The steady state theory of cosmology claims that the Universe simply exists without changing with time. This theory presents many physical as well as philosophical difficulties. Evidence suggests that the Universe is expanding. While there are ways to explain expansion in a steady state universe, few astrophysicists believe this theory, because there is little evidence to support it. As the first widely held theory about the Universe it is included here for historical completeness.The big bang theory states that at some time in the distant past there was nothing. A process known as vacuum fluctuation created what astrophysicists call a singularity. From that singularity, which was about the size of a dime, our Universe was born.It is hard to imagine the very beginning of the Universe. Physical laws as we know them did not exist due to the presence of incredibly large amounts of energy, in the form of photons. Some of the photons became quarks, and then the quarks formed neutrons and protons. Eventually huge numbers of Hydrogen, Helium and Lithium nuclei formed. The process of forming all these nuclei is called big bang nucleosynthesis. Theoretical predictions about the amounts and types of elements formed during the big bang have been made and seem to agree with observation. Furthermore, the cosmic microwave background (CMB), a theoretical prediction about photons left over from the big bang, was discovered in the 1960's and mapped out by a team at Berkeley in the early 1990's.After some period of time following the big bang, gravity condensed clumps of matter together. The clumps were gravitationally pulled towards other clumps and eventually formed galaxies. It is extremely difficult to model how this clumping may have occurred, but most models agree that it occurred faster than it should have. A possible explanation is that right after the big bang the Universe began a period of exaggerated outward expansion, with particles flying ...
