Olbers’ paradox (also known as the “dark night sky paradox”) is the argument that the darkness of the night sky conflicts with the assumption of an infinite, eternal, and static universe.
If the universe were infinite, unchanging, and filled with an infinite number of stars, then every single line of sight from Earth should eventually hit the surface of a star. As a result, the night sky shouldn’t be dark at all; it should be completely and blindingly bright—as bright as the surface of the Sun.
The Paradox Explained
Imagine standing in a very dense forest. In every direction you look, your vision is eventually blocked by a tree trunk. You cannot see past the forest because the trees form a solid wall of bark.
In an infinite and static universe, stars would act like those trees. No matter which point in the night sky you point your eyes toward, you should be looking directly at a star. Even though distant stars appear fainter, there would be mathematically more of them in each layer of space you look through, perfectly balancing out the dimness.
Why is the Night Sky Dark? (The Resolution)
The paradox is solved because the universe does not fit the traditional, static assumptions made by early astronomers. The modern resolution relies on two main pillars:
- The Universe Has a Beginning (Finite Age): The universe is about 13.8 billion years old. Because light travels at a finite speed, we can only see light that has had enough time to travel to us since the dawn of time. The “observable universe” is finite, meaning there are not an infinite number of stars visible to us.
- The Universe is Expanding: Space itself is expanding, which causes light from incredibly distant stars and galaxies to stretch out as it travels toward us. This stretching shifts the light from the visible spectrum into the infrared and microwave spectrums (redshift), making it invisible to the naked eye.