The CMB Dipole: The Most Recent Measurement And Some History

Abstract

The largest anisotropy in the cosmic microwave background (CMB) is the $\approx 3$ mK dipole assumed to be due to our velocity with respect to the CMB. Over the past ten years the precision of our knowledge of the dipole has increased by a factor of ten. We discuss the most recent measurement of this dipole obtained from the four year COBE Differential Microwave Radiometers (DMR) as reported by Lineweaver \etal (1996). The inferred velocity of the Local Group is $v_{LG}= 627 \pm 22$ km/s in the direction $\ell = 276°\pm 3$, $b= 30°\pm 2$. We compare this most recent measurement to a compilation of more than 30 years of dipole observations.