This is based on the theory that space is curved. As an analogy, imagine that you depart on a powerful long-range starship in one direction. Given enough time, you will come back to your starting point (though it may look different since stars and planets will probably have moved).
Word, man! Just word!
There are a few misconceptions in this thread that I think haven’t been fully cleared up that I’d like to address.
First of all, the universe is not infinite. Think about it: how can something be both infinite and expanding? You can’t make something bigger that’s already as big as big can be.
The universe is, however, PRACTICALLY infinite, in the sense that we could never reach or see the edge. This is because of a period of the universe’s early life after the Big Bang known as inflation. During inflation, the speed of the universe’s expansion was faster than the speed of light. Because we can never go faster than the speed of light, we can never catch up to that accelerating edge of the universe, because there was a time when it was moving away from us faster than we can catch up to it. So, you could not, as a few posters have suggested, circumnavigate the universe, given enough time (at least, not under the most widely-accepted model of the universe). You could never reach the edge that is moving away from you. The balloon analogy is still a good one, provided that you assume someone is constantly blowing the balloon up faster than you can travel across it.
Time in the universe is not infinite, either, in the sense that there is not an unlimited time for things to happen. Sadly, life in the universe (and possibly the universe itself) will end, although exactly how it will end depends on a number of factors about which science currently doesn’t have enough information to state with certainty. The primary determining factor that we just don’t know much about is dark energy, the mysterious observed phenomenon that the universe is not only expanding, but accelerating. The force of gravity of all the matter in the universe should be slowing down the expansion, but we observe the opposite. The force driving this acceleration is called dark energy.
If dark energy is strong enough to continue the acceleration, then eventually the space in the universe will become too large, matter and energy (one and the same, as described in E=MC^2) will be spread too thin, star formation ceases, the temperature everywhere approaches absolute zero, and the entire universe freezes to death. This is known as the “Big Freeze.”
If the force of gravity eventually overcomes the expansive force of dark energy, eventually the expansion will slow down, stop, and reverse, resulting in an end of the universe model know as the “Big Crunch,” which I believe is a pretty self-explanatory name.
There are other models, too, such as the Big Rip, the Big Bounce (cosmologists like “Big” things), Heat Death, and something fun called a false vacuum (it’s not fun; it makes my head hurt. Investigate at your own peril).
The moral of the story is, the universe has a theoretical boundary in both space and time, but the latter we don’t know, and the former we can’t know. Otherwise, the basic premise of most of the posts (the universe is space, it isn’t expanding into anything, the observable universe is different from the actual universe, etc) is accurate. I just wanted to make a few points I felt hadn’t already been made. Since this post is already too long, there are a few things I may not have explained fully, so feel free to respond with any questions, since I recognize I may have left a few holes unfilled. Also, I study astrophysics as a hobby, nothing else, so I could be mischaracterizing some information. This is just a layperson’s understanding of the situation. If anyone with a PhD feels I’ve done the scientific community wrong and wants to correct me, please do so.
It’s entirely possible for infinite sized things to expand. Like even numbers are just as infinite as natural numbers, even if spaced further apart. Or the hotel with infinite rooms that can make a vacancy by switching all the occupants to the next higher room number, freeing up room 1. Or just imagine an infinite balloon, the dots can still separate indefinitely. If you think of the big bang as a moment infinite density expanding towards lower density, it doesn’t really matter if the volume was infinite all along.
While the evidence might currently point towards a finite universe, I don’t think an infinite universe has been conclusively ruled out yet. But, hey, I’m just a layman too.
But remember that there are as many even numbers as natural numbers (since you can match them up 1-1 without leaving out any) and Cantor’s Infinite hotel still has the name number of residents after another one checks in (since you can match each member of the old population up 1-1 with a member of the new population without leaving out any), so it doesn’t make a lot of sense to talk about “infinite sized things” expanding.
