Is the Universe Truly Infinite and Ever-Expanding?

[Supaiku]

I'm wondering if there's science that supports an infinite (in some way) universe

 

[Danger]

I believe you'll find that it's the other way around. Since we know when the universe began, and how fast it's expanding, we know that it has to be of finite size. It's generally thought to be unbounded, though, so you can go in the same direction forever without hitting the end.

 

[Macro]

The only thing infinite about the universe is its future.
We see that it is expanding at an accelerating rate.
This implies that temporally its infinite; no future crunch.

 

[Mr. dude]

ARRRRGGGG! If the universe is finite, what is Outside the universe?!? This puzzels me.

 

[Danger]

It puzzles everyone. We just have to get over it.

The problem is that there isn't really any 'outside'.

 

[Chronos]

Well put, danger. The universe is a pretty weird place. It defies common sense - and offends just about every other sense sooner or later. How does something come from nothing, something not come from nothing ... or vice-versa? At some point there is no escape from saying 'dammit, nobody knows, that's just the way it is so live with it. So it's perfectly OK to say the universe is unbounded and there is nothing outside the universe. These premises are irrefutable by definition.

 

[saltydog]

I don't feel it is a problem: Critical points in my opinion can offer some assistance in understanding some perplexing questions about the Universe. It makes perfect sense to me to accept a bifurcation as the cause of the Big Bang: a critical point separating the pre-existence from our own existence without needing to understand what "tipped" the pre-existence past this bifurcation point. Our world is FULL of non-linearities, critical points, and abrupt qualitative changes. I'm convinced the non-linear world I see outside of my window is but a thumbprint of a larger phenomenon that created our Universe. This too allows me to come to terms with an "endless" and "timeless" Universe: At some point (in time and space), a bifurcation point is reached which changes the qualitative nature of existence rendering our metric concepts inapplicable in the same manner as rendering "swimming" so when the water temperature drops below freezing. Works for me anyway.

 

[Ro69]

ARRRRGGGG! If the universe is finite, what is Outside the universe?

Well, another Universe is outside ours. Just ask any Graviton.

 

[EL]

Since we know when the universe began, and how fast it's expanding, we know that it has to be of finite size.

I will not agree with this. The universe could as well be (spatially) infinite. Most observations actually points at a flat universe, i.e. an infinite universe.
Why do you say the universe has to be of finite size?
The size of the universe could be infinite already at the Big Bang (which is when the universe had an infinite density).

 

[The_Bled]

My opinion is that the universe is what is called in mathematics, a "three-dimensional torus". There are no edges, but it isn't infinite. Think of an ant on a beachball walking around on its surface. The universe is a three-dimensional flat surface that wraps around a four-dimensional sphere.

 

[EL]

My opinion is that the universe is what is called in mathematics, a "three-dimensional torus". There are no edges, but it isn't infinite. Think of an ant on a beachball walking around on its surface. The universe is a three-dimensional flat surface that wraps around a four-dimensional sphere.

Sure, that's also possible. No one knows wheter the universe is infinite or not. (And it certainly does not care about "opinions" )

 

[The_Bled]

Sure, that's also possible. No one knows wheter the universe is infinite or not. (And it certainly does not care about "opinions" )

Well, since there are so many theories and nobody has ever flown to the edge of the universe, I think I'm stickin' to that.

 

[EL]

Well, since there are so many theories and nobody has ever flown to the edge of the universe, I think I'm stickin' to that.

I notice the "wink", but may I ask why you choose to stick to just one option?

 

[The_Bled]

I notice the "wink", but may I ask why you choose to stick to just one option?

I don't just stick to one option... that just seems to be like the most plausible theory to me. But... you don't know which one is right, so you talk about all of them... Whatever, back to the infinite universe topic.

 

[SpaceTiger]

After 100 years of observational cosmology, I would say that the (spatially) finite/infinite universe controversy is still completely unresolved. Not only are the data fully consistent with either possibility, but we have a successful theory (inflation) that suggests that the present-day universe would look pretty much the same either way.

 

[mars2]

I think it is a philosophic problem .And ...the universe is finite ,but it have no boundary.It seems like ...you walking on the ground but never fall over the cliff and leave our Earth ~~

^_^

 

[saltydog]

. . . we have a successful theory (inflation) that suggests that the present-day universe would look pretty much the same either way.

Is this a cause for pause? Should not a "successful" theory account for only ONE possibility? I don't enjoy criticizing, esp. a field I love but to me, that's like saying the results suggest it's less than one and bigger than one at the same time.

 

[EL]

I don't just stick to one option... that just seems to be like the most plausible theory to me. But... you don't know which one is right, so you talk about all of them... Whatever, back to the infinite universe topic.

