Cosmic Microwave Background — Observation and Interpretation

1. Standard Definition

In standard cosmology, the Cosmic Microwave Background (CMB) is the faint thermal radiation left over from the time when the universe cooled enough for electrons and protons to form neutral hydrogen.
At that moment (≈ 380 000 years after the Big Bang, at ≈ 3000 K), photons ceased to scatter continuously and began to travel freely.
The resulting radiation, red-shifted by the ongoing expansion, reaches us today as a 2.725 K blackbody in the microwave band.