Maps of the sky as seen by NASA’s COBE satellite, after different stages of image processing.

The top panel shows (in false colour) the temperature of the sky after removing a uniform (2.7 K) component due to the CMB; the large-scale diagonal feature (the so-called dipole) is caused by the motion of the Sun with respect to the CMB; and the faint horizontal
smudge is due to emission from the Milky Way. The bottom map results when these two components are removed. What is left is residual galactic emission (seen as a bright horizontal band), and a background of hot and cold spots, due largely to a mixture of instrument noise and the CMB.