Measuring cosmic bulk flow with kinetic Sunyaev-Zel'dovich (kSZ) velocity reconstruction

Abstract

The average large-scale velocity of matter in the universe, known as **bulk flow**, is a fundamental test of the Cosmological Principle. Traditionally, this has been measured only out to $R\lesssim 100$ megaparsecs (Mpc). We present an application of **kinetic Sunyaev-Zel'dovich (kSZ) velocity reconstruction** to constrain bulk flow on cosmological scales more than $10\times$ larger, extending out to $R\sim 2000$ Mpc. kSZ velocity reconstruction isolates the Doppler shifting of CMB photons scattered by the electron plasma in galaxies to reconstruct the underlying velocity field. We use galaxy data from two catalogs (unWISE and WISExSCOS) combined with CMB maps (from Planck) to reconstruct velocities in six redshift bins ranging from $0.1\lesssim z \lesssim 1.5$. We place some of the tightest constraints on bulk velocity at $500 \lesssim R\ [{\rm Mpc}] \lesssim 2000$, finding results fully consistent with the standard cosmological model, $\Lambda$CDM. Furthermore, our constraints are relevant for the **cosmic dipole anomaly**, a persistent tension where measurements of galaxy number counts imply a bulk flow $\gtrsim 5\times$ larger than the standard theoretical expectation. Our constraints are in $\sim 2\sigma$ tension with the leading number-count dipole measurement from CatWISE, challenging their interpretation of the dipole anomaly as an excess coherent bulk flow, and reinforcing the standard cosmological model.