Speaker
Description
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 an order of magnitude larger, extending out to $R\sim2000\ {\rm Mpc}$. This technique isolates the Doppler shifting of Cosmic Microwave Background (CMB) photons scattered by electrons in galaxies to reconstruct the underlying velocity field.
We use galaxy data from two surveys (unWISE and WISExSCOS) combined with CMB maps (from Planck) to reconstruct large-scale velocities in six redshift bins ranging $0.1\lesssim z \lesssim 1.5$. We place the tightest upper limits to date 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 "CMB dipole anomaly", a longstanding tension where measurements of galaxy number counts imply a bulk flow significantly larger than the standard theoretical expectation (e.g., from CatWISE). 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.
| Keyword-1 | cosmology |
|---|---|
| Keyword-2 | cosmic microwave background |
| Keyword-3 | large-scale structure |