We know that the universe is expanding since Edwin Hubble measured the distance and redshift of far away galaxies. For many years, many people thought that the question of whether the universe would expand forever or not, would come down to how much matter and energy is contained in it. There are three cases: the “closed”, the “open” or the “flat” case. In the closed case, the universe is so heavy that it will eventually collapse back to a point under its own weight – very much like the Big Bang in reverse; in the open case it is so light that it will expand forever, but more and more slowly, growing beyond all boundaries; the flat case is something like the marginal case between the two: the total mass of the universe is just right so that the universe will grow indefinitely, but so slowly that it will not grow beyond a certain point. The names “closed”, “open” and “flat” come from the geometry the universe will have in each case: It would be curved positively, not at all, or in a negative way, respectively.
KeywordsDark Matter Dark Energy Cosmological Constant Cosmic Microwave Background Cold Dark Matter
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