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What did the COBE satellite discover about tiny fluctuations in the cosmic microwave background?

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The Cosmic Background Explorer (COBE) satellite, launched in 1989, provided critical evidence for the big bang theory by mapping the cosmic microwave background radiation.

Using its Differential Microwave Radiometer (DMR), COBE detected "wrinkles" in the radiation field, revealing that it was isotropic to 1 part in 100,000. These tiny temperature fluctuations, which appeared as patches of light and dark, represented the first evidence of spatial variation in the early universe.12

While these fluctuations were minute, they were statistically significant and distinct from random noise. Cosmologists had long predicted such departures from exact isotropy, as they were necessary to explain how gravitational separation could occur in an otherwise structureless early universe, eventually leading to the formation of galaxies and clusters of galaxies. These observations confirmed that the early universe was slightly denser in some regions than in others, providing the seeds for the large-scale structure of the universe observed today.21