How do we know space is not infinite?


by zeffur7
Tags: infinite, space
Imax
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#19
Sep10-11, 07:03 PM
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Quote Quote by phinds View Post
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
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#20
Sep10-11, 07:38 PM
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Quote Quote by Imax View Post
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
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#21
Sep10-11, 08:05 PM
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Billions of light years across, but itís not static. Space seems to be expanding, and that expansion seems to be accelerating.
phinds
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#22
Sep10-11, 11:50 PM
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Quote Quote by Imax View Post
Billions of light years across, but itís 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
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#23
Sep11-11, 10:16 PM
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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
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#24
Sep11-11, 10:26 PM
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Quote Quote by Imax View Post
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
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#25
Sep11-11, 11:12 PM
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Itís not small. Light from ghosts can take billions of years to reach us
Chronos
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#26
Sep12-11, 01:22 AM
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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
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#27
Sep12-11, 07:39 AM
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Quote Quote by Imax View Post
Itís 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
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#28
Sep12-11, 07:53 AM
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...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
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#29
Sep12-11, 08:12 AM
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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, 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 existance 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
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#30
Sep12-11, 08:30 AM
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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.
Completely true. No use debating things that cannot be verified in any way, and infiniteness is inherently such thing.
chrisbaird
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#31
Sep12-11, 10:41 AM
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Quote Quote by phinds View Post
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.
Oldfart
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#32
Sep12-11, 01:23 PM
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Quote Quote by Naty1 View Post
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.
zeffur7
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#33
Sep12-11, 02:42 PM
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Quote Quote by phinds View Post
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.

Quote Quote by phinds View Post
...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.
zeffur7
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#34
Sep12-11, 03:11 PM
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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.
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.

Quote Quote by chrisbaird View Post
"...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?
zeffur7
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#35
Sep12-11, 03:29 PM
P: 38
Quote Quote by phinds View Post
... the rest of us have an interest in discussing the existance 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.
jbar18
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#36
Sep12-11, 06:03 PM
P: 53
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?


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