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How does the cosmic microwave background support the big-bang model?

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Oval map of the cosmic microwave background radiation from NASA's WMAP mission, showing a mottled pattern of blue, green, yellow, and red representing temperature variations. — Cosmic microwave background A full-sky map produced by the Wilkinson Microwave Anisotropy Probe (WMAP) showing cosmic background radiation, a very uniform glow of microwaves emitted by the infant universe more than 13 billion years ago. Color differences indicate tiny fluctuations in the intensity of the radiation, a result of tiny variations in the density of matter in the early universe. According to inflation theory, these irregularities were the “seeds” that became the galaxies. WMAP's data support the big bang and inflation models, and cosmic microwave background is at the farthest limits of the observable universe.

The cosmic microwave background (CMB) serves as a foundational pillar of the big-bang model by providing a "fossil" record of the early, hot, and dense state of the universe.12

Approximately 380,000 years after the big bang, the universe cooled sufficiently for neutral atoms to form, a process known as recombination or decoupling. Before this, the universe was opaque; afterward, it became transparent, allowing electromagnetic radiation to travel freely.32

This "first light" originally possessed a high-energy blackbody spectrum at a temperature of 3,000 K. As the universe continued to expand, this radiation was stretched, or redshifted, to the microwave wavelengths observed today, which correspond to a temperature of approximately 2.7 K.312

The discovery of this uniform, persistent signal by Arno Penzias and Robert W. Wilson in 1965 provided the critical evidence that distinguished the big-bang theory from competing models, such as the steady-state theory, which could not account for this universal background radiation.2