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Once atoms had formed the radiation in the Big Bang was able to fly around the Universe freely. As the Universe continued to expand the radiation was stretched out and its energy density dropped. The radiation grew weaker and dropped down the radiation spectrum towards the low energy end. Gamma rays became X rays, then ultra-violet and so on.
We can still detect this radiation today, left over from the Big Bang, as a uniform background microwave radiation which looks the same in all directions. This is one of the main reasons for believing that inflation and the Big Bang really happened. |
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