What is the cosmic microwave background radiation?
The cosmic microwave background radiation is the faint, uniform glow of microwave energy that fills the entire universe. It's the thermal remnant of the Big Bang—the heat left over from the initial expansion of space about 13.8 billion years ago. When we first detected it, the signal corresponded to a blackbody temperature of about 3.5 Kelvin, later refined to 2.725 Kelvin. This radiation is essentially the cooled-down light from when the universe became transparent, about 380,000 years after the Big Bang. Before that, the universe was a hot, dense plasma that scattered light like a fog. As the universe expanded, that light stretched into longer wavelengths, now in the microwave range. The data show it's incredibly isotropic—nearly the same in every direction—which is exactly what the Big Bang model predicted. It's a beautiful example of how a simple measurement can confirm a grand theory.
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