How do we know space is not infinite?

  • Thread starter Thread starter zeffur7
  • Start date Start date
  • Tags Tags
    Infinite Space
AI Thread Summary
Space is currently understood to be expanding, but it remains uncertain whether it is infinite or finite. The observable universe is finite, with light from regions up to 46 billion light-years away reaching us, but beyond that lies a "wall of light" from the Big Bang. Discussions highlight that infinite space and expanding space are not contradictory, and models suggest that compact space can explain cosmic microwave background radiation better than infinite models. The concept of infinity is complex, with various interpretations that challenge traditional notions of measurement and size. Overall, while the observable universe is finite, the nature of the unobservable universe remains a topic of ongoing debate and exploration.
zeffur7
Messages
39
Reaction score
0
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?
 
Last edited:
Space news on Phys.org
zeffur7 said:
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.
 
Last edited:
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.
 
Last edited:
chrisbaird said:
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"
 
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.
 
Last edited by a moderator:
zeffur7 said:
How do we know space is not infinite?

Cosmic microwave background radiation?
 
Last edited:
Imax said:
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?
 
phinds said:
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?
 
Infinite is unbounded, so I fail to see your point.
 
  • #10
zeffur7 said:
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.
 
  • #11
Rishavutkarsh said:
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.
 
  • #12
jackmell said:
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.
 
  • #13
Rishavutkarsh said:
... it's already infinite and it's getting bigger infinite?

exactly
 
  • #14
phinds said:
exactly

Bigger infinite?? Wouldn't that imply a bounded space?
 
  • #15
Oldfart said:
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].
 
Last edited by a moderator:
  • #16
yenchin said:
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.
 
  • #17
Oldfart said:
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.
 
  • #18
phinds said:
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.,,
 
  • #19
phinds said:
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.
 
  • #20
Imax said:
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.
 
  • #21
Billions of light years across, but it’s not static. Space seems to be expanding, and that expansion seems to be accelerating.
 
  • #22
Imax said:
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.
 
  • #23
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.
 
  • #24
Imax said:
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.
 
  • #25
It’s not small. Light from ghosts can take billions of years to reach us
 
  • #26
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.
 
  • #27
Imax said:
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.
 
  • #28
...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.
 
  • #29
Chronos said:
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?
 
  • #30
Chronos said:
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.
 
  • #31
phinds said:
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.

You don't have to think about infinity as some eternally inaccesible twilight zone. You can get a very good feel for what infinity means by going large compared to the system, but still small numerically. For instance, x + 1 = x becomes very close to true when x = 1 million.
 
  • #32
Naty1 said:
Like oldfart, I too had a misconception about infinity...

Whoa there! What's this "we" stuff, white man? (Tonto to the Lone Ranger, as they find themselves surrounded by hostile indians.)

I actually don't admit to having a misconception, at least relative to the real world/universe, but am fine with infinity being an useful mathematical concept.
 
  • #33
phinds said:
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.

I did not think I made such an assertion.

phinds said:
...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.

Visible objects in solar systems obviously have orbits and the contents of galaxies appear to be accelerating away from each other, but how can we be certain that the explanation for those galaxies moving away from each other is caused by space expanding? Consider the conjecture that the big bang was a singularity that began to expand at some position and after ~14 billion years it has increased in volume to all that we are able to perceive now.
 
  • #34
chrisbaird said:
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.

Excellent points!

If the big bang originated as a singularity (which some theorize) can we estimate the volume of the observable universe at 1 billion year increments since the beginning?
If yes, what was the rate of expansion? Did 90+% of it expand in a nanosecond or how did it expand, if we know.

chrisbaird said:
"...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.

It seems to me that we should have been receiving light from the moment light was being created--which should include the light from the wall of light that you essentially describe as the barrier that we cannot see past. Light from points closer to us should arrive in shorter periods of time than light from points further from us. If the light we are now receiving is 14 billion years old, how can we be certain it is from the beginning of the big bang? Or do we make our estimate of the age of the universe based on the light wall barrier that you have described?
 
  • #35
phinds said:
... the rest of us have an interest in discussing the existence of things outside the OU,...

I'm digging your vibe, wo?/man :)

I'd like to see everyone's thoughts on the subject, as long as that have a modicum of sense with them.
 
