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Loading contentThe moment the cooling Universe let electrons and protons combine into neutral atoms, allowing light to travel freely for the first time.
As the Universe expanded and cooled to about 3,000 K, electrons and nuclei combined into neutral hydrogen. Space became transparent, and the light released at that moment streams to us today, stretched by expansion into the cosmic microwave background.
Facts on this topic will be cited from these primary and reference sources.
Mission data, planetary science, space telescopes, and public-domain imagery.
Most NASA-produced imagery is in the public domain; individual items are checked for usage terms before publication.
Cosmic microwave background maps and the cosmological parameters of the ΛCDM model.