This lesson explains how the early universe changed from a hot, opaque environment into one that allowed light to travel freely. That transition made it possible for us to observe the universe’s ancient afterglow today.
For its first few hundred thousand years, the universe was extremely hot and dense. Although light was present, it could not travel freely. Free electrons scattered photons repeatedly, changing their directions before they could move very far. The universe was therefore opaque, much like a dense crowd through which it is impossible to see clearly.
All flashcards
What made the early universe opaque?
A dense population of free electrons scattered photons repeatedly, so radiation could not travel far without changing direction.
What is recombination?
The formation of stable, neutral atoms when electrons combined with atomic nuclei as the expanding universe cooled.
What is photon decoupling?
The transition when photons began traveling freely because they interacted much less often with matter.
When was the CMB emitted?
The radiation we observe as the CMB began traveling freely at photon decoupling, about 380,000 years after the Big Bang.
Why is the CMB so cold today?
The expansion of space stretched the wavelengths of the ancient radiation and lowered its temperature to about 2.73K.
Quiz questions
What did Alpher, Herman, and Gamow predict about radiation from the early universe?Answer: A cooled blackbody afterglow from the hot early universe.
What did COBE observations establish about the CMB spectrum?Answer: It closely matches a blackbody spectrum at about 2.73K.
What does the CMB's presence in every direction indicate?Answer: The early-universe glow filled space rather than coming from one nearby source.
What do small CMB temperature anisotropies trace?Answer: Small early density differences that gravity later amplified into cosmic structure.
Based on Astronomy 2e by Andrew Fraknoi, David Morrison, Sidney Wolff (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.