Why is the night sky not completely white ?
In: 10
The light from stars is incredibly dim when it reaches us due to the inverse square law, despite the fact that stars are bright when you’re nearby.
It’s like spreading peanut butter on bread. As you cover a wider area, the amount of available peanut butter decreases. Given infinitely large bread, you’d eventually have such a thin layer of peanut butter that it wouldn’t be noticeable.
Two main reasons.
1. However far a star is away from us, it takes time from the light of that star to reach us. So there are stars that are so far away that their light hasn’t reached us yet.
2. The universe is expanding, and the rate of that expansion increases the further away you get. This creates a kind of “horizon” beyond which no light from anything past it can ever reach us, because it is expanding away from us faster than the light is approaching us.
Because the universe is expanding.
This is actually an important question in the history of early 20th century physics and the theories of relativity and quantum mechanics that came out of that era. The question is called Olber’s paradox.
If the universe was infinite, uniform, and infinitely old as they believed at that time then every direction you look would end at a star, and the sky would be blinding.
In an expanding universe of finite age then the paradox goes away: light from distant stars hasn’t had time to reach us, and light is redshifted out of the visible spectrum. In some ways the Microwave Background is exactly what you describe: a nearly uniform light throughout the sky.
Because there are huge distances between the stars and the light fans out it is why you can’t see a torch on a hill 3 miles away at night.
In addition to the answers given, try looking up Olber’s Paradox, dealing with this specifically. Essentially it states that, if the universe is static, homogenous, and infinite (simplifying a bit), the night sky should be bright. Since it’s not, one or more of those suppositions must be false.
in fact, it is full of light. the only reason why you cannot see it is that your eyes are build for daylight. they need much more light to work than there is at night.
Isn’t it because it is redshifted out of visible spectrum. This is what the CMB is? “light”(redshifted) everywhere.
[This is “Olbers’ Paradox”](https://en.wikipedia.org/wiki/Olbers%27_paradox), or dark night sky paradox first put forth by [Thomas Digges](https://en.wikipedia.org/wiki/Thomas_Digges). Long story short, cause space is really really big and it just keeps getting bigger faster. So fast in fact, the stars farthest away from us will never be seen because the space between them and us get bigger faster than light ! ( You need to see [Quark Science on Amazon](https://www.amazon.com/Quark-Science/dp/B07CTFML98) first hour of the first part of “Everything and northing” tackles this very problem.
Space is very, very big and light from stars doesn’t always reach us because they are that far away
It sort of is. The Cosmic Microwave Background Radiation is coming from every “black” point in the sky. It’s just very redshifted so we can’t see it.
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