Challenges to the standard cosmological model from large-scale bulk flow estimates

The cosmological principle demands that the motions of galaxies averaged over a large enough sphere should become small. Thus, the large-scale bulk flow is an important test of this principle and of the standard cosmological model more generally. While hints of anomalous bulk flows have been reported for decades, in the past few years, the quality and quantity of peculiar velocity data have improved to where bulk flows can put meaningful constraints on our models. We present estimates of the bulk flow in volumes of increasing radii using the minimum variance (MV) method with data from the CosmicFlows-4 (CF4) catalogue. Contrary to expectations, we find that the bulk flow amplitude increases with increasing radius, with the bulk flow amplitude in a volume of radius 200 h − 1 Mpc being large enough to have only a 0.003% chance of occurring in the standard model. We discuss the detailed characteristics of the large-scale bulk flow with an eye towards a better understanding of its origin.

This article is part of the discussion meeting issue ‘Challenging the standard cosmological model’.