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How did cosmic microwave background measurements help astronomers study the early universe?

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Radio telescope Lovell Telescope, a fully steerable radio telescope at Jodrell Bank, Macclesfield, Cheshire, England.

The cosmic microwave background (CMB) is the residual electromagnetic radiation from the big bang, serving as a critical tool for understanding the early universe.12

Initially discovered in 1965 by Arno Penzias and Robert Wilson using a radio antenna, the CMB provided essential evidence for the big-bang theory by confirming that the universe was once in a hot, dense, and opaque state. As the universe expanded and cooled, it reached a "decoupling time" when neutral atoms formed, allowing radiation to travel freely through space. Over billions of years, this radiation has been redshifted into the microwave region of the electromagnetic spectrum.321

By using satellite and ground-based radio telescopes to measure tiny temperature fluctuations (anisotropies) in this background, astronomers have been able to:

Refine the determination of the universe's age, size, and geometry.3
Identify the early seeds of structure, explaining how galaxies and clusters eventually condensed from a structureless state.4
Determine the composition of the early universe, including the proportions of dark matter, atoms, and neutrinos.4