Humankind has made stories about the origin of the world since prehistoric times. These creation stories often have a grand beauty and are sometimes richly detailed. It is only in the present century that such myths and images have been supplanted by a well-established scientific description of the origin of the world. ‘World’ is now understood to mean the Universe as a whole, not just the Earth or the solar system, and the modern picture of its origin and evolution is the Hot Big Bang model. This book describes one crucial piece of astronomical evidence supporting the Big Bang model, namely the cosmic microwave background radiation, heat radiation left over from a hot and dense phase early in the history of the Universe.

The cosmic background radiation (CBR) was discovered, by accident as it happens, a quarter of a century ago. Within a few years, the basic properties of the radiation had been established. Those properties, especially the thermal 3 K spectrum and the very uniform distribution of the CBR across the sky, have convinced virtually all astrophysicists that the radiation is a relic of the Hot Big Bang, and that it comes to us from a very early time in the history of the Universe. It thus provides information about the early history of the Universe obtainable in no other way.