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Q & A: light after Big Bang

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Most recent answer: 01/02/2016
Q:
Hi,My question is: if light came about 400,000 years after the Big Bang, what made possible its existence?Thanks.Gian
- Gian (age 39)
London
A:

Think of anything very hot- say a coil in an electric heater. It radiates light. That's because the charged particles in it can trade energy with light, so when they're hot energy will flow out into the light. When the universe was young it was very hot and there was lots of light. The reason we only see light from when it was about 400,000 years old, not earlier, is that the before that the universe was not at all transparent. Earlier light would get emitted and then right away would be absorbed again. 

Why was the universe so opaque? When it was hot enough, hydrogen atoms would fall apart into electrons and protons, charged particles. Charged particles that can move freely easily emit and absorb light. 400,000 years was the approximate age at which things cooled enough for these particles to combine into hydrogen atoms, which are nearly transparent. So after that time the light just kept on going, although stretched out and diluted by the expansion of the universe.

Mike W.


(published on 01/02/2016)

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