Hubble flow variance and the cosmic rest frame

Details Hubble flow variance and the cosmic rest frame Hubble flow variance and the cosmic rest frame

2012-01-25

arxiv.org/abs/1201.5371v1

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

24 pages, 3 tables, 9 figures

Scientific paper

We characterize the radial and angular variance of the Hubble flow in the COMPOSITE sample of 4534 galaxy distances. Independent of any cosmological assumptions other than the existence of a suitably averaged linear Hubble law, we find with decisive Bayesian evidence (ln B >> 5) that the Hubble constant averaged in spherical radial shells is closer to its global value when referred to the rest frame of the Local Group rather than to the standard rest frame of the Cosmic Microwave Background (CMB) radiation. Angular averages reveal a dipole structure in the Hubble flow variance, correlated with structures within a sphere of radius 30/h - 60/h Mpc. Furthermore, the angular map of Hubble flow variance is found to coincide with the angular map of the residual CMB temperature dipole in the Local Group rest frame, with correlation coefficient -0.92. This suggests a new mechanism for the origin of the CMB dipole: in addition to a local boost it is generated by differences in the distance to the surface of last scattering, of a maximum +/- 0.35/h Mpc, which arise from foreground structures within 60/h Mpc, a 0.6% effect. The dipole feature is accounted for by our position in a filamentary sheet between Local Voids and Local Walls, producing a foreground density gradient on scales up to 60/h Mpc on opposite sides of the sky. This result potentially eliminates problems of interpretation of "bulk flows". Furthermore, anomalies associated with large angles in the CMB anisotropy spectrum, and also the dark flow inferred from the kinetic Sunyaev-Zel'dovich effect on small angular scales, need to be critically re-examined.

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