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The main piece of evidence that supports Big Bang Theory is basically evidence that the expansion of the universe started long time ago (not necessarily from the begining of the universe).

Is it possible then that the universe started to expand at a moment when the universe was already there in a similar state?

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  • $\begingroup$ it seems you confuse expansion and expansion acceleration. For the remainder, the scenario can be narrated with the steps one wants, without forgetting that theories evolve quickly nowadays. $\endgroup$ – user46925 Dec 12 '15 at 18:58
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The universe has to obey General Relativity, and that means its expansion rate can't just change randomly. The expansion rate is related to the energy density of the universe.

The expansion rate doesn't have to change smoothly if there is some mechanism to change the energy density. For example we think the expansion rate temporarily increased massively during a period called the inflationary epoch, though exactly why the energy density changed during this period is uncertain. However if we wind time backwards then barring (currently unknown) effects from quantum gravity we expect the universe to shrink to zero size in a finite time - this moment of time being the Big Bang. There isn't any known mechanism that could cause the universe to remain stable at a constant size then suddenly start expanding.

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