The thought of a finite universe makes me sleep better at night too, it's easier for me to accept a universe with a finite amount of matter than an infinite amount...but, who says I'm supposed to sleep well at night .
Anyway, as you say, back to the main topic...

 

[SpaceTiger]

Is this a cause for pause? Should not a "successful" theory account for only ONE possibility? I don't enjoy criticizing, esp. a field I love but to me, that's like saying the results suggest it's less than one and bigger than one at the same time.

Well, no, I wouldn't say it's like that at all, nor do I think that every theory has to give us definite answers. Inflation is saying that we shouldn't be surprised that we're having trouble distinguishing between the finite and infinite universe. One of the main points of inflation was that it explained, without fine-tuning, how the universe could appear almost perfectly flat. In doing so, it also made other predictions that were testable, some of which have been verified.

If it happens that the universe conspires to make certain things difficult to observe, then we'll just have to live with that. If we rejected all such theories, there would be no quantum mechanics or chaos theory.

 

[EL]

I think it is a philosophic problem .And ...the universe is finite ,but it have no boundary.It seems like ...you walking on the ground but never fall over the cliff and leave our Earth ~~
^_^

Think of a 2D universe situated on the surface of a sphere. The area of this universe is finite, but anyway it has no boundaries where the 2D beings living there can "fall over a cliff".
Now you of course have to generalize this picture to a universe with 3 spatial dimensions, where the "area" is replaced by the "volume". Although our brains cannot really handle this generalization, this would give you a hint of how a universe could be finite and still unbounded.

 

[ComputerGeek]

I believe you'll find that it's the other way around. Since we know when the universe began, and how fast it's expanding, we know that it has to be of finite size. It's generally thought to be unbounded, though, so you can go in the same direction forever without hitting the end.

actually, I think it is infinite yet bounded.

 

[Danger]

Now you're just teasing...

What I was getting at is that if it formed a finite time ago, and is expanding at finite speed, it can't be infinite. There's a maximum size that it could have reached by now. 'Infinite' implies that it had no beginning, not just no end.
Perhaps I'm using the wrong definition of 'infinite' here, but that's what it means to me.

 

[Garth]

actually, I think it is infinite yet bounded.

How?

Garth

 

[alfredblase]

The universe is defined by what we as individuals percieve. As we all believe that we can only percieve up to a certain distance, then the universe is finite without question. And since there is no diference as to whether I look in this direction or in that direction, we can see equally far in all directions, so at any given moment in time the universe is spherical with me as the origin of that sphere.

Further I would add that speculating about what is outside our universe, as we believe that it can never be interacted with, is an exercise in philosophy at best. And philosophy can be very ...

 

[mars2]

Think of a 2D universe situated on the surface of a sphere. The area of this universe is finite, but anyway it has no boundaries where the 2D beings living there can "fall over a cliff".
Now you of course have to generalize this picture to a universe with 3 spatial dimensions, where the "area" is replaced by the "volume". Although our brains cannot really handle this generalization, this would give you a hint of how a universe could be finite and still unbounded.

Oh~~It is so involuted.:zzz:

 

[EL]

What I was getting at is that if it formed a finite time ago, and is expanding at finite speed, it can't be infinite.

Why can it not? If the universe was (spatially) infinite at Big Bang, which may have been the case, it should of course be infinite now also. (Or are you just talking about the today observable universe?)

 

[EL]

Oh~~It is so involuted.:zzz:

Sorry, I can't find "involuted" in my dictionary...what does it mean?

 

[Danger]

If the universe was (spatially) infinite at Big Bang, which may have been the case,

You lost me on that one. How could it have been infinite when it formed? If it started at zero volume/infinite density, and is now at medium volume/medium density, then it had to have passed through small volume/high density on the way.

 

[EL]

How could it have been infinite when it formed?

Why not? Why must it have been finite?

If it started at zero volume/infinite density

Why must it have startet at zero volume?
The Universe started as a singularity of infinite density, but not necesary of zero volume. Think of an infinite plane with small dots on it, representing the matter distribution today. Now proceed backwards in time, i.e. move the dots closer and closer together, until you reach an infinite density, i.e. the singularity. The plane still has an infinite area though! You get my point?
This of course requires an infinite amount of matter in the universe.

Hence the Universe could always have been spatially infintie, although the density has changed with time.

Edit: Just to make it clearer: When I say "move the dots closer and closer", I of course mean that the space between the points shrinks, not that some force acts on the dots.

 

[Danger]

I'm going to wait for an expert's take on this. Again, I might be misusing the term 'infinite'. To my mind, the only way that something can have infinite density is if it has zero volume.