The cosmic microwave background (CMB) is a faint glow of radiation that permeates the entire observable universe. It is the oldest light in the universe, originating around 380,000 years after the Big Bang, when the universe had cooled enough for photons to decouple from matter and travel freely through space.

The CMB carries important information about the early universe, revealing details about its temperature, density, and the distribution of matter and energy at that time. Studying the CMB has been instrumental in supporting the Big Bang theory and providing insights into cosmic inflation, dark matter, and the formation of the first stars and galaxies.

The podcast episodes 'The Dark Ages of the Cosmos' and 'The First Fraction of a Second' both explore the CMB and its significance in understanding the early universe.