Cosmic background radiation generally refers to electromagnetic radiation that fills the entire universe. The most notable component is the Cosmic Microwave Background (CMB), which is relic radiation left over from the early universe shortly after the Big Bang. The CMB is a nearly uniform glow of microwave radiation filling all space, with an almost perfect black-body spectrum at about 2.7 Kelvin. It was emitted around 380,000 years after the Big Bang, when the universe became transparent and photons could travel freely (the recombination epoch). This radiation provides critical evidence supporting the Big Bang theory. There are slight fluctuations in the CMB's temperature, which correspond to the primordial density variations that seeded the formation of galaxies and large-scale structures in the universe. Other forms of cosmic background radiation exist across different wavelengths, including infrared and X-ray backgrounds, but the CMB is the most studied and significant for cosmology. The CMB's discovery in 1965 was accidental but pivotal, proving the universe was once in a hot, dense state. In summary, cosmic background radiation, mainly the CMB, is the faint microwave radiation left from the early universe that pervades all space and forms a cornerstone for understanding the origin and evolution of the cosmos.
