The cosmic microwave background (CMB) radiation is composed of microwave radiation rather than any other type of radiation for several fundamental reasons. These reasons are deeply rooted in the early history and evolution of the universe. Let's explore them:
Redshift: The universe has been expanding since the Big Bang, causing wavelengths of photons to stretch over time due to the cosmological redshift. As the universe expanded, the wavelengths of the CMB photons have been stretched to longer wavelengths, shifting them from initially much shorter wavelengths (such as ultraviolet or visible light) to the microwave region of the electromagnetic spectrum.
Recombination Epoch: In the early universe, the universe was a hot and dense plasma of charged particles, mainly electrons and protons. Photons couldn't travel freely because they constantly scattered off charged particles, making the universe opaque. However, as the universe expanded and cooled, it eventually reached a point where the temperature dropped enough for electrons and protons to combine and form neutral hydrogen atoms. This event is known as "recombination."
Decoupling of Matter and Radiation: After recombination, the universe became mostly transparent to photons, and they could travel freely without constant scattering. This moment, when matter and radiation decoupled, occurred about 380,000 years after the Big Bang. The photons that were "released" at this epoch are the ones we observe as the CMB today.
Blackbody Spectrum: During the recombination epoch, the universe was incredibly hot, with a temperature around 3000 Kelvin. At this temperature, the photons and matter were in thermal equilibrium, and the CMB took on a blackbody spectrum, which is typical of a system in thermal equilibrium. The blackbody spectrum peaks in the microwave region of the electromagnetic spectrum, making the CMB predominantly microwave radiation.
Cosmic Age: The CMB radiation we observe today has been traveling through the expanding universe for around 13.8 billion years since the Big Bang. During this time, the photons have experienced significant redshift, and the once-hot radiation has cooled to a temperature of approximately 2.7 Kelvin, which corresponds to microwave frequencies.
In summary, the CMB is composed of microwave radiation because of the redshift, the evolution of the universe from an opaque plasma to a transparent state, and the cooling of the blackbody spectrum over billions of years. These factors have led the CMB to manifest as a faint but remarkably uniform glow of microwave radiation that permeates the entire observable universe, providing us with essential insights into the early history and nature of the cosmos.