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How to prove the stretching of space

by timmdeeg
Tags: prove, space, stretching
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ConformalGrpOp
#73
Jan25-13, 02:20 PM
P: 40
Naty, in general, I would agree that, in some sense, we are talking about a distinction without a difference in cosmological terms. The distinction is made to emphasize the nature of the red shift due to the Hubble relation. But, there are galactic systems which are in relative motion to us separate and apart from that due to "Hubble flow". Accordingly, the spectra of the light observed from these bodies will exhibit doppler shifts distinct from the red shift due to the Hubble relation.
Chalnoth
#74
Jan25-13, 05:50 PM
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Quote Quote by ConformalGrpOp View Post
In this regard, I would simply quote Weinberg on this subject:

"[H]ow is it possible for space, which is utterly empty, to expand? How can nothing expand? The answer is: space does not expand. Cosmologists sometimes talk about expanding space, but they should know better."
I'd have to see a lot more context to know exactly what he was saying here. But suffice it to say that the description of the evolution of the universe as expanding space is quite accurate. There are other accurate descriptions, but that is one. And in practice, that way of describing how the universe has changed over time doesn't seem to lead to many practical errors (except for the occasional person that doesn't realize that the expansion is an average effect that does not occur in gravitationally-bound systems).
ConformalGrpOp
#75
Jan25-13, 09:45 PM
P: 40
Quote Quote by Chalnoth View Post
But suffice it to say that the description of the evolution of the universe as expanding space is quite accurate.
It appears that we are working with a common understanding of what cosmologists mean by expanding space. That is, you don't seem to be suggesting that the increase in the "volume" of the "expanding" Universe is the result of (caused by) a physical (force-like) property of empty space.

I take Weinstein's comment to arise from the insight that it is the presence of matter/energy in space which defines the universe. It is only meaningful to state that it is matter/energy that is "flowing" in the models of an expanding universe, and that the statement that "empty space" is "flowing" and expanding in all directions has no physical meaning. Empty space doesn't flow and we do not attempt to measure any characteristic of empty space's nothingness to prove that this nothingness is the source of some physically relevant effect.

It is meaningful to say, in a physical sense, that the distances (the space) between celestial bodies is increasing over time based on our observation of physical events involving matter and energy. It is in this sense that we can say that such "space" is expanding.

In this regard, the analogy of the rising bread dough (expanding space) which "carries" the raisins (matter) out in all directions (before the whole thing is placed in the oven) can be misleading.
timmdeeg
#76
Jan26-13, 04:02 AM
P: 246
Quote Quote by ConformalGrpOp View Post
It is only meaningful to state that it is matter/energy that is "flowing" in the models of an expanding universe, and that the statement that "empty space" is "flowing" and expanding in all directions has no physical meaning. Empty space doesn't flow and we do not attempt to measure to measure any characteristic of empty space's nothingness to prove that this nothingness is the source of some physically relevant effect.ing.
Let's have a look on the empty FRW-Universe. Though being empty it's metric is expanding, regardless whether or not one imagines massless testparticles, whose distances are increasing. The particles are helpful to have a vision of the "flow", but they don't create it.

One can moreover imagine a large non-expanding (due to internal forces) ruler. Then you can visualize the increasing distances between the particles travelling along the ruler. The volume of the cube defined by the ruler isn't interpretable, but the increasing distances are (expansion vs. motion), as pointed out several times by Chalnoth. Admittedly in the case of the empty universe it seems much easier to interpret the recession of "galaxies" as being due to a motion, because in the Milne model, which is equivalent to the empty FRW model, the redshift is purely a Doppler shift.
ConformalGrpOp
#77
Jan26-13, 10:06 AM
P: 40
Quote Quote by timmdeeg View Post
The particles are helpful to have a vision of the "flow", but they don't create it.
I suppose one might be able to say that, in the FRW model, the particles are "of" the flow, and to the extent they arise out of the original conditions and instability that produced the flow, in a sense, they are material to it. How their existence in some sense might "affect" the flow is something that is beyond my pay grade to consider, if its worth considering at all.

I've been reading Relativity Gravitation and World Structure (along with Cunningham's The Principle of Relativity. Last night, by happenstance, I come across a comment by Milne on the question of the "flow" which is pertinent to the discussion. He writes,

"In assigning structure to space, in restoring structure to the structurelessness, mathematical physicists had in effect reintroduced an ether."

Then in a footnote, he remarked:

"This is illustrated by Sir James Jean's remark that the great nebulae are 'mere straws floating in the stream of space', which 'ought to show us in what way, if any, its currents are flowing'."

