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 philosphy 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.

 

[hellfire]

The Universe started as a singularity of infinite density, but not necesary of zero volume.

I fail to understand how the universe could have been spatially infinite at the beginning of time. For every cosmological model the volume contained within every boundary at the big bang is zero because for a zero scale factor a(t=0) = 0 the volume element is zero (or all distances are zero in the singularity). A different situation arises as soon as one considers any time slightly greater than zero, avoiding the singularity.

 

[EL]

I fail to understand how the universe could have been spatially infinite at the beginning of time. For every cosmological model the volume contained within every boundary at the big bang is zero because for a zero scale factor a(t=0) = 0 the volume element is zero (or all distances are zero in the singularity). A different situation arises as soon as one considers any time slightly greater than zero, avoiding the singularity.

I agree with that using the term "spatially infinite" at t=0 may not really have any meaning, since as you say the scale factor is zero. However, what I wanted to point out is that it need not have started as what we intuitively think of as a point either. I.e. the reasoning Danger is using, that it has to be of finite size, does not hold.
The point I wanted to make is that even though the Universe started as a singularity, it may be infinite at any t>0. Wheter one call it infinite or not at t=0 is more of a personal taste.
Anyway, we do not really know how to describe the universe as time approaches zero, since we don't know about what laws will hold.

 

[SpaceTiger]

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.

I think Ned Wright addresses this in a fairly straightforward manner:

http://www.astro.ucla.edu/~wright/infpoint.html"

 

[EL]

I think Ned Wright addresses this in a fairly straightforward manner:
http://www.astro.ucla.edu/~wright/infpoint.html"

Yes! And the question adressed is what confuses many people, since it is simply not the whole thruth. The Universe (i.e. the whole Universe) doesn't have to be concentrated into one point at the Big Bang.

 

[Danger]

Thanks, ST. It's not all that straight-forward to me, though, because I'm missing most of the educational background to follow it properly. I'll certainly read as much as I can when I have more time, but the math is really going to mess me up.

 

[SpaceTiger]

Thanks, ST. It's not all that straight-forward to me, though, because I'm missing most of the educational background to follow it properly. I'll certainly read as much as I can when I have more time, but the math is really going to mess me up.

Are you sure you're looking at the right thing? The link only contains two paragraphs and a diagram.

 

[mars2]

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

My English is poor.So ...The threads I read is more than I reply .When I meet lion in threads ,I often use this on line http://cb.kingsoft.com/".
"involuted" mean "hard to learn" 、"complex".

I want to improve my English .^_^
The universe is so strange .I think if I fly in one direction,and many years later I can back ~~~ kidding^^

 

[EL]

My English is poor

Well, in this case it was my English which was poor! Thanks for the clarification.

 

[Danger]

Are you sure you're looking at the right thing?

Oops!
I was reading the linked tutorials at the bottom of the page. Unfortunately, I still can't quite grasp the concept. It was always my understanding that the entire universe erupted from the BB. This makes it look as if BB happened inside a pre-existing universe. What the hey am I missing here?

 

[Dmstifik8ion]

Infinity does not exist in nature (out there in reality land). It is a creation of the human mind, (an abstract in mathematics, like zero), to help us to comprehend the relationship between nothing and everything else. Things, (that are inclusive in the set of existents) seem infinite when we are no longer able to attach any concepts or ideas to what something is (its nature). Learning about, understanding and defining the universe will lead us to an appreciation of its finite quality.
This is my humble attempt to clarify something I know very little about. My hope is that it contributes less confusion to the subject than it eliminates. Please feel free to clarify this 'clarification' on a really big subject.

 

[Dmstifik8ion]

When we have achieved the ability to wrap our minds around the universe we will then, thereby, have encircled it finite quality.

 

[EL]

Oops!
It was always my understanding that the entire universe erupted from the BB.

You're correct in that!

This makes it look as if BB happened inside a pre-existing universe.

I cannot really see that. I think you may still be stuck to the idea that the BB started as what we intuitively think of as a "point". Anyway, it's kind of hard for our minds to grasp how the BB really looks like. What matters in this case is that GR allows for the existence of an infinite universe with a singularity as starting point.

 

[Danger]

Yeah, I'm stuck on the point thing. That's the only way that I've ever seen it described. I'll try to get caught up on the rest of it. Thanks for your patience.

