The Direction of Space

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Les Sleeth
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Main Question or Discussion Point

Here is something I wonder about:

If the big bang has created spherical expansion, with all resulting galaxies residing on the suface of the sphere, does celestial observation ever involve looking "across" the vast (assumed) empty middle to the other side of the sphere? Does anyone think light might travel around the periphery rather than straight through it? Is there a chance all observed galaxies are only those on the surface all around us, and much further away, on the other side of the expanding sphere, are galaxies we can never see?
 
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  • #2
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i see what you're getting at, but i think your error is it the assumtion that "all galaxies reside on the surface of the sphere". you should think of it in terms that all current galaxies reside on the surface, but there are older galaxies further to the center. and remember, that there is no "center of expansion" so there really is no center of the spherical expansion. one could just as easily say we're the center of expansion as they could another point in space.
 
  • #3
Les Sleeth
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My understanding is that part of the inflation model explains the universe's matter as expanding away from the big bang point, and existing in a huge spherical shape something like the surface of a balloon.

I could see this "surface" being quite thick, but with inflation there should also be an empty core. I suppose I was asking if light could travel across that core, or if it is too vast and expansion too rapid for that light to ever reach us.
 
  • #4
marcus
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Originally posted by LW Sleeth
My understanding is that part of the inflation model explains the universe's matter as expanding away from the big bang point, and existing in a huge spherical shape something like the surface of a balloon.

I could see this "surface" being quite thick, but with inflation there should also be an empty core. I suppose I was asking if light could travel across that core, or if it is too vast and expansion too rapid for that light to ever reach us.
It is quite remarkable how many different visions of space there are!

The "expanding balloon" model of a space with positive curvature was thrown out by cosmologists some time ago

but they apparently did not bother to tell the rest of us!

so people still picture the galaxies as dots painted on the surface of a balloon which is gradually expanding so the dots are getting farther apart----a lovely picture but unfortunately confusing because very wrong.

a better picture, my cosmology professor told me years ago, is of a rising loaf of raisin-bread dough
the galaxies are raisins
and as the 3D loaf of space expands the raisins get farther apart.

as cosmologists talk, for space to be "spatially flat" means that it is ordinary Euclidean 3D space----only expanding in time.

It doesnt mean that it is flat like a 2D thing like a flat piece of paper but it has an analogous 3D flatness in that the sum of the angles in any triangle is 180 degrees

maybe in small scale around stars and blackholes etc it is curved by local concentrations of mass-energy but in large scale space has an overall flatness-----or so they tell us the best and most recent observations show.

so ordinary 3D space, like raisinbread dough extending to infinity in all directions, seems by far the best model

and there is no HOLLOW that it curves around and encloses!!!!
that way lies madness. Perish the thought!!!
Alan Guth, who graciously invented the "Inflation Scenario"
for our edification, never said anything about expanding balloon shape or hollow. This is not part of any currently credible big bang or inflation picture at least that I have heard of.
 
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  • #5
Les Sleeth
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Originally posted by marcus
This is not part of any currently credible big bang or inflation picture at least that I have heard of.
See, I knew this was a dumb question.

Thanks Marcus/Maximus
 
  • #6
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It doesnt mean that it is flat like a 2D thing like a flat piece of paper but it has an analogous 3D flatness in that the sum of the angles in any triangle is 180 degrees
If the universe is flat, it doesn't mean that it has a center?
 
  • #7
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Originally posted by meteor
If the universe is flat, it doesn't mean that it has a center?
Not if it's infinite.
 
  • #8
Phobos
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LW Sleeth - As already well explained (so why am I repeating it?!?), the Big Bang was not an expanding sphere from a central point with galaxies on the outer edge (that image is the result of an inadequate balloon analogy...the raison bread analogy is slightly better but still incorrectly implies an outer expanding edge). The Big Bang happened everywhere in the universe, not from a central point. The points of space across the universe just used to be a lot closer together than they are now.

meteor - Like Mentat said, a "flat" universe is believed to be infinite (or at least boundless)....no center & no edge in 3D space.
 
