The Big Bang Theory
Let us examine the most successful theory of the birth and evolution of the universe: the Big Bang Theory.
We know now that matter is highly structured. Bodies are made of molecules, molecules of atoms, atoms of nucleons and electrons, nucleons of protons and neutrons, protons and neutron of quarks. At the beginning of the universe (according to the Big Bang Theory and supported by physical theories of elementary particles), matter was an undifferentiated soup of quarks. There was only one type of force (or interaction) among these particles. As the universe expanded, it cooled and different forces and particles emerged. As an amorphous body of gas (water vapor) cools and becomes more structured forming first a liquid (water) and then a solid (ice), so as the universe cooled, the unified force froze first into two components (gravity and the strong-electroweak force); then the strong-electroweak force split into strong force and electroweak force (both forces regulate processes occurring in the nucleus of an atom), and finally the electroweak split into electromagnetic force and weak force. This happened in the first billionth of a second.
As the universe expanded, its temperature became low enough so nuclei first (after the first three minutes), and then atoms and molecules formed (after a few hundred thousand years). The first stars were formed when giant gas clouds of hydrogen atoms (millions of light years across) collapsed under gravity to densities high enough to initiate thermonuclear reactions.
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