  • #36
I like the theory that the universe might be some weird 4-D shape. If you travel in the same direction on the 2-D surface of the earth, you would eventually end up in the same place. It's not infinite, but you would never find a boundary. If the universe were 4D it could be that you could keep going in one direction through 3D space, and in a similar way never find a boundary, just end up in the same place. I guess that would make it bounded in the 4th dimension, but unbounded in the other 3.

That may even lead to ideas like all of the other galaxies we see are just our own galaxy from the various times in the past, with the light having passed different distances through the whole universe before we see it again.

Pure speculation really, I don't think there's any evidence for it, there may in fact be evidence to the contrary. I just think it's quite a neat, wacky idea that seems to tie in quite nicely. How could it be tested?
 
  • #37
zeffur7 said:
I did not think I made such an assertion.
OK, I guess I misunderstood you.


... Consider the conjecture that the big bang was a singularity that began to expand at some position ...

That is a totally incorrect description of what is currently believed to be the case, in that there was no "position" in space that the expansion happened from, it happened everywhere at once.
 
  • #38
jbar18 said:
I like the theory that the universe might be some weird 4-D shape. If you travel in the same direction on the 2-D surface of the earth, you would eventually end up in the same place. It's not infinite, but you would never find a boundary. If the universe were 4D it could be that you could keep going in one direction through 3D space, and in a similar way never find a boundary, just end up in the same place. I guess that would make it bounded in the 4th dimension, but unbounded in the other 3.

That may even lead to ideas like all of the other galaxies we see are just our own galaxy from the various times in the past, with the light having passed different distances through the whole universe before we see it again.

Pure speculation really, I don't think there's any evidence for it, there may in fact be evidence to the contrary. I just think it's quite a neat, wacky idea that seems to tie in quite nicely. How could it be tested?

Note that what you are talking about doesn't have to require extra spatial dimension, just imposing non-trivial topology will do. For example on some video game, you disappear into the right side of the screen and emerges from the left, effectively that world is *the same* as a cylinder, obtained by gluing left and right edges of the computer screen. Such a topological effect can be looked for via brute force statistical methods (like you say, essentially by looking for patterns which correlate at different directions, not just galaxies but also CMB), but we have not found anything conclusive yet. See e.g.

http://plus.maths.org/content/os/issue10/features/topology/index"

http://www.csulb.edu/~scrass/Teaching/math355/articles/dodecaSpace.pdf"

http://www.maths.lse.ac.uk/Personal/mark/topos.pdf"
 
Last edited by a moderator:
  • #39
yenchin said:
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.

yes i accept infinite can get bigger infinite. But no finite thing can become infinite after finite amount of time (or after finite number of steps). for example the sequence {nc},n=1 to infinity (where c is a positive real number, in case you want the sequence to grow faster you can choose a big c ) after any finite number of steps, never becomes infinity . then after 14 billion years how could the universe become infinite ? starting from a point?
my point is " some thing infinite can get bigger or smaller but it could have not been finite in the past, and will never become finite in the future "
 
Last edited:
  • #40
jbar18 said:
I like the theory that the universe might be some weird 4-D shape. If you travel in the same direction on the 2-D surface of the earth, you would eventually end up in the same place. It's not infinite, but you would never find a boundary. If the universe were 4D it could be that you could keep going in one direction through 3D space, and in a similar way never find a boundary, just end up in the same place. I guess that would make it bounded in the 4th dimension, but unbounded in the other 3.

I like this idea too, and it's possible that 3D space at the last scatering surface was compact. You don't need 4D space, only 4D spacetime.
 
  • #41
I see the Universe's "diameter" mentioned in two above posts.
I'm pretty sure the Observable Universe only has radius, and not diameter. To measure a diameter you need to be on the edge of the Universe (or any circle or sphere) and that's not possible in any version of the Universe. You can't simply multiply the radius by two and say it's diameter.
 
Last edited:
  • #42
I'll quote these two above posted arguments as very convincing:
---

"how can you prove something like "infinite". It would take an infinite amount of time to measure something infinitely large."

"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."

---
The first argument can be given against anything being infinite, not just the Universe.
 
