How did the light forming the Cosmic Microwave Background (CMB) achieve its current microwave frequency?
It was redshifted from hot, glowing plasma by 13.8 billion years of space expansion
The Cosmic Microwave Background (CMB) represents the earliest observable light in the universe, originating from a time when the universe was a hot, dense plasma. As space has expanded continuously for approximately 13.8 billion years since that epoch, the initial high-energy radiation has been stretched dramatically. This enormous cosmological redshift has lowered the energy and shifted the wavelength of that ancient radiation from visible or near-infrared light down into the microwave part of the spectrum. Observing the CMB today as microwave radiation is a direct demonstration of the immense power of cosmic expansion acting as a cosmological tool over cosmic timescales.
