Cosmic microwave background. Coloured infrared all-sky map showing the large-scale variation of temperature of the cosmic microwave background (CMB). This is due to the motion of the Local Group of galaxies (including the Milky Way) relative to the CMB. Radiation from the direction in which the Local Group is travelling is doppler-shifted to shorter wavelengths, and thus appears hotter (red, upper right), whilst radiation from the opposite direction appears cooler (purple). The amount of doppler shift suggests a velocity of 600 km per second relative to the CMB. The data for this image were gathered by the COBE satellite over four years.