eli5:with billions of stars emitting photons why is the night sky not bright?
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Three reasons :
1) Light faints the further it is. It doesn’t mean it disappears , it means it loses energy. Our human eyes are not accustomed to this sort of light, we can only see things with a certain energy because that is the way humans evolved. If we could see in the microwave spectrum, we could see that fainted light and the universe would be all bright.
2) Some stars and galaxies are very far away from us. It takes time for light to travel to us, and a lot of this light hasn’t reached us yet.
3) Space itself is expanding. The further space is, the faster it’s expanding, even to the point where space is expanding faster than the speed of light. So for the stars very, very far away, light will never reach us because it’s too slow.
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