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If the universe is infinite, it always had to be spatially infinite since at the big bang. And I'm assuming that cosmologists don't think that a Big Bang singularity actually occur in the real universe therefore the universe has to always be infinite from the beginning.

In the big bang theory says that the universe evolved from a denser early stage at the big bang. But there was already infinite space even at the big bang. So how can the infinite space be dense?

parker
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  • When in doubt about the Big Bang, refer to (mostly) John Rennie's excellent answer to the question Did the Universe happen at a a point /. At the end he specifically mentions the issue of the universe being infinite. There's also Luboš Motl's answer to How can something finite become infinite. – StephenG - Help Ukraine Oct 25 '17 at 21:34
  • I cannot be technical enough to post an answer. Consider that a spatially infinite universe does not imply density ----> 0. Current understanding is that universe was denser before because space did expand and it keeps expanding. In simple words the infinite size of NOW is bigger than the infinite in the past. Thinking of density (m/V) should help you. 1 g in a cubic meter is less than 10g in a cubic meter. Nothing is said about the total volume. Indeed it seems to me that without referring to density there will be no cosmological models at all. – Alchimista Oct 25 '17 at 21:40
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    Possible duplicate of Did the Big Bang happen at a point? – JMLCarter Oct 25 '17 at 22:52
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    I don't quite understand this question. Is it claiming that a spatially infinite universe must have (1) constant density, (2) increasing density, (3) zero density, but not (4) decreasing density? – Alfred Centauri Oct 26 '17 at 00:53
  • Could you explain why you claim that "If the universe is infinite, it had to be spatially infinite right after the big bang"? – Dr. Ikjyot Singh Kohli Oct 26 '17 at 03:47
  • @Dr See Luboš Motl's answer to How can something finite become infinite – parker Oct 26 '17 at 06:08
  • @AlfredCentauri I mean : How can a universe that has always been spatially infinite be dense ? – parker Oct 26 '17 at 06:29
  • It was denser locally. We have to talk about local density. The local density in every point was greater in the past. – Nemo Oct 26 '17 at 07:34
  • @parker Think of it in terms of coordinate systems. First make a coordinate system. You can only draw a little bit of course, but you can convince yourself that it is infinite in all directions, right? Then make another one, this time with the numbers spaced ten times as far apart, so 1 goes where 10 is in the first one, 2 where 20 was etc. Do we agree that the second is also infinite, just like the first? Do we also agree that the second is in some sense ten times larger than the first? – Thriveth Oct 26 '17 at 09:30
  • No it is not clearer. I think you are confusing the mathematical description with extrapolated coordinates for spacetime, with the reality under study. The only measurement/observation we have is at the here and now. We extrapolate to an infinite universe in the future extrapolating the present matter which defines space-time , with a diminishing density , and extrapolates to an infinite matter density in the past at a (0.0.0.0) point, calling it the Big Bang singularity (which quantum effects will fuzz over). read this https://physics.stackexchange.com/a/136861/104964, – anna v Oct 31 '17 at 18:05

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According to current theories, space itself is expanding. The galaxies that were closer together are now traveling (moving away) faster than light away from each other because the space between them is more.

Imagine this: you traveled 1 km. But now you are 10 kms away from the point you started because the extra 9kms just came into existence. Now things are less denser than before.

But there is another hypothesis, that space was always infinite. But now even though it is still much bigger than before and it is still infinite.

LostCause
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  • Not sure why I got a negative vote and also why this question is on hold. Not a single statement I made is incorrect, I challenge anyone who can prove otherwise. – LostCause Oct 26 '17 at 17:56