(My question might be asked a lot on this stack and I thank you if you mind mentioning it.)
So we know that bigbang wasn't a firecracker explosion but it was the expansion or more precise, inflation of universe. Considering that the real infinity is impossible but universe is NOT limited, so it's kind of infinite AND at the time(a fraction of a fraction of time after bigbang) of bigbang universe not only WASN'T small, also it was still infinite.
Now my question is if we get closer and closer to the "exact" time of bigbang(knowing that we actually can't so it means we're going forever)the universe was still infinite in size AND it means that universe is infinite in time
(please consider that when scientists say the universe started at 13.8 billion y.o, it is an approximation, just like when we say two things reach a temperature equilibrium and is wrong since it takes infinite time for two things to become thermally balanced. And by that I mean the length of universe was so small that we can't calculate further)
Am I wrong or missing something?
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and also it means that universe is not expanding or inflating like a balloon since there is nothing beyond the universe and expansion of universe only caused the space in between objects to become larger(like what we are seeing now)– MpH81679Commented Mar 30, 2023 at 22:16
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Being infinite wouldn't prevent a universe from expanding. Infinity is not a fixed size.– D. HalseyCommented Mar 30, 2023 at 23:44
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1Can you ask on a physics or astrophysics forum? It seems astrophysics is the right way to settle this.– FrankCommented Mar 31, 2023 at 1:23
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2@user4894 Isn't that exactly what he means? Also, isn't "distances get bigger" and "space gets larger" one and the same thing?– kutschkemCommented Mar 31, 2023 at 6:36
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1@Conifoild Friedmann universes can be either finite or infinite, depending on the total density of matter & energy. Current estimates of the parameters suggest that the universe could, in fact, be infinite. If so, it has always been infinite - never changing from finite to infinite. There are many related questions on the physics site. The best one (in my opinion) is this one: physics.stackexchange.com/questions/136860/…– D. HalseyCommented Mar 31, 2023 at 11:52
1 Answer
Based on current observations, it appears that the Universe is mostly empty space with a bunch of objects (matter or energy) scattered in it. So when you ask about the extent of the Universe, are you asking about the objects, or the space?
It seems plausible that the space just goes on forever. But it has not yet been proven that the space somehow "ends" at one point. It could also be that the space itself is finite but it loops back on itself. These are unanswered question and nobody knows the truth, it is beyond contemporary physics or cosmology (not to say there isn't interesting speculation, though).
The other question is whether the matter, as in the stars and planets and so on, goes on forever. So far it's only been observed up to a point, because the speed of light and the age of the universe are both finite. It doesn't look like there's any sign of it stopping or even thinning out. As far as you can see, the stars and the planets also seem to just go on and on. You have to keep in mind as well that looking at stars far away is looking back in time - nobody really knows what state these distant galaxies are in today, you can only see how they were millions of years ago.
Trivially, if the space is infinite, and since photons are fairly stable, eventually at least some rays of light would fly out very far into "space", which would push out the "boundary" of the physical "stuff" of the universe. Up to you if you consider that finite or not.
If the matter did go on forever, at a rough look it seems like that would imply there's infinite matter in the Universe. And if that came from the Big Bang, then it would imply the original, contracted Universe was infinitely massive. Alternatively, the mass could have been created somehow after the Big Bang. I believe these are all open questions as well. It is not known exactly how much mass participated in the creation of the Universe, and how likely it is that more mass or energy was created afterwards.
However, interestingly, from our perspective the Universe is very much finite, because we are limited by relativity in how much of it we can observe or visit. Perhaps one day, new physics will be discovered that allow us to travel faster than light or observe more distant objects, but currently there is not a known way.
