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.
Mapping the CMB in ever finer detail has become one of cosmology’s great engineering projects. NASA’s COBE satellite first confirmed its near perfect blackbody spectrum in the early 1990s, work that earned the 2006 Nobel Prize in Physics, and later missions like WMAP and the European Space Agency’s Planck satellite mapped its tiny temperature variations across the entire sky with increasing precision. Those measurements have pinned down the universe’s age, composition, and geometry to within a percent or two, numbers that would have been unimaginable to measure directly just a few decades earlier. The CMB effectively works as a baby picture of the universe, frozen at the moment it first became transparent, and it remains the single most information-rich signal cosmologists have ever gotten their hands on.