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Why is the cosmic microwave background in the microwave part of the electromagnetic spectrum?

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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 in the microwave part of the electromagnetic spectrum because of the expansion of the universe over the last 13.8 billion years.12

Approximately 380,000 years after the big bang, the universe cooled sufficiently for protons and electrons to form neutral hydrogen atoms, a period known as recombination. At this time, the universe became transparent to light, releasing radiation that initially had a blackbody spectrum with a temperature of approximately 3,000 K.324

As space has continued to expand, the wavelengths of these original photons have been stretched—a process known as cosmological redshift—to much longer, microwave-length wavelengths. Consequently, this "first light" has cooled to its current temperature of approximately 2.7 K, shifting its peak intensity into the microwave region of the electromagnetic spectrum.32415