Cosmic Flow from 2mass Redshift Survey: the Origin of Cmb Dipole and Implications for Λcdm Cosmology

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. We estimate the mean matter density to be Ωm = 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 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 HydraCentaurus-Norma supercluster at around 40h Mpc. Although most of the amplitude of the CMB dipole seems to be recovered by 120h Mpc, we observe no convergence up to this scale. Due to dominant superclusters such as Horologium-Recticulum in the southern hemisphere at scales above 120h Mpc, one might need to go well beyond the Shapley supercluster 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 50h Mpc, assuming a Local Group velocity of 627 km s, we estimate Ωmh 2 = 0.085± 0.06 and σ8 = 1.92± 0.97, in agreement with WMAP5 measurements at the 1 to 2σ level. However, for scales up to 100h Mpc, we obtain Ωmh 2 = 0.075± 0.04 and σ8 = 1.72± 0.56, which agrees only at the 2 to 3σ level with WMAP5 results. Weighing our likelihood analysis by the probability of occurrence of the velocity of the Local Group does not change the aforementioned disagreement. Subject headings: dark matter — methods:analytical, numerical and observation