Interpreting the CMB aberration and Doppler measurements: boost or intrinsic dipole?

TL;DR

This paper examines the assumptions behind interpreting CMB aberration and Doppler measurements, revealing degeneracies with primordial gravitational effects and proposing methods to distinguish between velocity and intrinsic dipole contributions.

Contribution

It demonstrates that Doppler and aberration signals can arise from both velocity and gravitational effects, highlighting the conditions under which these effects are degenerate or distinguishable.

Findings

Doppler-like couplings can result from velocity and gravitational potential effects.

Degeneracy between effects depends on initial conditions and potential profiles.

Additional signatures from second-order terms can help disentangle scenarios.

Abstract

The aberration and Doppler coupling effects of the Cosmic Microwave Background (CMB) were recently measured by the Planck satellite. The most straightforward interpretation leads to a direct detection of our peculiar velocity $\beta$, consistent with the measurement of the well-known dipole. In this paper we discuss the assumptions behind such interpretation. We show that Doppler-like couplings appear from two effects: our peculiar velocity and a second order large-scale effect due to the dipolar part of the gravitational potential. We find that the two effects are exactly degenerate but only if we assume second-order initial conditions from single-field Inflation. Thus, detecting a discrepancy in the value of $\beta$ from the dipole and the Doppler couplings implies the presence of a primordial non-Gaussianity. We also show that aberration-like signals likewise arise from two independent effects: our peculiar velocity and lensing due to a first order large-scale dipolar gravitational potential, independently on Gaussianity of the initial conditions. In general such effects are not degenerate and so a discrepancy between the measured $\beta$ from the dipole and aberration could be accounted for by a dipolar gravitational potential. Only through a fine-tuning of the radial profile of the potential it is possible to have a complete degeneracy with a boost effect. Finally we discuss that we also expect other signatures due to integrated second order terms, which may be further used to disentangle this scenario from a simple boost.