How do we know space is not infinite?

[zeffur7]

How do we know space is not infinite? How can we be sure that the objects we see moving through space aren't just moving away from each other through space, rather than 'expanding space' as they change positions in outer space?

 

[phinds]

How do we know space is not infinite? How can we be sure that the objects we see moving through space aren't just moving away from each other through space, rather than 'expanding space' as they change positions in outer space?

You have a fundamental misconception here if you think "moving away IN space" and "moving away due to space expanding" cannot be happening at the same time.

We do NOT know for sure that space is or is not infinite but we DO know that space is expanding. If I understand it correctly, all galaxies are moving IN space in random directions but their motion relative to each other (small) is totally dominated (NOW) by their apparent motion (LARGE) due to the expansion of space.

EDIT: if galaxies were NOT moving IN space in random directions, and in particular if they were all moving away from US (or from ANY single point) IN space, this would imply a point-position for the big bang and I don't think you'll find any support in science for that point of view.

FURTHER EDIT: Just in case you don't get another point, expanding space and infinite space are NOT in any way contradictory terms.

 

[chrisbaird]

The observable universe is finite. The big bang happened about 14 billion years ago, so light from points in space that are 14 billion-light years away (actually 46 because of expansion) are just now reaching us and showing us the big bang. If we try to see farther than 46 billion light years away, we can't, because there is a wall of light caused by the big bang we are trying to see passed.

The unobservable universe may be infinite, but we can't know because we can't see it. But even if we could, how can you prove something like "infinite". It would take an infinite amount of time to measure something infinitely large.

 

[phinds]

The observable universe is finite. The big bang happened about 14 billion years ago, so light from points in space that are 14 billion-light years away (actually 93 because of expansion) are just now reaching us and showing us the big bang. If we try to see farther than 93 billion light years away, we can't, because there is a wall of light caused by the big bang we are trying to see passed.
.

Zeffur, the "wall of light" mentioned here is best understood if you look up "surface of last scattering"

 

[soulcoma]

Just to be a bit more concise...yes, the Observable Universe is always finite, but also increasing every year.
At present the "[[PLAIN]http://en.wikipedia.org/wiki/Comoving_distance[/URL] has a radius of about 47 billion light years, while the diameter is, of course, doubled to around 93 billion light years. We can't actually see objects 93 billion light years away in one direction.

 

[Imax]

How do we know space is not infinite?

Cosmic microwave background radiation?

 

[phinds]

Cosmic microwave background radiation?

The CMB tells us absolutly nothing about whether the universe is infinite or not. What is it that makes you think it does?

 

[Rishavutkarsh]

The CMB tells us absolutly nothing about whether the universe is infinite or not. What is it that makes you think it does?

but does infinite expand? i mean it's already infinite and it's getting bigger infinite?

 

[Chronos]

Infinite is unbounded, so I fail to see your point.

 

[jackmell]

How do we know space is not infinite?

I suspect it will turn out to be more like a paradigm shift. Doesn't that make more sense rather than the two options of finite or infinite especially when you consider how our views of the Universe have changed in history? Our understanding of Nature I'm optimistic will continue to improve, perhaps we'll replace General Relativity with something more broad, new discoveries will be made, and our views of the Universe will change once again. It may not be just more of the same like 2000 years ago when one considered walking along the "flat" earth. The spherical Earth was a paradigm shift and resolved the paradox of "falling" off. Perhaps will will reach another in the future which may resolve our puzzle about the "size" of the Universe. That to me makes more sense than wondering if it's finite or infinite.

 

[yenchin]

but does infinite expand? i mean it's already infinite and it's getting bigger infinite?

Infinitely large can expand to still be...well...infinitely large. For example, the set of all natural number has the same "size" as the set of all even natural numbers. Check out the idea of cardinality of infinite set.

 

[yenchin]

I suspect it will turn out to be more like a paradigm shift. Doesn't that make more sense rather than the two options of finite or infinite especially when you consider how our views of the Universe have changed in history? Our understanding of Nature I'm optimistic will continue to improve, perhaps we'll replace General Relativity with something more broad, new discoveries will be made, and our views of the Universe will change once again. It may not be just more of the same like 2000 years ago when one considered walking along the "flat" earth. The spherical Earth was a paradigm shift and resolved the paradox of "falling" off. Perhaps will will reach another in the future which may resolve our puzzle about the "size" of the Universe. That to me makes more sense than wondering if it's finite or infinite.

It is possible that the universe is finite but with non-trivial topology.

 

[phinds]

... it's already infinite and it's getting bigger infinite?

exactly

 

[Oldfart]

exactly

Bigger infinite?? Wouldn't that imply a bounded space?

 

[yenchin]

Bigger infinite?? Wouldn't that imply a bounded space?

Why would it imply a bounded space? Forget about the universe for now and read about "[URL Hotel[/URL].

 

[Oldfart]

Why would it imply a bounded space?

Well, it just seemed to me that if space was already infinite, it would be meaningless to consider that it was becoming infiniter. Anyway, thanks -- I read about Hilbert's Hotel, no joy. I evidently have a personel conceptual problem with infinity, possibly stemming from incorrectly thinking that if something increases, it increases from a defined point in space and time. and the amount of increase would be measured from that point.

