Britannica Sources used in this answer

How did the cosmic microwave background form?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Examine the observable universe's place within the whole universe — Cosmology Learn about defining and measuring the observable universe within the “whole” universe.

The cosmic microwave background (CMB) is a remnant of the big bang that serves as a foundational piece of evidence for the big-bang theory.12

In the early, extremely hot stages of the universe, matter existed as a dense, ionized plasma. During this period, the universe was opaque to electromagnetic radiation because photons were constantly scattering off free electrons.23

As the universe expanded and cooled, it reached a point—roughly 380,000 years after the big bang—where temperatures dropped sufficiently to allow the formation of neutral atoms through a process called recombination. At this "decoupling time," the universe suddenly became transparent to radiation, allowing photons to travel freely through space.23

These primordial photons, originally of very high energy and short wavelength, have since been stretched by the continued expansion of space. Today, they are observed as low-energy microwave radiation filling the entire sky.12