The Cosmic Microwave Background, often shortened to CMB, is the oldest light in the observable universe. It was released about 380,000 years after the Big Bang, when the universe had finally cooled enough for atoms to form and for light to travel freely instead of being constantly scattered by a hot, dense plasma.
That ancient light has been stretching and cooling ever since, and today it shows up not as visible light but as faint microwave radiation, arriving from every direction in the sky with a temperature of about 2.7 degrees above absolute zero. It was discovered by accident in 1965 by two radio engineers who initially thought it was noise from their antenna.
Tiny temperature variations in the CMB, differences of just a few parts in a hundred thousand, map out the seeds of structure that would eventually grow into every galaxy, star, and cluster we see today. Studying those variations in detail is one of cosmology’s most precise tools for understanding the universe’s earliest moments.