[ELi5]If the universe is full of stars why is it so dark?
In: Physics
It isn’t dark, it is full of amazing things that emit a lot of light & heat, quasars, Stars, nebulas, galaxies, supernovas, there are new stars blazing into life as old ones are dying, many of the stars you see in the night sky have already dimmed out of existence , the light from them has taken so long to get here. The universe has clouds of gas, rainbows of light, stars that are yellow giants like ours to red dwarfs that are a fraction of the size. It looks dark to the naked eye because the stars are so far apart that the space in between appears empty.
Artificial light reflecting into our atmosphere masks a lot of what is in the night sky.
For the same reason night time is still dark even if you turn on a flashlight. Because most of the universe isn’t stars.
Because the universe is mainly empty. Although there are billions and billions of stars, the universe is so big the light is not that present. Also, some stars are so far away their light hasn’t arrived to us yet.
Imagine your room filled with about a 100 fireflies. They produce light, but your room will still be dark because the intensity of their light pales in comparison to the size of your room.
It is the same case with the universe, although the galaxies are huge, and the stars they have produce enormous amounts of light and energy, it just pales in comparison to the size of the universe.
Also, even if your room has 100 firefiles, it will not be a lot, because there is still more than 99% empty space.
Similarly, although there are a lot of stars, more than 99% of the universe is still empty.
Because you only see the light that travels to your eyes. So it either has to travel directly from the star to your eyes or it has to bounce off of something like a planet and then you’ll see the planet. But if light is just floating around in space and not traveling to your eyes, you don’t see it.
None of the other answers covers Olber‘s Paradox. If we assume the universe is infinite and has an infinite number of stars, then no matter where you look you should see a star. Every line you draw to the sky should at some point hit a star at some distance away.
Distance is key here. Even if it is true that there are an infinite number of stars, some of them are going to be so far away that their light just couldn‘t have reached us yet, since light travels at a finite speed and the universe has only existed for some 13 billion years. Therefore those spots in the sky are dark.
There are a few lines throughout the Hitchhiker’s Guide to the Galaxy that put some poetry to this. Many references to the idea that there is a LOT of stuff in the universe, but there is also A LOT of not stuff in the universe. Cubic light years with an average density of a single atom of hydrogen per cubic meter. The space is dark because that light gets more spread out the farther it travels and most of it, no mater where you are, has traveled a VERY long way.
Or if you aren’t into that and you want to get a bit more sciency and technical. Any light source is producing little bits of light. There are a lot of them but not infinitely many of them, and they aren’t all traveling in EXACTLY the same direction. No matter how tightly focused (which most light isn’t anyway) they spread out. The father away you get from that star or neon sign, the light has had more room to spread out. Go get a piece of paper and hold it REALLY CLOSE to a light. Now back it off a bit. Again, same distance as before. Again.
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