"Relic radiation," also known as cosmic microwave background (CMB) radiation, refers to a faint radiation that permeates the universe. It is a form of electromagnetic radiation that exists in the microwave portion of the electromagnetic spectrum.
The cosmic microwave background radiation is considered the afterglow of the Big Bang, the event that is believed to have initiated the expansion of the universe approximately 13.8 billion years ago. Prior to the formation of the CMB, the universe was hot and dense. As the universe expanded and cooled down, the primordial plasma of protons, electrons, and photons combined to form neutral atoms. This allowed photons to travel freely without being constantly scattered by charged particles.
The relic radiation we observe today is the remnant of those primordial photons. However, due to the expansion of the universe, the wavelengths of these photons have stretched over time, resulting in a shift towards longer wavelengths, specifically in the microwave region of the electromagnetic spectrum. This is why it is referred to as cosmic microwave background radiation.
The discovery of the cosmic microwave background in 1965 by Arno Penzias and Robert Wilson provided strong evidence for the Big Bang theory. The CMB is nearly uniform in all directions and has a characteristic temperature of approximately 2.7 Kelvin (or -270.45 degrees Celsius), making it one of the coldest phenomena in the universe. Studying the CMB has provided valuable insights into the early universe, its composition, and its evolution, as well as supporting the theory of cosmic inflation and the formation of large-scale structures in the universe.