What is the Cosmic Microwave Background (CMB) relic from the early universe?

Answer

Visible light from recombination, redshifted by expansion to a 2.7 Kelvin microwave glow

The Cosmic Microwave Background (CMB) represents the oldest light we can detect, originating from roughly 380,000 years after the Big Bang, an epoch called recombination when the universe cooled enough for atoms to form and light could travel freely. This radiation was initially extremely hot, emitting light in the visible or even higher energy spectrum. However, due to the subsequent 13.8 billion years of universal expansion stretching spacetime, this powerful light has been significantly redshifted. This redshift has lowered its energy dramatically, resulting in a faint, uniform glow detectable today only in the microwave portion of the spectrum, corresponding to a temperature of about 2.7 Kelvin.

What is the Cosmic Microwave Background (CMB) relic from the early universe?

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night sky brightness