Measuring our velocity from fluctuations in number counts

TL;DR

This paper proposes a new method using bipolar spherical harmonics to detect the Doppler boost effect on galaxy number count fluctuations, aiming to measure our velocity relative to the cosmic microwave background.

Contribution

It introduces the first analysis of Doppler imprint on galaxy number counts using bipolar spherical harmonics, and predicts detectability with future SKA surveys.

Findings

Doppler boost induces a measurable signature in galaxy number counts.

Next-generation surveys like SKA can detect this effect at over 3 sigma.

Detection is feasible with Phase 2 of SKA, marginal in Phase 1.

Abstract

Our velocity relative to the cosmic microwave background (CMB) generates a dipole from the CMB monopole, which was accurately measured by COBE. The relative velocity also modulates and aberrates the CMB fluctuations, generating a small signature of statistical isotropy violation in the covariance matrix. This signature was first measured by Planck 2013. Galaxy surveys are similarly affected by a Doppler boost. The dipole generated from the number count monopole has been extensively discussed, and measured (at very low accuracy) in the NVSS and TGSS radio continuum surveys. For the first time, we present an analysis of the Doppler imprint on the number count fluctuations, using the bipolar spherical harmonic formalism to quantify these effects. Next-generation wide-area surveys with a high redshift range are needed to detect the small Doppler signature in number count fluctuations. We show that radio continuum surveys with the SKA should enable a detection at $\gtrsim 3 \sigma$ in Phase 2, with marginal detection possible in Phase 1.