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FRW universe, expanding space or spacetime? 
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#19
Feb2912, 06:01 PM

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Let's next go to the literature for some guidance on the terminology. One paper I'm aware of is http://arxiv.org/abs/0707.0380 "Expanding space  the root of all evil". First note the title  it's "Expanding space" not "Expanding spacetime". The internals of the paper follow the usage of "expanding space" not "expanding spacetime". Aside from the title, I think it's a rather wellwritten paper on the topic, though there are a few things that it doesn't say that I wish it would say, so while I agree with all the points it makes, I think it misses making at least one important point. It's still worth a read, however (it's not terribly technical for the most part). I'll quote the abstract here to provide some information on what the paper is about, with the hope that it will motivate some people to read it (and I'll repeat that I think that for the most part it's fairly accessible without a lot of math). Being able to switch with both points of view is particularly important in explaining why Brooklyn is not expanding, even though the universe as a whole is. It would be possible to use coordinates where Brooklyn expands, but they would be inconvenient, and wouldn't represent the physics well. Contrawise, you can use "Brooklyn" coordinates for cosmology, (the technical name here would be Fermi normal coordinates BTW), but the math gets awfully, horribly, messy  it's just not a good fit. 


#20
Feb2912, 06:05 PM

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Also does this expanding space only works for curved spacetime? Or is it not related to whether curved or flat? Meaning flat spacetime can expand too? 


#21
Feb2912, 06:24 PM

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BTW, read pervect's post #19 and the paper he linked to. 


#22
Feb2912, 07:04 PM

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I think what atyy is saying is this.
Expanding space produces our universe. Expanding curved spacetime doesn't produce our universe because it is invalid. Expanding flat spacetime doesn't produce our universe because it is invalid (Milne). Hence. Expanding space can either have curved spacetime or flat spacetime embedded in it. Is this correct analysis? 


#23
Feb2912, 07:06 PM

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In the expanding balloon analogy, the 3 dimensions of space are represented by the 2 dimensions of the balloon surface, and time is represented by time. In my orange analogy, the 3 dimensions of space are represented by just one dimension (circumference) of a circle, and time is represented by the "latitude" of the circle. The 4 dimensions of spacetime are represented by the 2 dimensions of the orange skin. The "North Pole" represents the big bang. The circle expands as it moves down but the orange is static. 


#24
Feb2912, 07:28 PM

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Expanding space (not expanding spacetime) is just curved spacetime. Spin2 on flat spacetime produces curved spacetime. So in the instance of space expanding. It is composed or combinations of the dynamics of spin2 and flat spacetime. So how does each interact separately with space expanding to produce our universe? Or is there some conceptual flaw somewhere? 


#25
Feb2912, 07:42 PM

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http://luth.obspm.fr/~luthier/gourgo...pohang081.pdf 


#26
Feb2912, 07:54 PM

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#27
Feb2912, 08:43 PM

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#28
Feb2912, 08:47 PM

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I think it might be worthwhile to ask a few partially rhetorical questions here:
1) What experiment would you do to tell whether empty space was moving or flowing? 2) What experiment would you do to tell whether or not space is "expanding" 3) What experiment would you do to tell whether empty space was curved? I'll add my own (spoiler) answers below.... I should also add that the "empty" space I'm envisioning allows you to create and label events in it  it's just that it doesn't have any predefined structure to start with. I should add the metaphysical point that if there isn't any experiment to test something, the question is philosophical in nature and not scientific. A corollary of this is that you can argue about such questions forever without settling the issue.  1) Once upon a time, it was thought that you could measure the "ether velocity", but we now expect any experiment that we can perform not to be able to detect the motion of space will have a null result. So, if special relativity is right, there is no apparatus that can detect empty space moving. So it's possible to imagine an experiment that could detect moving space, but relativity says that all such experiments will show that it's not moving. 2) None  as far as I know, at least, there isn't any experiment (at least none compatible with relativity) to tell whether or not empty space is expanding or not. This may be debatable, I suppose  just because I've never seen it doesn't mean it exists. But if we can't tell that empty space is moving, how would we tell that it's expanding? 3) A lot of people aren't aware of the details, but this can in fact be measured. To tell if a plane is curved, for example, you'd construct a quadrilateral with four equal sides and equal diagonals, and measure the diagonals to make sure they're sqrt(2). This presuposes that you do know how to measure distances  it's necessary to define how you measure distances before you can measure curvature. A good example is using this technique to detect the fact that the surface of a sphere is curved. You should be able to do something similar with the diagonals of a cube in 3d  check if they are both sqrt(3). Someone posted a good reference in the literature about a different geometric construction to measure spatial curvature by measuring only distances, but I'm forgetting both the poster. ((I think it was some book by Synge)) It's also easy to to find constructions if you're confident in your ability to measure angles, but measuring distances is really more fundamental IMO. 


#29
Feb2912, 08:51 PM

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#30
Feb2912, 10:30 PM

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Since expanding space is automatically curved spacetime (right?).. and since curved spacetime is just spin2 field on flat spacetime. Then expanding space is composed of spin2 field and flat spacetime. Therefore expanding space is related to expanding space&spin2 field and expanding space&flat spacetime. How does one imagine or model expanding space&spin2 field for example? Or expanding space&flat spacetime which is a Milne model that isn't valid. Can one say that when one adds spin2 to Milne model. It becomes valid? Does anyone see if there is something wrong with my analysis and directly address what I'm talking about. Thanks. 


#31
Feb2912, 11:46 PM

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Also field gravity is not the same as GR. They are two different theories, both claim to explain the observed cosmological phenomena but in different ways. In fact I don't think FTG needs expanding space but supposes a fractal distribution of mass. So you can't talk about splicing them together in the way you suggest. 


#32
Mar112, 12:00 AM

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#33
Mar112, 12:04 AM

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#34
Mar112, 04:27 AM

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You seem to think the big bang happened in some place, it's wrong...



#35
Mar112, 04:51 AM

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If you're looking for a way to quantize gravity using the standard treatments, you'd do better with teleparallel gravity. 


#36
Mar112, 05:20 AM

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