 

[CJames]

I'm pretty sure that COBE and WMAP both suggest, though don't prove, that the universe is flat and therefore most likely infinite. That is, it started out infinite and will always be infinite. At the beginning of the universe, the universe was infinitely dense and infinitely vast. Now the density has dropped but it is still infinite. That is the prevailing theory at this time, although it is possible to have a finite universe that "wraps around" like a pac-man screen without curving, fulfilling the expectation that the universe is indeed flat.

 

[hellfire]

I'm pretty sure that COBE and WMAP both suggest, though don't prove, that the universe is flat and therefore most likely infinite. That is, it started out infinite and will always be infinite. At the beginning of the universe, the universe was infinitely dense and infinitely vast. Now the density has dropped but it is still infinite. That is the prevailing theory at this time, although it is possible to have a finite universe that "wraps around" like a pac-man screen without curving, fulfilling the expectation that the universe is indeed flat.

We may have also an universe that is nearly flat. This would be consistent with inflation, that drives the energy density towards the critical energy density but does not necessarily lead to \Omega = 1. If \Omega \simeq 1, the radius of curvature of the universe is much larger than the Hubble radius. We would measure a flat universe, but it might be, however, curved. A value \Omega = 1 seams to be a very fine tuning.

 

[alfredblase]

There is something I don't understand here. How can physicists and astronomers be talking about anything but the observable universe? I mean, you can conjecture about what is outside the observable universe but never prove your conjectures... right? The most you can say is we cannot, in anyway, describe the universe outside our observable universe; as far we can measure the observable universe is flat and by definition finite.

 

[EL]

There is something I don't understand here. How can physicists and astronomers be talking about anything but the observable universe? I mean, you can conjecture about what is outside the observable universe but never prove your conjectures... right? The most you can say is we cannot, in anyway, describe the universe outside our observable universe; as far we can measure the observable universe is flat and by definition finite.

Often when scientists talk about "the universe" they mean just the observable universe, for instance when one here someone estimating "the number of particles in the universe", or "the total mass of the universe".

As you say, it may sound meaningless to talk about something which we cannot observe. However, we can see from the CMB that parts of the universe which are now causaly disconnected must have been connected at some earlier time. The solution to this problem is probably inflation.
Hence, the parts of the universe which is at the time not observable for us, may have been observable before inflation, and hence it makes sense, at least in certian cases, to discuss the "whole" universe.

 

[alfredblase]

Are you saying that before inflation the "whole" universe was causally connected, or just that more of the universe was causally connected? In either case it only makes sense to talk about outside our currently observable universe when referring to a period of time before inflation right? If that is true are you saying that by knowing how at least some of the currently unobservable universe was before inflation, that we can predict how it is today? I tend to disagree, simply because predictions that cannot be proven or disproven are not to be accepted as truth in physics.

 

[EL]

Are you saying that before inflation the "whole" universe was causally connected, or just that more of the universe was causally connected?

That must depend on the geometry of the universe.

In either case it only makes sense to talk about outside our currently observable universe when referring to a period of time before inflation right? If that is true are you saying that by knowing how at least some of the currently unobservable universe was before inflation, that we can predict how it is today? I tend to disagree, simply because predictions that cannot be proven or disproven are not to be accepted as truth in physics.

I agree with you that it is actually not very scientifically correct to talk about something which cannot be proven by observations. But the point is that if we agree on sticking to GR and standard cosmology, the theory will model a whole universe, not just some observable part. Actually I think that Dangers statement that the universe must be finite included the "whole" universe as modeled by standard cosmology. But as you say, by strictly defining the universe as the observable one, it of course is finite by definition.

 

[hellfire]

The homogeneity we see today in the CMB should had been impossible to reach without inflation, because at the time of the formation of the cosmic background (400.000 years after the big-bang), a causal connection was given only in patches of 1 angular degree in the CMB sky-map we observe today. However, the CMB map is extremely homogeneous throughout the whole sky. To explain this inflation comes into play.

According to inflation there was a causally connected partch that inflated immediately after the big-bang to a size that is equal or larger than the observable universe today. Therefore the causal connection of in our inflated bubble was given before the formation of the cosmic background.

There may be deviations from homogeneity and flatness at larger scales. However, it would be a great coincidence if the inflated bubble we are located in would be exactly equal to the size of the observable universe today. The size of the current observable universe has no especial meaning in the theory of inflation and it is an essential feature of inflation to involve superhorizon physics during previous epochs of the history of the universe.