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Chapter 26: Problem 2

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

Short Answer

Expert verified
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

What is a cosmological constant? Why did Einstein introduce it into cosmology?

. What does it mean to say that the universe was once radiationdominated? What happened when the universe changed from being radiation-dominated to being matter-dominated? When did this happen?

What was the era of recombination? What significant events occurred in the universe during this era? Was the universe matter-dominated or radiation- dominated during this era?

Temperatures in the Early Universe. Access the Active Integrated Media Module "Blackbody Curves" in Chapter 26 of the Universe Web site or eBook. (a) Use the module to determine by trial and error the temperature at which a blackbody spectrum has its peak at a wavelength of \(1 \mu \mathrm{m}\). (b) At the time when the temperature of the cosmic background radiation was equal to the value you found in (a), was the universe matter-dominated or radiation- dominated? Explain your answer.

What does it mean when astronomers say that we live in an expanding universe? What is actually expanding?
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