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Big bang is not an explosion, but an expansion of space and time. Universe had almost infinite density... wait a moment. If it had ALMOST infinite density, it had a certain volume, and thus, space already existed. Am I commiting a mistake?

I had this question after reading this post.

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  • $\begingroup$ Very high density does not imply a finite volume. Is that what you're asking about? $\endgroup$
    – HDE 226868 ♦
    Jan 3, 2016 at 19:31
  • $\begingroup$ Yes. Well as far as I know, density = mass / volume. If density and mass are not infinite, volume must be different from zero $\endgroup$
    – Felix L.
    Jan 3, 2016 at 21:33
  • $\begingroup$ Yes, the Big Bang did not happen at a point. $\endgroup$
    – HDE 226868 ♦
    Jan 3, 2016 at 22:36
  • $\begingroup$ Well thanks, now I see! If you could write your last comment as an answer I would accept it :) One last question: may it be (and it might be just speculative) that our universe is a subset of a higher dimensional space? Well I'm pretty sure I am not the first to think about it. Multiverse theory? Just wondering... $\endgroup$
    – Felix L.
    Jan 4, 2016 at 1:03
  • $\begingroup$ So let's rewind time a bit, say roughly 1 billion years ago every galaxies and stars must be very close to one another. This time let's rewind the clock all the way... I'll throw in some random number say 14 billion years ago there must be a location in the pre-spacetime that current spacetime is born... Of course pre-spacetime or whatever it is called cannot be detected or can it? $\endgroup$
    – user6760
    Jan 4, 2016 at 5:02

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Krotanix : nobody really knows the answer to this. We have good evidence that the universe is expanding, and we extrapolate this back to the big bang. Then some people say the universe was once the size of a grapefruit. See for example this. Since WMAP evidence in 2013 suggested the universe is flat, some people have started saying the observable universe was once the size of a grapefruit. And that the universe was always infinite. But this is a non-sequitur, and a non-answer.

Yes, it looks as if 13.8 billion years space had a much higher density and started expanding. And it may have had a certain volume, so space may have already existed. But nobody knows for sure.

Re your follow-on question, nobody has discovered hollowness or emptiness anywhere in the centre of the universe. The expansion is thought to be like that of a raisin cake rising in an oven. There's no big hole in the middle.

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You're right; space did exist. In fact, the universe was infinite in size at the moment of the Big Bang, and it still is. This is because a common misconception is wrong: The Big Bang did not happen at a point. Thinking of it like an explosion isn't correct.

You asked in a comment about the multiverse. This is speculative, to say the least. There are some theories involving objects called branes that posit that we are on the surface of some object floating through a higher dimensional "bulk". However, these theories of so-called large extra dimensions have yet to be supported by experimental evidence.

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  • $\begingroup$ Thanks, I'll solve it at some point of my life, If I find a moment to have a coffee and spend an afternoon thinking about it! XD $\endgroup$
    – Felix L.
    Jan 4, 2016 at 1:11
  • $\begingroup$ We do not know that the universe was infinite in size at the moment of the Big Bang, and it still is. This idea is not supported by experimental evidence. $\endgroup$
    – John Duffield
    Jan 11, 2016 at 8:06
  • $\begingroup$ @JohnDuffield The curvature of the universe is nearly zero, and the added assumption of homogeneity means that there can't really be an edge. Put the two together, and evidence shows that if we don't live in an infinite universe, it's certainly very big. $\endgroup$
    – HDE 226868 ♦
    Jan 11, 2016 at 21:26
  • $\begingroup$ @HDE 226868 : it's an assumption of homogeneity and isotropy. For all you know some guy 46 billion light years away is looking up at the clear night sky wondering why one half is black, or a mirror-version of the other, or something else. We just don't have any evidence for a universe that's a lot bigger than the observable universe, or an infinite universe. $\endgroup$
    – John Duffield
    Jan 12, 2016 at 13:28

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