How old was the universe when the light of the cosmic microwave background was released?

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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 light of the cosmic microwave background (CMB) was released approximately 380,000 years after the big bang.1

This event, often referred to as the time of recombination or decoupling, occurred when the universe had cooled sufficiently to allow for the formation of neutral atoms. Prior to this, the universe was filled with ionized matter that rendered it opaque to electromagnetic radiation; once neutral atoms formed, the universe became transparent, allowing this "first light" to travel freely through space. Over the subsequent billions of years, the expansion of the universe has stretched these original high-energy photons into the microwave wavelengths observed today.12