False-colour microwave map of the whole sky, taken by the Cosmic Background Explorer (COBE) satellite at a wavelength of 5.7 mm and colour-coded to show variations in the temperature of the microwave background. The microwave background is a faint, cold (2.7K) echo of the radiation emitted during the Big Bang, when the universe was a hot fireball. The smooth variation across the sky from blue (cool) to pink (warm) is due to the motion of the Solar System through the universe and the pink horizontal band is emission from the Milky Way. Once these are subtracted, the microwave back- ground appears remarkably smooth, indicating the homogeneous nature of the primaeval fireball.