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Chapter 26: Problem 2

What is Olbers's paradox? How can it be resolved?

Short Answer

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Olbers's paradox questions why the night sky is dark if an infinite, eternal, and static universe should be filled with starlight. It can be resolved by considering that the universe is not infinite in age, so only the light from stars within the observable universe reach us. Furthermore, the universe's expansion red-shifts this light, reducing its overall intensity and explaining the darkness of the night sky.

Step by step solution

01

Understanding Olbers's Paradox

Olbers's Paradox is a question that explores the contradiction between what we observe in the night sky and what we would see, given an infinite and eternal static universe filled with stars. If the universe is such, every line of sight in the sky would end on a star and the sky would be bright instead of dark.
02

Why is all the starlight not reaching us?

Despite the abundance of stars in the universe, not all light from these stars reaches us on Earth. This is due to a number of reasons: (1) Stars have a finite lifetime. They do not shine indefinitely. (2) Some light is absorbed by interstellar and intergalactic dust and gas. (3) Not all regions of the universe contain stars. There are vast empty regions, voids, that are devoid of luminous matter.
03

Resolving Olbers's Paradox

The paradox can be resolved by understanding two fundamental aspects of the universe. The universe is not infinite in age - it has a finite age of about 13.8 billion years. Because of this, we can only see stars that are within our 'observable universe', meaning the light of many stars hasn't had time to reach us. Additionally, the universe is expanding, effectively red-shifting starlight into longer wavelengths and reducing its intensity, creating the dark night sky.

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Most popular questions from this chapter

Can you see the cosmic background radiation with the naked eye? With a visible-light telescope? Explain why or why not.

How was the Big Bang different from an ordinary explosion? Where in the universe did it occur?

Suppose we were living in a radiation-dominated universe. Discuss how such a universe would be different from what we now observe.

(a) For what value of the redshift \(z\) were representative distances between galaxies only \(20 \%\) as large as they are now? (b) Compared to representative distances between galaxies in the present-day universe, how large were such distances at \(z=8\) ? Compared to the density of matter in the present-day universe, what was the density of matter at \(z=8\) ? (c) If dark energy is in the form of a cosmological constant, how does its present-day density compare to the density of dark energy at \(z=2\) ? At \(z=5\) ? Explain your answers.

As a group, create a four- to six-panel cartoon strip showing a discussion between two individuals describing why the sky is dark at night.
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