The Cosmic Microwave Background

The CMB is the oldest light in the universe — a faint microwave glow coming from every direction. It was emitted when the universe cooled enough for atoms to form, making space transparent for the first time. The CMB started as visible light but has been stretched by expansion into microwaves over 13.8 billion years.

Tiny temperature variations in the CMB — differences of just 1 part in 100,000 — map the density fluctuations of the early universe. These fluctuations are the seeds of all structure: every galaxy, cluster, and void in the cosmic web can be traced back to these minute variations.