Like with anything in mathematics/physics you just need to make sure when you talk about the expansion of space you’ve got the rigourous definitions to back it up. Really when talking about the expansion of space we’re talking about the behaviour of FLRW coordinatesand in that context it makes sense to talk about expansion of space whether the foliation of spacetime by FLRW coordinates leads to spatial slices of finite or infinite volume.
First point: It is not known whether the Universe is finite or infinite. If it is finite, then it is larger than our capability of measuring. Or best measurements indicate that it is flat or very close to flat, and the simplest models of a flat universe are infinite, but models which are only slightly more complicated would allow for the Universe to be both flat and finite. The Universe being infinite, meanwhile, would in no way be in conflict with the observed fact that it’s expanding.
Second point: The Universe has no boundary. Either it is infinite, or it wraps around upon itself in some way (or, possibly, both, in different directions). If it wraps around upon itself, this might be a consequence of the curvature of space, but it’s possible for a flat space to wrap around upon itself, and likewise possible for a curved space to be infinite.
Third point: If the Universe is infinite, then it has always been infinite, at any time after the moment of t = 0. At the moment of t = 0 itself, the size of the Universe (or indeed, any physical property of anything) is undefined.
When I was in junior high, I was under the impression that Einstein’s statement that the universe is “finite yet unbounded” meant that it is the 3-dimensional surface of an expanding 4-dimensional space, ostensibly a perfect sphere. Now I know that this is not believed to be true—or is it? Are there any cosmologists who adhere to this simple model? From what I understand, wiki being my primary source, it’s not universally accepted that all extra dimensions are compactified. I guess the answer to my question (which I’m already beginning to suspect is extremely poorly worded) is no, because we now believe the universe is flat, right?
OK, I should have made it clearer that the the finity or infinity of the universe is currently undetermined in science, and is a matter of debate within the scientific community, and I mostly just described one model. I was trying to describe the model that seemed to jibe most with the original question asked, and I didn’t make it clear enough that it was still an unanswered question. I also slightly mischaracterized the boundary of the universe. You’re right, Chronos, that the universe doesn’t have a defined boundary. I was just trying to highlight that you could never reach the “edge” because any edge would be moving away from an observer faster than that observer could approach it.
That being said, I admit that I am unclear on the relationship between an infinite universe and an expanding one. I characterized the two as being somewhat contradictory. I admit that could be wrong, but I’m having some trouble seeing how. I think your explanation, DrCube, is a little off. You’re talking about rearranging infinite things, but the infinite objects you describe aren’t actually getting bigger. The numbers aren’t expanding, just matching, and the hotel isn’t actually getting bigger, the guests are being rearranged. If anyone can explain this relationship a little more clearly, I think that would clear this up for anyone else who might be confused.
Quoth Washoe:
That is one possible model for such a universe, but there are many others. If that model is in fact true, then the characteristic scale for the curvature (you can think of it as the radius of that sphere) must be absolutely immense, to reconcile with the observation that space, on cosmological scales, is extremely close to flat.
Quoth Thugs And Kisses:
The analogy I prefer to use is an infinite sheet of graph paper. If you went from an infinite sheet of graph paper ruled at five squares to an inch to one ruled four squares to an inch, one might reasonably say that the paper grew by 25% in each of its dimensions, despite being infinite.
Well what exactly did Einstein mean when he said that? Or did he even have a specific model in mind?
Well, my ability to read folks’ minds is pretty rusty even when they’re still living, so take this with a grain of salt, but if Einstein ever said that the Universe is finite but unbounded, he probably had a fairly large set of possible models in mind. Frankly, though, it doesn’t sound characteristic of Einstein to ambitiously claim without evidence that the Universe be finite.
You are assuming the universe acts like a box of marbles or something existing in something even larger. If the universe is infinite then it’s not really getting “bigger”. However all the objects in it can continue to move further apart from each other.
Well, if the universe was infinite AND infinitely old, the sky would appear as a solid white light all the time.