Olbers' Paradox & Cosmic Expansion: Nature of the Universe

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  • 0:49 Olbers' Paradox
  • 3:30 Cosmic Expansion
  • 4:02 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Why is the night sky dark if there are so many stars in the universe? It used to be thought our universe had several properties that we now know not to be true. This lesson will explain what all that means and why the night sky isn't as bright as the sun.

Taking a Hike in the Woods

I love to hike in the woods. The pretty sounds, the cool breeze, it's all very beautiful. But when you are truly in the woods, not just on a trail, you can only see so far. That's because every line of sight you take inside the forest will eventually lead to a tree. Since we don't have X-ray vision, we won't be able to see past it. Trees closer to you will be easily seen, and the ones far away will be a bit fainter and blurrier. But beyond a certain distance, you simply can't see anything at all because each line of sight will be blocked by a tree.

This notion of every line of sight falling on a tree actually has to do with a paradox, called Olbers' paradox, the main subject of this lesson.

Olbers' Paradox

We take it for granted that the night sky is dark. But why? I mean, the universe is filled with God only knows how many shining stars. Shouldn't the night sky be extremely bright in that case? Shouldn't every point in our sky be a shining star?

Long ago, Isaac Newton proposed that we must be living in a static, or unchanging, infinitely sized universe, of infinite age, and one that's filled with stars. But, physician and astronomer Heinrich Olbers was puzzled by this because observation and theory didn't seem to match.

The clashing contrasts of theory and observation for why the night sky should or shouldn't be dark became known as Olbers' paradox.

You see, if Newton's version of our universe was correct, then by the forest analogy I just made, observation should dictate that every line of sight you take in the sky should, at one point or another, land upon a shining star like it lands upon a tree in the forest. Meaning, if we used our line of sight, the stars should crowd the night sky as if they're next to one another. This would then make the night sky as bright as our sun.

The paradox lies in the fact that the sky is actually dark and not amazingly bright at night. But why? Well, Olbers' paradox tells us that Newton's idea of our universe isn't truly correct. While the universe might be infinite in its size, it's not infinitely old, and it's not unchanging. That means the night sky is dark because our universe has a beginning. Because our universe has a finite age, there simply hasn't been enough time for the entire universe to become filled with light.

Put another way, the really distant stars in our universe are too far away for their light to have reached Earth. Furthermore, if you look far away into the night sky right now, the look-back time will be about the age of our universe, a time when stars had yet to be born.

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