Early history of the universe. Artwork showing the cooling and expansion of the early universe from its origin in the Big Bang (upper left). The first stage (orange area) is a mixture of protons (blue balls), neutrons (yellow balls) and light photons (yellow arrows). The following four stages (seen from left to right as curved bands) are: a line of hydrogen and helium atoms (379,000 years); the universe becoming transparent to radiation (the straight lines of light); the formation of the first stars (400 million years) and gas clouds (blue area); and the formation of the galaxies (final band, with galaxies shown).
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