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We know, everything is in existence due to big bang from singularity. Time also started from that big bang point.

So, did big bang happen only once or it might happen again somewhere million time away from our observable universe ?

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There is a fundamental difficulty in answering this question because, while theories exist about multiple repeating events similar to the Big Bang it is difficult, if not impossible, to design experiments that would (dis)prove them - in other words they are potentially metaphysical theories. Roger Penrose, an eminent cosmologist/mathematician, recently proposed such a theory - which he sought to popularise in his book "Cycles of Time" - so certainly it has been suggested.

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We don't know it, because the question isn't feasible thus far to experiments or observation.

But it's possible to develop theories far beyond our observable universe, even far beyond the multiverse theories described here in Wikipedia. There exist theories about a tranfinite hierachy of meta-levels of theories in mathematical physics, each level comprising all possible theories on the level below (beyond any notion of infinity) and all their interconnections (including hierarchies of interconnections between interconnections). To explain this in reasonable detail would need hundreds of mind-boggling pages, at least. So I won't even try to explain more details. At the end it's not even clear, whether there is a difference between mathematics (including a transfinite number of meta-levels) and physics (including a transfinite number meta-levels).

The anthropic principle tries to answer the follow-up question "Why do we live in exactly this universe with exactly these laws of nature?". But this kind of answer isn't satisfying from a scientific point of view, when looking for cause and effect.

If you're looking for a final answer comprising everything: There is no, because the notion "everything" leads to paradoxa, when used in an absolute sense. (See Gödel's incompleteness theorems, if you need more details.)

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Time, as we know it, started with Big Bang, so there is no "before". And if there is "another" Big Bang (which I doubt as the universe is likely to expand forever) it will not be "another" Big Bang, but "the" Big Bang, as time will start anew with it.

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"which I doubt as the universe is likely to expand forever"... could you expand a bit? I thought entropy of the universe was a scientific fact and as a bit of research shows , it not that set in stone –  Jonathan Jan 28 at 13:22

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