The 3 K background radiation is just the redshifted gamma wavelength radiation of the decoupling.

Question

Quizplus · Accepted Answer

The 3K (Kelvin) background radiation, also known as the Cosmic Microwave Background (CMB), is indeed the redshifted radiation from the time of decoupling, when photons were first able to travel freely through the universe. This radiation originated as high-energy photons (including gamma wavelengths) but has been stretched into the microwave range due to the expansion of the universe.

The 3 K Background Radiation Is Just the Redshifted Gamma