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What is the present average temperature of the CMB?

The cosmic microwave background is about 2.73 K.

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  1. 273 K
  2. 2.73 K ✔
  3. 0.0273 K

Why: Cosmic expansion stretched the early universe's hot radiation to microwave wavelengths. Its spectrum now closely matches a 2.73 kelvin blackbody.

Huh, didn't know that: The CMB arrives from every direction with tiny temperature variations.

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The CMB is cool now, but it has a very hot backstory.
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References

Answer: 2.73 K. Cosmic expansion stretched the early universe's hot radiation to microwave wavelengths. Its spectrum now closely matches a 2.73 kelvin blackbody.

  1. Astronomy 2e Chapter 29 Summary openstax.org/books/astronomy-2e/pages/29-summary
    the CMB acts like a blackbody with a temperature of 2.73 K
    Checked 2026-10-10.

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