During this period the universe began a sudden burst of exponential expansion—faster than the speed of light. This expansion is called "inflation" and explains why the universe we observe is so uniform. Temperatures were so hot (1027 K) before inflation that the familiar particles that make up atoms today (protons and neutrons) were not stable—the universe was a hot soup of quarks (particles that are hypothesized to make up baryons), leptons (electrons and neutrinos), photons, and other exotic particles.
The History of the Universe: Formation of the Elements, Stars, and Galaxies, and the Cosmic Microwave Background
As the universe expanded after inflation it continued to cool. For the first three minutes conditions everywhere were similar to those at the center of stars today, and fusion of protons into deuterium, helium, and lithium took place. Most of the helium we see today in stars is believed to have been produced during these early minutes. The universe was an extremely opaque plasma, and photons dominated the mass density and dynamical evolution of the universe. When the universe cooled sufficiently to allow the free electrons to recombine with the hydrogen and helium nuclei, suddenly the opacity dropped, and the photons were free to stream through space unimpeded.
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