The Cosmic Microwave Background (CMB) is a steady low-level microwave radiation that glows throughout the entire universe. It is the last echo of the heat of creation. For the first few hundred thousand years or so after the Big Bang, space was crammed full of a plasma of subatomic particles, so hot that they could not combine into atoms. But as it all expanded, the temperature dropped until suddenly it was cool enough that electrons could pair up with protons to form the first atoms. That let space became transparent (the way it is today) and all that thermal radiation was suddenly able to go somewhere. Over the past 13.8 billion years, the photons of that radiation have been losing energy fighting against the expansion of the universe so that today they are red-shifted way down into the microwave band and since the source of this radiation is the entire universe, it is almost perfectly uniform in all directions. The CMB was predicted by proponents of the Big Bang Theory back in 1948, when astronomers were still arguing over whether the universe was expanding or not. It was first observed accidentally, in the 1950’s by radio technicians who were simply trying to tune a sensitive antenna, and remains the single most convincing proof to scientists that the Big Bang happened.
Written by Allen Versfeld
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