Last edited:
  • #43
Constantin said:
I see the Universe's "diameter" mentioned in two above posts.
I'm pretty sure the Observable Universe only has radius, and not diameter. To measure a diameter you need to be on the edge of the Universe (or any circle or sphere) and that's not possible in any version of the Universe. You can't simply multiply the radius by two and say it's diameter.

Now that's just silly. The OU is not a physical object, it's just a way of describing the sphere which represents the fartherst out in any direction that we can "see". To say that it has a radius but no diameter is profoundly nonsensical.
 
  • #44
simoncarl said:
No one knows right now, our technology is to young to discover if the Universe is infinite or finite. The only thing we know is, it is expanding and we already proved that.

We also know that that expansion is accelerating.
 
  • #45
vrmuth said:
yes i accept infinite can get bigger infinite. But no finite thing can become infinite after finite amount of time (or after finite number of steps). for example the sequence {nc},n=1 to infinity (where c is a positive real number, in case you want the sequence to grow faster you can choose a big c ) after any finite number of steps, never becomes infinity . then after 14 billion years how could the universe become infinite ? starting from a point?
my point is " some thing infinite can get bigger or smaller but it could have not been finite in the past, and will never become finite in the future "

If the big bang reverses at some point & then become the big crunch (i.e. a singularity), what say ye of the min & max of that system, if it were true?
 
  • #46
Chronos said:
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 by observable universe do you count the CMBR sphere or just the highest redshifted galaxies?

The observable universe is shrinking all the time and eventually only our local group will be visible from our position. However isn't it pretty certain that the unobservable universe still exists both now and at that later time - unless we believe that the universe not homogenous and isotropic?
 
  • Like
Likes zeffur7
  • #47
Tanelorn said:
The observable universe is shrinking all the time and eventually only our local group will be visible from our position. However isn't it pretty certain that the unobservable universe still exists both now and at that later time - unless we believe that the universe not homogenous and isotropic?

Seems to me that statement is both right and wrong. The OU isn't actually shrinking in terms of the number of light-years it encompasses, but it IS "shrinking" in the way you mean, which is that it contains less and less stuff because everything is moving out of it.

What Chronos has said in other posts is that there basically ISN'T anything outside the OU because we can't TELL directly whether there is or not, but I think that's an overly restrictive point of view. I think the UN-observable universe exists now and will continue to exist but in practical terms, that doesn't seem to mean much since as Chronos always points out (correctly) we just can't detect it.

It's not at all clear to me that there will EVER be any way to detect it, although I have heard, vaguely, that there are some theories that say there will be / may be observational evidence left over from the earliest times after the singularity that we WILL be able to observe remnants of. I don't get how this works but it would be neat if such things ARE ever observed.
 
  • #48
Perhaps I wasnt being clear. Try less of the matter in the universe will be observable in the future. However this wasnt the point I was trying to make.
 
  • #49
zeffur7 said:
If the big bang reverses at some point & then become the big crunch (i.e. a singularity), what say ye of the min & max of that system, if it were true?

then please give me the initial conditions to formulate the differential equations, then i will tell you the max and min
 
Last edited:
  • #50
chrisbaird said:
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.

Ok now tell me will you take infinite amount of time to say "The set of all natural numbers" is finite or infinite? :smile: Actually only if you want to prove an Infinitely large thing as "Finite" you will take infinite amount of time
 

Similar threads

B Expansion of Space Itself, rather than due to an Explosion like the Big Bang?
Replies
9
Views
2K
B Big Bang and Infinite Universe
Replies
20
Views
1K
I How do we know space is expanding?
Replies
24
Views
2K
B Can the "creation" of space in the Universe affect the speed of light?
Replies
7
Views
3K
I How Does Space Expansion Affect the Velocity of Galaxies?
2
Replies
65
Views
6K
B How do we know that the Universe is expanding?
Replies
16
Views
2K
B The infinite and the infinitesimal
Replies
10
Views
3K
I Black Hole Diameter: Is It Finite or Infinite?
Replies
36
Views
6K
B Does an infinite universe contradict a Big Bang origin?
2
Replies
65
Views
6K
I Why is the observed local region of space flat?
Replies
25
Views
3K

Hot Threads

Recent Insights

Back
Top