Summary
The CMB exhibits an almost perfect blackbody spectrum, matching theoretical predictions for thermal radiation from a hot early universe with extraordinary precision.
Observation / Fact
Measurements by COBE, WMAP, and Planck show the CMB’s spectrum conforms to a blackbody curve at approximately 2.725 K, with deviations smaller than one part in 10⁵.
Why This Is Non-Trivial
Blackbody spectra arise only when radiation has been in thermal equilibrium. No known astrophysical foreground processes can reproduce this spectrum at cosmic scales.
What It Implies
The precision of the blackbody spectrum indicates the universe was once in a hot, dense, equilibrated state. This observation effectively rules out models lacking such a phase.
Related
- Discovery of the CMB
- Hot Big Bang