  • #9
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Not if it's infinite.
You can't say that. The universe started in a singularity, and has been growing since that, and only can reach an infinite size in an infinite period of time, that is not the case
 
  • #10
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Originally posted by meteor
You can't say that. The universe started in a singularity, and has been growing since that, and only can reach an infinite size in an infinite period of time, that is not the case
If the entire Universe, all of spacetime, began as a singularity, then you are right, it cannot be infinite now (IMO). However, it didn't necessarily all start out as a singularity, as it could be just "local expansion" (in that there could be many Universes, that "bubbled up" at the beginning of time, and our's just happens to be expanding).
 
  • #11
russ_watters
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Originally posted by marcus
It is quite remarkable how many different visions of space there are!

The "expanding balloon" model of a space with positive curvature was thrown out by cosmologists some time ago

but they apparently did not bother to tell the rest of us!

so people still picture the galaxies as dots painted on the surface of a balloon which is gradually expanding so the dots are getting farther apart----a lovely picture but unfortunately confusing because very wrong.

a better picture, my cosmology professor told me years ago, is of a rising loaf of raisin-bread dough
the galaxies are raisins
and as the 3D loaf of space expands the raisins get farther apart.

and there is no HOLLOW that it curves around and encloses!!!!
that way lies madness. Perish the thought!!!
Alan Guth, who graciously invented the "Inflation Scenario"
for our edification, never said anything about expanding balloon shape or hollow. This is not part of any currently credible big bang or inflation picture at least that I have heard of.
Marcus, the balloon analogy is just that - an ANALOGY. Part of the analogy is that you must translate the 2D balloon surface into 3D space. It requires some mental gymnastics. It was never, AFAIK, meant to say that everything in the universe resides on a thin shell of a sphere - that would not account for Hubble's observations.

The loaf of rasin bread analogy is also flawed because it includes an edge for the galaxy, something that is not observed.

The expanding balloon analogy is often simply misstated and thats why the confusion exists. IMO, the balloon anaogy is the better analogy - it is borderless, finite, and uniformly expanding.

Not if it's infinite.
AFAIK, the universe is not theorized to be infinite.
 
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  • #12
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The Big Bang happened everywhere in the universe, not from a central point. The points of space across the universe just used to be a lot closer together than they are now.
I have read that the aparition of the universe is interpreted as a fluctuation of the vacuum, more or less like the appearance of virtual particles in quantum mechanics. Can a fluctuation have an infinite size?
 
  • #13
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Originally posted by meteor
I have read that the aparition of the universe is interpreted as a fluctuation of the vacuum, more or less like the appearance of virtual particles in quantum mechanics. Can a fluctuation have an infinite size?
First off, that is just one hypothesis on the origin of the Universe.

And, secondly, the fluctuation wouldn't really have size, as their would not yet be any space.
 
  • #14
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The expanding balloon analogy is often simply misstated and thats why the confusion exists. IMO, the balloon anaogy is the better analogy - it is borderless, finite, and uniformly expanding.
Definitely, the balloon analogy is a very bad analogy. It would be a good analogy if the curvature of the universe was different from zero, but it has been showed that the curvature of the universe is zero, and the cake analogy is billions times better. The dream of Einstein of look at a telescope and watch his own back neck is not plausible

First off, that is just one hypothesis on the origin of the Universe.

And, secondly, the fluctuation wouldn't really have size, as their would not yet be any space.
Yes, but this hypothesis first introduced by Tryon is the standard hypothesis. You only have to Google and you will see that the majority of the webs says clearly that the Universe started in a very little fluctuation, and after undergo inflation in a fraction of second, reached the size of a tennis ball
 
  • #15
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Inflation seems to be the standard hypothesis, but is different than the an ex-nihilo varient floating around these days. While inflation doesn't explain the origins of the pre-existing expanding space-time, some like Alexander Vilenkin have proposed that the entire universe was created out of literally nothing at all, as a quantum fluctuation. Inflation occurs after this fluctuation has already brought the universe into existence.

I'm not sure how popular this idea is among physicists.
 
  • #16
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Originally posted by Mentat
If the entire Universe, all of spacetime, began as a singularity, then you are right, it cannot be infinite now (IMO).

how can it ever be infinite? it will always be expanding at the max of a very finite speed (the speed of light). maybe you mean it will exist forever.(if it doesn't recollapse) but at no point in time (now or ever) will somebody observe the size of the universe to be infinite.
 
