| flatness problem | has definition Poses the question: why, out of an infinite number of possibilities, is our Universe so close to the one special case: the "flat" Universe? |  |
| has definition The riddle of why the universe is neither dramatically open nor closed, but appears to be almost perfectly balanced between these states. | |
| has definition A problem of the traditional big bang theory (without inflation) related to the precision required for the initial value of omega, the ratio of the actual mass density to the critical mass density. If the description is started at one second after the big bang, for example, omega must have been equal to one to an accuracy of fifteen decimal places, or else the resulting universe would not resemble our own. Yet the traditional big bang theory offers no explanation for this special value, which must be incorporated as an arbitrary postulate about the initial conditions. See also horizon problem. | |
| has definition The puzzle of why the universe today is so close to the boundary between open and closed, that is, why it is almost flat. Equivalently, why should the average mass density today be so close to the critical mass density, but not exactly equal to it? If omega begins bigger than 1, it should get bigger and bigger as time goes on; if it begins smaller than 1, it should get smaller and smaller. For omega to be near 0.1 today, about 10 billion years after the big bang, it had to be extraordinarily close to 1 when the universe was a second old. Some people consider such a fine balance to have been highly unlikely according to the standard big bang model, and thus are puzzled as to why the universe today is almost flat. (See closed universe; critical mass density; flat universe; open universe.) | |
| is a kind of problem | |
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