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Why is the cosmic microwave background considered evidence for the big bang?

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The cosmic microwave background (CMB) is considered a cornerstone of the big bang theory because it represents the "first light" of the universe, providing a direct observation of its primordial state.12

During the early stages of the universe, temperatures were so high that matter existed as an ionized plasma, rendering the universe opaque to electromagnetic radiation. As the universe expanded and cooled, it reached a point known as "decoupling" or recombination (approximately 380,000 years after the big bang), where neutral atoms formed and the universe suddenly became transparent.231

The radiation released at this time was initially very high-energy, but as space continued to expand, these photons were stretched to the microwave wavelengths observed today. This residual radiation, predicted by big-bang theorists, was discovered accidentally in 1965. Its existence, along with its precise blackbody spectrum and high degree of isotropy, provides compelling evidence for a hot, dense origin of the universe that has been expanding ever since.42513