CMB Aberration and Doppler Effects as a Source of Hemispherical Asymmetries

Understand

Our peculiar motion with respect to the CMB rest frame represents a preferred direction in the observed CMB sky since it induces an apparent deflection of the observed CMB photons (aberration) and a shift in their frequency (Doppler).

• Both effects distort the multipoles $a_{\ell m}$'s at all $\ell$'s.
• Such effects are real as it has been recently measured for the first time by Planck according to what was forecast in some recent papers.
• However, the common lore when estimating a power spectrum from CMB is to consider that Doppler affects only the $\ell=1$ multipole, neglecting any other corrections.