How do we know space is not infinite?


by zeffur7
Tags: infinite, space
phinds
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#37
Sep12-11, 06:17 PM
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Quote Quote by zeffur7 View Post
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.
yenchin
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#38
Sep12-11, 11:37 PM
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Quote Quote by jbar18 View Post
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/iss...topology/index

http://www.csulb.edu/~scrass/Teachin...odecaSpace.pdf

http://www.maths.lse.ac.uk/Personal/mark/topos.pdf
vrmuth
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#39
Sep17-11, 07:59 PM
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Quote Quote by yenchin View Post
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 "
Imax
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Sep17-11, 09:23 PM
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Quote Quote by jbar18 View Post
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.
Constantin
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#41
Sep19-11, 05:20 PM
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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.
Constantin
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#42
Sep19-11, 05:35 PM
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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.
phinds
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#43
Sep19-11, 07:23 PM
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Quote Quote by Constantin View Post
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.
zeffur7
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#44
Sep22-11, 04:44 PM
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Quote Quote by simoncarl View Post
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.
zeffur7
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#45
Sep22-11, 04:49 PM
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Quote Quote by vrmuth View Post
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?
Tanelorn
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#46
Sep23-11, 12:55 PM
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Quote Quote by Chronos View Post
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 isnt 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?
phinds
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#47
Sep23-11, 01:14 PM
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Quote Quote by Tanelorn View Post
The observable universe is shrinking all the time and eventually only our local group will be visible from our position. However isnt 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.
Tanelorn
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#48
Sep23-11, 01:40 PM
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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.
vrmuth
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#49
Sep25-11, 05:22 AM
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Quote Quote by zeffur7 View Post
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
vrmuth
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#50
Sep26-11, 08:01 AM
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Quote Quote by chrisbaird View Post
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? Actually only if you wanna prove an Infinitely large thing as "Finite" you will take infinite amount of time
chrisbaird
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#51
Sep26-11, 11:28 AM
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Quote Quote by vrmuth View Post
Ok now tell me will you take infinite amount of time to say "The set of all natural numbers" is finite or infinite? Actually only if you wanna prove an Infinitely large thing as "Finite" you will take infinite amount of time
I meant it is not possible to physically prove something is infinite. A mathematical proof is a different beast. We can go ahead and make mathematical models of the universe and from some postulates mathematically prove it is infinite. But to see if our model matches reality, we have to do physical experiments and make observations. No physical experiment can measure something infinite. If the universe is finite, then we could measure it. If the cosmic background radiation stopped coming at a certain point in time, that would tell us the universe is finite. The last glimpse of CMB radiation would be coming from the literal edge of the universe. Such a finding would shake the cosmology world up a bit, but I doubt it will happen.
phinds
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#52
Sep26-11, 12:06 PM
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Quote Quote by chrisbaird View Post
... If the cosmic background radiation stopped coming at a certain point in time, that would tell us the universe is finite. The last glimpse of CMB radiation would be coming from the literal edge of the universe. Such a finding would shake the cosmology world up a bit, but I doubt it will happen.
Uh ... say WHAT? Could you elaborate on that, please. I'm certainly willing to believe it's just me, but that makes no sense to me.
chrisbaird
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#53
Sep26-11, 12:34 PM
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Quote Quote by phinds View Post
Uh ... say WHAT? Could you elaborate on that, please. I'm certainly willing to believe it's just me, but that makes no sense to me.
That me try to do better. Imagine that the universe is finite spatially, and always has been, and is not periodic (it does not wrap around and connect to itself). In other words, imagine that there is a physical edge to the universe beyond which nothing exists. Shortly after the big bang, the cosmic background radiation is created effectively uniformly at all points in the universe and spreads out from each point in all directions. CMB radiation that travels a long ways before hitting a planet or space probe's camera needs a longer time to do so, and is thus absorbed/observed at a much later time than the big bang. The CMB radiation created at points in space close to the earth were absorbed long ago by the earth (or were not absorbed and long ago zipped away from the earth, out of our region of interest). But CMB radiation created at points very far away from the earth are just now reaching us and being detected. If there were a physical edge to the universe, there is a physical limit to points in space that were in existence to create CMB radiation. After enough time has elapsed that the CMB radiation created at one edge of the universe has had a chance to travel to the opposite edge of the universe, it will be gone. All of it will have been absorbed or flowed beyond the universe's edge at this point. (Unless the edge of the universe consists of giant and perfect mirrors, so that the universe in a giant resonant cavity.) While I do not believe this to be the case in reality, I am trying to imagine the implications of a finite universe. Have I gone wrong anywhere?
Tanelorn
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#54
Sep26-11, 03:36 PM
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Chris, I think I understand what you are suggesting. If the universe has an edge and we are relatively near it, then eventually we would see no more CMBR in that direction because it had all passed us by. I suppose we can keep looking!

I think that the observable universe is small compared to the whole universe because the CMBR is so flat in every direction implying homogeneity. However, if there was an edge of some kind I would not expect it to be a sudden transition either, there again we cannot discount any possibility, however remote, when we cannot make any observations.


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