In today’s universe there are four fundamental forces; gravitation, electromagnetism, and two forces that act within the nucleus, a strong and weak. But at the colossal temperatures of the newly born universe, these were all merged into one super force felt equally by all particles. However, 10-43 seconds after the beginning, gravity became a separate force. Then, at 10-35 seconds, there was an enormous “inflation” of the universe, in which it blew up from the size on an atom to that of a beach ball in a split second. Then the strong nuclear force separated out, and finally the weak force and electromagnetism split, all well within a billionth of a second.
As the universe expanded, it thinned and cooled. Short lived, exotic particles ‘decayed’ into longer lived, more familiar particles that we recognize today. Quarks were built up into the protons and neutrons that were to be the building blocks of atoms. But they were still too hot to stick to each other or to join up with electrons to form atoms.
All this took place within the first hundredth of a second. When the universe was 25 seconds old it consisted mostly of radiation at a temperature of several billion degrees and a thousand times denser than water is today, out weighing matter a hundred thousand times. Matter was knocked around by energetic photons, “bullets” if radiation
During the first three minutes, neutrons stuck to proton’ to form nuclei of helium, with traces of one or two other kinds of light atom. But helium nuclei were outnumbered 12 to one by single protons, or hydrogen nuclei. The temperature had fallen below one billion degree centigrade.
For 300,000 years the universe continued to expand. Then, as the temperature fell to about 3000C; electrons joined with atomic nuclei to form uncharged atoms. Radiation no longer interacted strongly with matter. After another few hundred million years, the thinning gas broke up into giant lumps the pro galaxies. The first stars may have begun to shine at this time.