Easily Interpretable Bulk Flows: Continuing Tension with the Standard Cosmological Model

TL;DR

This paper introduces an improved, interpretable method for measuring cosmic bulk flows from peculiar velocity data, revealing some tension with the standard cosmological model especially at larger scales.

Contribution

It develops a new minimal variance method that is insensitive to small-scale flows and Hubble constant variations, applied to CosmicFlows-3 data to test cosmological model predictions.

Findings

Bulk flows at smaller scales agree with the standard model.

A 150 Mpc/h bulk flow shows tension with model expectations.

Only 2% chance of such a large flow under Planck parameters.

Abstract

We present an improved Minimal Variance (MV) method for using a radial peculiar velocity sample to estimate the average of the three-dimensional velocity field over a spherical volume, which leads to an easily interpretable bulk flow measurement. The only assumption required is that the velocity field is irrotational. The resulting bulk flow estimate is particularly insensitive to smaller scale flows. We also introduce a new constraint into the MV method that ensures that bulk flow estimates are independent of the value of the Hubble constant $H_o$; this is important given the tension between the locally measured $H_o$ and that obtained from the cosmic background radiation observations. We apply our method to the \textit{CosmicFlows-3} catalogue and find that, while the bulk flows for shallower spheres are consistent with the standard cosmological model, there is some tension between the bulk flow in a spherical volume with radius $150$\hmpc\ and its expectations; we find only a $\sim 2\%$ chance of obtaining a bulk flow as large or larger in the standard cosmological model with \textit{Planck} parameters