The Cosmic Microwave Background (CMB) is the faint, uniform radiation that fills the entire universe—often described as the afterglow of the Big Bang. It is the oldest light we can observe, dating back to about 380,000 years after the Big Bang, when the universe had cooled enough for atoms to form and light to travel freely.
Before this time, the universe was a hot, dense plasma of particles and radiation, constantly scattering photons. As it expanded and cooled, electrons and protons combined to form neutral hydrogen—a process called recombination—allowing photons to decouple and move freely through space. These photons have been traveling ever since, stretched to microwave wavelengths due to the continued expansion of the universe.
Discovered in 1965 by Arno Penzias and Robert Wilson, the CMB provides a snapshot of the early universe, revealing tiny temperature fluctuations that correspond to the seeds of galaxies and cosmic structures we see today.
The CMB is a crucial piece of evidence for the Big Bang theory, confirming that the universe began from a hot, dense state and has been cooling and expanding over billions of years.