Olbers' Paradox

In the early 1800's, there was a quandary. Scientists did not know about the size of the universe or the speed of light. There was an assumption that the universe was infinite in size, age, and mass. Not only that, they assumed that light itself travelled instantaneously from place to place. They did not know about the Big Bang, or Hubble's Law, or even Relativity. So from these assumptions, there arose a question: why is the night sky dark?


They believed the universe was infinite and static. Because of this, they wondered where all the stars were. Imagine looking in any direction in the sky. If the universe is infinitely populated with stars, no matter where you look, you will see a star, despite the great distances involved (remember, light is instantaneous). Therefore, you should see light from that star. Because of this, at night, when the Sun is not dominating the sky, the sky itself should be lit up like daytime. Though this question has been posed before, Heinrich Olbers was the first to formulate the question and try to answer it, and we call this question, Olbers' Paradox.






He came up with a few reasons why the night sky is not dark. I'm only going to mention two for simplicity.

1. The universe is not infinite. There is a limit to the size as well as the age of the universe, though the size may be larger than the age of the universe. We still don't know exactly how big the universe really is since we can only see about 14 billion light-years in any direction.
2. The speed of light is not infinite. He didn't know how fast it was, but he could conclude that light travelled at a finite speed. Therefore, light from stars that are farther away from us than the age of the universe have not had enough time to reach us since the star/galaxy/whatever was formed.

From these two reasons, we can see the night sky is dark because the light from most of the stars in the universe has not had enough time since the light left the star to reach us.