  • #17
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Originally posted by maximus
how can it ever be infinite? it will always be expanding at the max of a very finite speed (the speed of light).
First off, it is (according to modern measurement) expanding faster than the speed of light.

Secondly, if all of space is infinite, but our local Universe is expanding, then the whole "Universe" (everything) is infinite, but we would still observe expansion.

maybe you mean it will exist forever.(if it doesn't recollapse) but at no point in time (now or ever) will somebody observe the size of the universe to be infinite.
Well, no one can ever actually conclude that it is infinite (since it could just be really big), but it (spacetime) could still be infinite.
 
  • #18
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Originally posted by Mentat
Secondly, if all of space is infinite, but our local Universe is expanding, then the whole "Universe" (everything) is infinite, but we would still observe expansion.
my understanding is that as the universe expands, space expands with it. so what's beyond it? nothingness: no time/matter, ect...
 
  • #19
Draco
Originally posted by maximus
my understanding is that as the universe expands, space expands with it. so what's beyond it? nothingness: no time/matter, ect...
Thats true.
Scientists already know that the Universe expands. They can see galaxy's spreading far apart, they move out into space. So i guess this means that the Universe does expand.

I doubt whether the Universe if infinite.....When i think of it, i get all weird inside:wink: for some reason.

I don't think that the Universe if a sphere, if it is, then what is on the other side? Nothing? Some sort of wall? Hmmm!
Or is the Universe just a black hole going in reverse. Well thats what Stephen Hawking said, a massive black hole thats in reverse....you know, time starts to go...and eventually it dies. Well i don't know if it will result in the supposedly 'big crunch'
 
  • #20
Les Sleeth
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Originally posted by maximus
my understanding is that as the universe expands, space expands with it. so what's beyond it? nothingness: no time/matter, ect...
I agree with you that time and space are defined by the beginning and, if it happens, the end of the universe. So they are not infinite and can never be.

The question of "nothing" I think is a really important one which has no satisfactory answers. To say nothing does anything whatsoever, even something as apparently insignificant as a quantum fluctuation, means it wasn't "nothing" after all.

In the philosophy area I argued that since everthing which exists must be preceded by the potenial to exist, we can look at what exists now and say something about the potentiality it sprang from.

Even being conservative about it, we can say potentiality can produce a universe, life and consciousness. So is that really "nothing"?
 
  • #21
477
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humanity lacks both the mental and verbal ability to describe absolute nothingness.
 
  • #22
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Originally posted by maximus
my understanding is that as the universe expands, space expands with it. so what's beyond it? nothingness: no time/matter, ect...
There is no beyond. And it's really that simple. Either the universe is finite or infinite. If it's finite, there can be no beyond at all, since that location would neccessarily be defined by space.
 
  • #23
Eh
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Originally posted by LW Sleeth
I agree with you that time and space are defined by the beginning and, if it happens, the end of the universe. So they are not infinite and can never be.
Not quite. Spaces are defined by their points, not their edges. The same applies to time when treated as an extra coordinate. Space may well be infinite in spatial extent, and is not incompatible with the big bang theory. There are also some models for a cyclic universe that would allow time to be without beginning.

The question of "nothing" I think is a really important one which has no satisfactory answers. To say nothing does anything whatsoever, even something as apparently insignificant as a quantum fluctuation, means it wasn't "nothing" after all.
The void of pre big bang theories is something - it's a state where spacetime does not exist, but the laws of physics do.
 
  • #24
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The void of pre big bang theories is something - it's a state where spacetime does not exist, but the laws of physics do.
Are you suggesting that the Big Bang is a quantum fluctuation of infinite size?
 
  • #25
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Originally posted by maximus
my understanding is that as the universe expands, space expands with it. so what's beyond it?
I had the same problem before, the truth is that you are right: if the Universe is finite, it will always be finite. However, if it (spacetime) started out infinite, then it will always be infinite and is not expanding altogether - there are just small parts of it that are expanding ("local expansion"). IOW, if space is infinite, but our local "Universe" is expanding (into that empty space), then our local Universe will never become infinite, but the whole Universe (all of spacetime) is infinite.

nothingness: no time/matter, ect...
"Nothingness" doesn't really mean anything. See the following two threads
Why "Nothingness" Is Not A Word and An Exercise in "Nothing" Semantics.
 

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