Hubble flow variance and the cosmic rest frame

Abstract

We characterize the radial and angular variance of the Hubble flow in the COMPOSITE sample of 4534 galaxies, on scales in which much of the flow is in the nonlinear regime. With no 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 independent spherical radial shells is closer to its asymptotic value when referred to the rest frame of the Local Group, rather than the standard rest frame of the Cosmic Microwave Background. An exception occurs for radial shells in the range 40/h-60/h Mpc. Angular averages reveal a dipole structure in the Hubble flow, whose amplitude changes markedly over the range 32/h-62/h Mpc. Whereas the LG frame dipole is initially constant and then decreases significantly, the CMB frame dipole initially decreases but then increases. The map of angular Hubble flow variation in the LG rest frame is found to coincide with that of the residual CMB temperature dipole, with correlation coefficient -0.92. These results are difficult to reconcile with the standard kinematic interpretation of the motion of the Local Group in response to the clustering dipole, but are consistent with a foreground non-kinematic anisotropy in the distance-redshift relation of 0.5% on scales up to 65/h Mpc. Effectively, the differential expansion of space produced by nearby nonlinear structures of local voids and denser walls and filaments cannot be reduced to a local boost. This hypothesis suggests a reinterpretation of bulk flows, which may potentially impact on calibration of supernovae distances, anomalies associated with large angles in the CMB anisotropy spectrum, and the dark flow inferred from the kinematic Sunyaev-Zel'dovich effect. It is consistent with recent studies that find evidence for a non-kinematic dipole in the distribution of distant radio sources.Comment: 37 pages, 9 tables, 13 figures; v2 adds extensive new analysis (including additional subsections, tables, figures); v3 adds a Monte Carlo analysis (with additional table, figure) which further tightens the statistical robustness of the dipole results; v4 adds further clarifications, small corrections, references and discussion of Planck satellite results; v5 typos fixed, matches published versio

Observational challenges for the standard FLRW model

“…While this result is completely unexpected if the clustering dipole is purely kinematic, the residual monopole and dipole variations of the Hubble expansion in the LG frame were found to be consistent with a 0.5% anisotropy in the distance-redshift relation on < ∼ 65 h −1 Mpc scales, 74 an effect which can be reproduced using nonlinear exact solutions of Einstein's equations with inhomogeneities no larger than the scales in question.…”

Gravitational, shear and matter waves in Kantowski-Sachs cosmologies

“…It is known that anisotropies in the Hubble and deceleration parameters cannot be excluded by present observations [22], [23] and [24]. Consequently, perturbations of anisotropic cosmological models have been considered by many authors, e.g., [25][26][27][28][29][30][31][32], using both gauge dependent methods (e.g., [33]) or Bardeen's gauge invariant formalism [34].…”

Observational challenges for the standard FLRW model

“…The dipole and quadrupole variations of the local Hubble expansion are then quantitatively compared to those of the COMPOSITE sample data as studied earlier in Ref. 74. It is found that the Szekeres model is able to more closely model observational features than either a FLRW model with kinematic boost from the LG to CMB frame, or Newtonian N -body simulations.…”

“…While this result is completely unexpected if the clustering dipole is purely kinematic, the residual monopole and dipole variation of the Hubble expansion in the Local Group frame was found to be consistent with a 0.5% anisotropy in the distance-redshift relation on < ∼ 65 h −1 Mpc scales [74], an effect which can be reproduced using nonlinear exact solutions of Einstein's equations with inhomogeneities no larger than the scales in question [76].…”

“…In contrast, a recent purely observational model-independent analysis [74] of the COMPOSITE sample of 4,534 group and cluster distances found that the spherically averaged Hubble expansion is very significantly more uniform in the rest frame of the Local Group (LG) as compared to the standard rest frame of the CMB e (The d The overall statistics [59] include a small fraction of larger voids and a large population of smaller minivoids [60], making the Universe void dominated at the present epoch with typical theoretical estimates of the void filling-fraction of up to 80% [61,62]. By volume the Universe is mostly empty, with a very spiky density distribution in the remaining tiny volumes.…”