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I have heard in multiple locations that in the case of a flat universe, the universe is infinite. This seems to present multiple issues:

a) If a flat universe were infinite, would would the space of the Big Bang then not be infinite?

b) If the above is not the case, that seems to suggest that the geometry of the universe can evolve over time?

c) If the space of the Big Bang was infinite, and since space, time, matter, and energy came from the Big Bang, wouldn't the density of the universe at the start of the universe be equal to that of the universe now?

d) If the above is true, why are we told that the universe began in a very dense state? And also, if the universe indeed was more dense initially, would this not suggest that space must not be flat?

Note: Some of these questions depend on the answer to previous questions.

Kieran Moynihan
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  • Hi Kieran. I've marked this as a duplicate because I think all your questions are answered by the linked question. If you feel this isn't the case perhaps you could edit your question to reference the earlier question and highlight the specific points you feel haven't been addressed. – John Rennie Jul 11 '16 at 05:06
  • You are talking about solutions in general relativity, but you aren't talking about "the universe". The observed universe is perfectly finite, even though it is of changing size. What's "behind" the event horizon of the big bang is unknown and in principle unknowable. It could be flat, it could be a pretzel, it could be finite or infinite, it could be the back of a turtle that rides on a turtle that rides on a turtle... we will most likely never know. Keep this in mind while you are reading the links that explain what general relativity (which is not the final theory) can (not) tell us. – CuriousOne Jul 11 '16 at 08:30

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