Why Is Space Black If Stars Are Emitting Light?

Have you ever looked up at the night sky and wondered why space appears to be black? Surely the billions of stars in it should be able to light it up? We picked out the best answers from the internet that explain why that’s not the case.


This is called Olbers’ Paradox.
In essence, if we posit that the universe is infinitely large and contains an infinite number of stars (and they are largely randomly distributed), then every line of sight an observer can see should eventually end at a star. So if the universe is infinitely old, every point in the sky should be as bright as the surface of a star.

Since it clearly isn’t – we have to discard one or more of our assumptions. (They are the universe is infinitely large, contains an infinite number of stars and is infinitely old)
This is evidence for the big bang – we discard the idea that the universe is infinitely old, so although every line of sight does end in a star, the light from those stars has not had time to arrive yet. (As the speed of light is slow compared to the size of the universe).


There’s nothing for the starlight to reflect off of (mostly). Imagine an empty room, without any light. If someone turns on a flashlight and points it towards you, the rest of the room still looks black, since that light isn’t hitting anything.


Why is the sky dark at night? This was a question raised by an astronomer many years ago. He reasoned that in any direction, there would eventually be a star. Why don’t we see them all, as a canopy of light.


Briefly, the reason is that the further away a star (nebula, galaxy, etc.) is from us, the faster it is retreating from us, producing a red-shift and this reducing the energy of the light from it. Eventually, there are things so far away and retreating from us so fast that we can’t see them at all.


It’s a combination of age and size. Since light travels at a finite speed, it takes time for light emitted arbitrarily far away to reach us. and the universe isn’t old enough that there’s been infinite time for that to happen, which means that light emitted from far enough away hasn’t had enough time to reach us. Combined with the expansion of the universe, which is growing faster than light can travel, that means some light never will reach us.


You may also like...