 

[phinds]

Well, it just seemed to me that if space was already infinite, it would be meaningless to consider that it was becoming infiniter. Anyway, thanks -- I read about Hilbert's Hotel, no joy. I evidently have a personel conceptual problem with infinity, possibly stemming from incorrectly thinking that if something increases, it increases from a defined point in space and time. and the amount of increase would be measured from that point.

Yes, that is DEFINITELY a misconception when it comes to infinities. Do you have a problem with the following algebraic statement?

infinity + 1 = infinity

The thing represented by the word "infinity" is EXACTLY the same on both sides of the equation. If you can't get your head around this, then you will not get any further with the concept of infinity. This, by the way, is just an algebraic version of Hilbert's Hotel.

 

[Oldfart]

Yes, that is DEFINITELY a misconception when it comes to infinities. Do you have a problem with the following algebraic statement?

infinity + 1 = infinity

The thing represented by the word "infinity" is EXACTLY the same on both sides of the equation. If you can't get your head around this, then you will not get any further with the concept of infinity. This, by the way, is just an algebraic version of Hilbert's Hotel.

Oh, I can get my head around that equation OK. The hard part is ascribing meaning to it. Like, what's the point? Can infinity be increased? Or is infinity plus 1 senseless?

My problem, not yours.,,

 

[Imax]

The CMB tells us absolutly nothing about whether the universe is infinite or not. What is it that makes you think it does?

CMB radiation is almost homogeneous, but it has small differences. If you build models were space can be infinite or can be compact, it turns out that compact space models can explain those small differences better that infinite space models.

 

[phinds]

CMB radiation is almost homogeneous, but it has small differences. If you build models were space can be infinite or can be compact, it turns out that compact space models can explain those small differences better that infinite space models.

How "compact" are we talking about here? I'm assuming that you are not implying small here, just not infinite.

 

[Imax]

Billions of light years across, but it’s not static. Space seems to be expanding, and that expansion seems to be accelerating.

 

[phinds]

Billions of light years across, but it’s not static. Space seems to be expanding, and that expansion seems to be accelerating.

"Billions" could be smaller than the diameter of the observable universe. I assume you DON'T mean that, but I now assume that you ARE talking about VERY small --- that is, not much bigger than the observable universe. I certainly can't argue persuasively that this is impossible, but I believe the consensus is that it is MUCH bigger than that at the very least.

 

[Imax]

It depends on whether or not we can identify ghost images. If space is compact, then light from a distant galaxy can travel in two directions, towards us or it can circumnavigate the universe and appear somewhere else as a ghost image.

 

[phinds]

It depends on whether or not we can identify ghost images. If space is compact, then light from a distant galaxy can travel in two directions, towards us or it can circumnavigate the universe and appear somewhere else as a ghost image.

I completely understand this concept, but would be astounded if it ever turned out to be the case. I DO believe it possible that the universe has the topology that you suggest but even if it does, the thought that it could be small enough that the light would actually reach us from both directions just strikes me as so unlikely as to be silly.

 

[Imax]

It’s not small. Light from ghosts can take billions of years to reach us

 

[Chronos]

The observable universe is finite. Given that is the only part observationally accessible, the rest is scientifically irrelevant until an observationally detectable effect on the observable part is confirmed.

 

[phinds]

It’s not small. Light from ghosts can take billions of years to reach us

since the current diameter of the observable universe is pushing 100 billion light years, I consider "billions" to be QUITE small.

 

[Naty1]

...but does infinite expand?

Like oldfart, I too had a misconception about infinity...turns out there are dozens of different infinity concepts discussed and easily accessible in Wikipedia...

One is enough to occupy my brain for the time being.

 

[phinds]

The observable universe is finite. Given that is the only part observationally accessible, the rest is scientifically irrelevant until an observationally detectable effect on the observable part is confirmed.

Chronos, I have seen you make this exact point numerous times, including one in a thread when I first joined many months ago. I was quite taken aback at the time because it seemed then, as it seems now, that you wish to shut down all discussion of the possibilities of what might exist outside the observable universe.

I have been very encouraged since then to see that your point of view is decidedly in the minority and I now find your point of view tiresome.

I do not mean that I think there is anything wrong with the technical accuracy of your point of view, but it strikes me as identical to a hypothetical critic of Columbus wanting to sail around the world because after all, our current experience says hey Chris, there's nothing out there.

I DO understand that this is a very flawed analogy since the Earth is observable and outside the observable universe is not, but my fundamental point is that it seems to me that you want to shut down discussion and have us hunker down in ignorance.

Since it seems to bother you that most of the rest of us have an interest in discussing the existence of things outside the OU, why don't you just ignore these threads, write us all off as willfully ignorant, and go about your merry way?

 

[Calimero]

The observable universe is finite. Given that is the only part observationally accessible, the rest is scientifically irrelevant until an observationally detectable effect on the observable part is confirmed.

Completely true. No use debating things that cannot be verified in any way, and infiniteness is inherently such thing.