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Western Slope Skies - June 10th - Olbers’ Paradox


Look up into the night sky and you will see mostly darkness, sprinkled with faint points of light. Most of those points are nearby stars, but some of the very faint points of light are distant galaxies, each consisting of hundreds of billions of stars. Have you ever wondered if there is an end to this vastness of space, or is the universe truly infinite? That dark night sky you see holds the answer to that age old question.

Until quite recently, astronomers have thought that the universe is infinite in size, eternal in age, unchanging, and is essentially the same everywhere. However, if this were true, the night sky would not be dark at all, but would actually be very bright… as bright as the daytime sun! Why?

If the universe is truly infinitely big and infinitely old, then no matter what direction in the sky we look, we should see light from a star.

Even if a star is extremely distant, we would eventually see the light from that star, however faint it might be. Distant stars are not very bright, but the farther in space we look, the more stars there will be. The light from many faint stars would add up to be quite bright.

This is like being in the middle of a dense forest. No matter what direction you look, you will always see trees.

The gaps between nearby trees will always be filled in with smaller, more distant, but more numerous trees.

This question, `Why is the night sky dark?’ is known as Olbers’ paradox, named after the German astronomer Heinrich Olbers.

The dark night sky is evidence that some assumption we have made about the universe must be false. Either the universe is finite in size or finite in age and therefore, changing in time.

The poet Edgar Allan Poe, surprisingly, was the first person to provide a correct resolution. He claimed that since the speed of light is finite and the universe is not infinitely old, the universe is simply too young to be filled with light, as the light from distant stars has not yet had enough time to reach us.

We now have evidence that the universe is indeed finite in age, expanding from a very small, dense state 13.8 billion years ago. In fact, we do see light coming from all directions in space, but the expansion of the universe has shifted the light to be invisible to the eye.

This is called the cosmic microwave background radiation, which is too faint to be a direct solution to Olbers’ paradox, but provides the evidence for the finite, changing universe that we observe.

Next time you look up at the night sky to observe the stars, also peer into the darkness. Recognize that it is not just vast empty space devoid of stars. It is the universe telling us that it had a beginning.


You’ve been listening to Western Slope Skies, produced by the Black Canyon Astronomical Society and KVNF Community Radio. This feature was written and recorded by Dr. Catherine Whiting, Assistant Professor of Physics and Astronomy at Colorado Mesa University.