The cosmic microwave background (CMB) is the “first light” from the time of recombination, 380,000 years after the big bang, redshifted to microwave wavelengths. At recombination, the first light had a blackbody spectrum with a temperature of 3,000 K. It now has a blackbody spectrum with a temperature of 2.7 K, providing evidence of the universe’s expansion.
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