Large-scale Cosmic Flows from Cosmicflows-2 Catalog

TL;DR

This paper analyzes the Cosmicflows-2 catalog of peculiar velocities, compares it with the COMPOSITE catalog, and finds significant bulk flow on large scales that challenges the standard cosmological model, while smaller scales agree with it.

Contribution

It provides a reanalysis of Cosmicflows-2 data ensuring consistency across measurement methods and compares it with the COMPOSITE catalog to test cosmological models.

Findings

Large-scale bulk flow inconsistent with ΛCDM at 98% confidence

Good agreement between datasets on smaller scales

Removing certain cluster samples aligns results with standard model

Abstract

The Cosmicflows-2 catalogue is a compendium of peculiar velocity measurements. While it has many objects in common with the COMPOSITE catalogue, a previously analysed collection of peculiar velocity data found to give an unexpectedly large bulk flow on large scales, the data in Cosmicflows-2 have been reanalysed to ensure consistency between distances measured using different methods. In particular, a focus on accurate distances led the authors of the Cosmicflows-2 to not correct for homogeneous or inhomogeneous Malmquist bias, both or which are corrected for in the COMPOSITE compilation. We find remarkable agreement between the COMPOSITE and the Cosmicflows-2 if the small EFAR sample of clusters located in two dense superclusters is removed from both surveys, giving results that are inconsistent with the $\Lambda$ cold dark matter standard model with Planck central parameters at the 98\% level. On smaller scales we find overall agreement between data sets and consistency with the standard model.