-
Asked by anon-267369 on 5 Nov 2020. This question was also asked by anon-267547.
-
Bradley Young answered on 5 Nov 2020:
The universe is dark because it has (approximately) nothing in most of it. You can only see light when it reflects off something and in to your eye. Most of the universe of empty, so the light doesn’t reflect off anything, so it looks dark. There is light there though which is produced by stars and reflects off planets.
-
-
Adrien Chauvet answered on 5 Nov 2020:
Because your eyes are not sensitive to most of what is out there!
You only see a narrow range of electromagnetic waves, from ~400 to ~750nm. Hence, we are simply blind to most “lights” that the universe is made of. -
Joshua McAteer answered on 5 Nov 2020:
This is related to a very interesting thought experiment called Olbers’ paradox. Olbers was a German astronomer who lived between 1758 and 1840. At the time it was thought that the universe was infinitely large and existed in a steady-state (in that there were not large scale changes like clumping together of groups of stars over time) and had existed forever.
Olbers argued that if there were an infinite extent of the universe and that it had stars distributed somewhat-evenly through it then the night sky should be as bright as the sun. He argued that in each direction that you look in the sky there should be a star and that the light from the star would reach our eyes. Light travels through space efficiently, if there isn’t an object which blocks the path of the light it will travel in a straight line from the source forever. Interestingly, surface brightness (which is the brightness divided by the apparent size of an object) doesn’t change with distance i.e. if we stood on Mars (or Venus or any other planet) the Sun would be the same brightness, however the apparent size would change.
The reason that distant stars appear much dimmer is that, if you think about a camera, the sun might take up 100 pixels on a camera sensor, but a very distant star is so small that it takes up only a small fraction of 1 pixel. The same is true for the light sensitive cells in your eye.
The reason that the universe is dark is the reason worked out by Olbers. The universe does not fit the model of the 1800s. The universe has not existed forever and there are plenty of gaps where, if you projected a straight line from your eye forever, you’d never hit a star. Because the universe has existed for only a finite time there are stars we can’t see yet because the light hasn’t reached us yet.
-
Daisy Shearer answered on 8 Nov 2020:
Most of the universe is empty space, so we see it as dark. When you look up at the night sky, the places where there are no stars are dark because there is no light source or particles to reflect light from a light source to our eyes (like when the moon reflects the sun’s light which means we can see it!).
-
Sam Geen answered on 9 Nov 2020:
If you could see microwaves, you might see the glow from the early universe shortly after the Big Bang! This is called the “Cosmic Microwave Background”, and it tells us a lot about the shape of the universe, how old it is, and what will happen to it in the distant future.
Comments