Olbers' Paradox - Why the Night Sky Is Dark

Olbers' Paradox Definition and Explanation

According to Olbers' paradox, there are stars in every direction, so space and the night sky should be light instead of dark.
According to Olbers' paradox, there are stars in every direction, so space and the night sky should be light instead of dark. ©//www.facebook.com/pete.lomchid, Getty Images

Question: What Is Olbers' Paradox? Why Is Space Dark? Why Is the Night Sky Dark?

The universe is so vast (even if not infinite) that no matter which direction we look, we should see a star. If this were the case, then the whole night sky should be nothing but a giant sheet of starlight. This begs the question: Why is the night sky dark?


When I first heard of this paradox, it didn't strike me as something that was really much of a concern. After all, distant stars and galaxies are just so faint that we can't see them with the naked eye, right? Doesn't that alone resolve the paradox?

Actually, it turns out that even when you consider that distant stars are fainter, there should still be so many stars that they'd overall be fairly bright. Because each little area of space represents more and more volume of space the further out you go. If you assume a vaguely even distribution of stars throughout the universe, there would still be plenty of light in each little patch to readily light up the night sky.

So what prevents it?

The paradox rests on the idea of a static and infinite (or nearly infinite) universe. It turns out that while our universe is extremely big, it's nowhere near that large. or static. We know this because of the evidence supporting the Big Bang.

Because the universe had an origin and is expanding, there is a definite horizon to how far we can see. When we look at a given section of the night's sky, we are not looking infinitely far into space, but a "mere" 13 or so billion light-years out. Beyond that, there's nothing else to see, except for the faint glow (invisible to the naked eye) of the cosmic microwave background radiation.

That is part of why the night sky is dark -- because there just isn't enough space and time for this particular paradox to have the room it needs to light up the night sky.

Another reason is because space isn't an empty void. While the pressure in space is much lower than that within the atmosphere, it's not devoid of ions, atoms, and molecules. These particles can absorb light, as well as scatter it. You can think of space as a dusty cloud that's almost infinitely thick. It's so thick, not that much light makes it all the way to us.

Other reasons for space to be dark include:

  • Some of the stars line up, so their light is hidden behind closer stars and other bodies.
  • The expanding universe red-shifts light from distant stars so their light is outside of the visible spectrum.
  • The universe is young, so light from distant stars hasn't had time to reach us yet.

Edited by Anne Marie Helmenstine, Ph.D.