Planck 2013 results. XXVII. Doppler boosting of the CMB: Eppur si muove

Abstract

Our velocity relative to the rest frame of the cosmic microwave background (CMB) generates a dipole temperature anisotropy on the sky which has been well measured for more than 30 years, and has an accepted amplitude of v/c = 0.00123, or v = 369km/s. In addition to this signal generated by Doppler boosting of the CMB monopole, our motion also modulates and aberrates the CMB temperature fluctuations (as well as every other source of radiation at cosmological distances). This is an order 0.1% effect applied to fluctuations which are already one part in roughly one hundred thousand, so it is quite small. Nevertheless, it becomes detectable with the all-sky coverage, high angular resolution, and low noise levels of the Planck satellite. Here we report a first measurement of this velocity signature using the aberration and modulation effects on the CMB temperature anisotropies, finding a component in the known dipole direction, (l,b)=(264, 48) [deg], of 384km/s +- 78km/s (stat.) +- 115km/s (syst.). This is a significant confirmation of the expected velocity.

Figures (2)

• Figure 1: Exaggerated illustration of the aberration and Doppler modulation effects, in orthographic projection, for a velocity $v = 260000{\rm km}{\rm s}^{-1} = 0.85 c$ (approximately $700$ times larger than the expected magnitude) toward the northern pole (indicated by meridians in the upper half of each image on the left). The aberration component of the effect shifts the apparent position of fluctuations toward the velocity direction, while the modulation component enhances the fluctuations in the velocity direction and suppresses them in the anti-velocity direction.
• Figure 2: Specific choice for the decomposition of the dipole vector $\vec{\beta}$ in Galactic coordinates. The CMB dipole direction $(l,b) =(263$0=$^{\circ}$.$^{\circ} \hbox{.}$^∘$99, 48$0=$^{\circ}$.$^{\circ} \hbox{.}$^∘$26)$ is given as $\vec{\beta}_{\parallel}$, while two directions orthogonal to it (and each other) are denoted as $\vec{\beta}_{\perp}$ and $\vec{\beta}_{\times}$. The vector $\vec{\beta}_{\times}$ lies within the Galactic plane.