Question

When did the photons in the cosmic microwave background first travel freely through the universe?

Answer 1

• This was the moment of first light in the universe, between 240,000 and 300,000 years after the Big Bang, known as the Era of Recombination. The first time that photons could rest for a second, attached as electrons to atoms. It was at this point that the universe went from being totally opaque, to transparent The light that was emitted from our location has been travelling away from us at the speed of light for the past 13.8 billion years, and owing to the expansion of the Universe, is now some 46 billion light-years away from us. Roughly 380,000 years after the Big Bang, matter cooled enough for atoms to form during the era of recombination, resulting in a transparent, electrically neutral gas, according to NASA (opens in new tab)