The Cosmic Microwave Background (CMB) radiation is redshifted down to microwaves primarily due to the expansion of the universe. To understand this, let's explore the concept of redshift and the expansion of space.
Redshift occurs when light waves are stretched as the source of light moves away from an observer. This phenomenon is similar to the Doppler effect observed with sound waves. When an object emitting light moves away from us, the light waves get stretched, causing the light to shift towards longer wavelengths, which correspond to lower frequencies. This shift towards longer wavelengths is known as redshift.
The CMB radiation was produced when the universe was much younger and hotter, about 380,000 years after the Big Bang. At that time, the universe was filled with a hot, dense plasma of charged particles. As the universe expanded, the plasma cooled down, allowing atoms to form. Once atoms formed, the photons were able to travel freely without constant interaction with charged particles.
Over time, as the universe continued to expand, the wavelengths of the CMB photons stretched due to the expansion of space itself. This stretching led to the redshift of the CMB radiation from its initial higher-energy state (in the form of gamma rays) to lower-energy states in the microwave range.
The reason why the CMB radiation is specifically observed in the microwave range is that the redshift caused by the expansion of the universe has shifted the original high-energy gamma-ray photons to longer wavelengths. Today, these photons are detected as microwaves, which have much lower energy and longer wavelengths.
It's worth noting that the CMB radiation was initially emitted at a much higher temperature, corresponding to the high-energy state of gamma rays. However, due to the expansion of the universe, the radiation has been redshifted to lower energies, resulting in its observation as microwaves.
In summary, the CMB radiation is redshifted to microwaves due to the expansion of the universe, which stretches the wavelengths of the original gamma-ray photons emitted during the early hot phase of the universe.