Detection of cosmological dipoles aligned with transverse peculiar velocities

Abstract

Peculiar velocities encode rich cosmological information, but their transverse components are hard to measure. Here, we present the first observations of a novel effect of transverse velocities: the dipole signatures that they imprint on the Cosmic Microwave Background. The peculiar velocity field points towards gravitational wells and away from potential hills, reflecting a large-scale dipole in the gravitational potential, coherent over hundreds of Mpc. Analogous dipoles will also exist in all other fields that correlate with the potential. These dipoles are readily observed in projection on the CMB sky via gravitational lensing and the integrated Sachs-Wolfe (ISW) effect -- both of which correlate with transverse peculiar velocities. The large-scale ISW dipole is distinct from the small-scale moving lens effect, which has a dipole of the opposite sign. We provide a unified framework for analysing these velocity-related dipoles and demonstrate how stacking can extract the signal from sky maps of galaxy properties, CMB temperature, and lensing. We show that the CMB dipole signal is independent of galaxy bias, and orthogonal to the usual direction-averaged correlation function, so this new observable provides additional cosmological information. We present the first detections of the dipole signal in (i) galaxy density; (ii) CMB lensing convergence; and (iii) CMB temperature -- interpreted as the ISW effect -- using galaxies from the SDSS-III BOSS survey and CMB maps from Planck. We show that the observed signals are consistent with $\Lambda$CDM predictions, and use the combined lensing and ISW results to set limits on linearised models of modified gravity.