranges and different areas of the sky. However, one possible cosmological explanation for this discrepancy is that our Galaxy is located in unusually over, or under, dense region of the Universe.