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SESK: Stuff Everyone Should Know · sesk.info/astro-cmb-release-time

When was the cosmic microwave background released?

About 380,000 years after the Big Bang.

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  1. About 380,000 years later ✔
  2. Exactly 4 minutes later
  3. About 4 billion years later

Why: When electrons joined nuclei to form neutral atoms, photons stopped scattering constantly. The universe became transparent and the radiation now seen as the CMB streamed freely.

Huh, didn't know that: The gas temperature at last scattering was about 3,000 kelvins.

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The early universe became transparent after clearing up its charged behavior.
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References

Answer: About 380,000 years later. When electrons joined nuclei to form neutral atoms, photons stopped scattering constantly. The universe became transparent and the radiation now seen as the CMB streamed freely.

  1. The Cosmic Microwave Background openstax.org/books/astronomy-2e/pages/29-4-the-cosmic-microwave-background
    a significant milestone in the history of the universe occurred about 380,000 years after the Big Bang
    Checked 2026-10-10.

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