Since the universe is very large, and the speed of light is constant, scientists know that when light arrives from the infant universe, it arrives as the oldest signal that can be detected.
Cosmic Background Radiation
With the help of radio and infrared telescopes, scientists have detected energy left over from the Big Bang. They have observed a faint, nearly uniform glow of microwave radiation coming from all directions of the sky. The glow is called cosmic background radiation and it provides strong scientific support for the Big Bang theories origin of the Universe. |
In the early Universe light did not have a clear path to travel. Light particles constantly scattered off electrons, which were not yet formed into atoms. About 380,000 years after the Big Bang, the Universe was cool enough for atoms to form. The light that filled the Universe was then free to travel through space. As the Universe appears to have expanded, the radiation shifted toward wavelengths longer than those of visible light, through what is known as the Doppler effect. The radiation is now in the form of microwaves.
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During the Big Bang, a lot of high-energy radiation was created. Then, the universe became bigger and colder. Therefore, the high-energy particles lost most of their original energy. Now, as a result, that radiation is in the microwave part of the electromagnetic spectrum which is a lower energy.