Just as an aside, in my estimation, the value of this forum would be inestimably diminished but for the willingness of contributors such as Chalnoth and others to offer their knowledge and insights for the benefit of everyone.
Naty1
#78
Jan26-13, 10:15 AM
P: 5,632
Conformal:

I take Weinstein's comment to arise from the insight that it is the presence of matter/energy in space which defines the universe. It is only meaningful to state that it is matter/energy that is "flowing" in the models of an expanding universe, and that the statement that "empty space" is "flowing" and expanding in all directions has no physical meaning. Empty space doesn't flow and we do not attempt to measure any characteristic of empty space's nothingness to prove that this nothingness is the source of some physically relevant effect.
The idea of 'matter/energy' flowing seems ok.....

but "empty space doesn't flow"....well, yeah, but who says that?? If you are saying 'empty space' is a lousy descriptive term, ok. but few people take the view space is 'empty'........space isn't empty, so whether you like the word 'space' or 'matter/energy' seems a matter of semantics....

second, the cosmological GR model is based on the flow of a 'perfect fluid' flow with varying pressures and densities....[or matter/energy in your terms] so what is described via the model should not be viewed as 'empty' anything.

good discussion!!
Chalnoth
#79
Jan26-13, 11:47 AM
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Quote Quote by ConformalGrpOp View Post
It appears that we are working with a common understanding of what cosmologists mean by expanding space. That is, you don't seem to be suggesting that the increase in the "volume" of the "expanding" Universe is the result of (caused by) a physical (force-like) property of empty space.
Let me try to be more clear: most cosmologists simply understand the mathematical way of describing the expanding universe, and do not concern themselves with any worries over how to translate that into words. That is, they only care that their equations work out, and consider it to be a pointless exercise to quibble over semantic arguments. And most cosmologists do, indeed, correctly apply the math in most cases.
ConformalGrpOp
#80
Jan26-13, 12:29 PM
P: 40
Quote Quote by Chalnoth View Post
Let me try to be more clear: most cosmologists simply understand the mathematical way of describing the expanding universe, and do not concern themselves with any worries over how to translate that into words. That is, they only care that their equations work out, and consider it to be a pointless exercise to quibble over semantic arguments. And most cosmologists do, indeed, correctly apply the math in most cases.
This makes perfect sense to me.
ConformalGrpOp
#81
Jan28-13, 02:58 AM
P: 40
Quote Quote by timmdeeg View Post
Okay, thanks, you gave me a better understanding of Chodorowski's paper.

And I want to thank all participants of this thread for helpful comments.

So, in simple words, one should use the notion of 'expanding space' with some care, as the 'stretching or the creation of space' is not measurable, or perhaps better per se is physically not meaningful.
Then the demystfied version could be the conclusion, "that the expansion of space is neither more or less than the increase over time of the distance between observers at rest with respect to the cosmic fluid", refering to the author's of the paper Expanding Space: The Root of all Evil?. The knowledge of the increasing distances results from the cosmological redshifts, which depend only on the space-time curvature. I hope, that's correct so far.
This is consistent with my understanding of the explanation of the red shift phenomenon in the FRW model; to wit, "..cosmological red shift is not, as is often implied, a gradual process caused by the stretching of the space a photon is travelling through. Rather, cosmological red shift is caused by the photon being observed in a different frame to that which it is emitted."

The key point, in terms of interpreting the results from the model is that what is relevant is that "the difference between frames relates to a changing background metric rather than a differing velocity."

As I understand the subject, this insight gets to the root of a fundamental issue about the red shift; that is, if light propagates with respect to the source with a metric that is Minkowskian, then the red shift can be understood as a characteristic time dilation with respect to the frame of the observer in an expanding universe.

This would seem to justify the proper distinction that is made between a doppler shift and a cosmological shift. That is to say, in the case of a doppler shift, the scale of the distance between the source and the observer in relative motion to one another is irrelevant to the red shift observed. In contrast, it is precisely the existence of the dependence of the observed red shift on distance that defines the Hubble relation.

(Note: all quotes in this post are from "Expanding space: Root of all Evil")
wdevlin
#82
Jan28-13, 12:35 PM
P: 4
When trying to image an expanding timespace frame, let's not neglect that matter itself occupies space and this space too would be accelerating into the 4th dimension. Matter's volume would increase concurrently with surrounding space. It would be virtually undetectable in any give temporal frame of reference. Gravity and time would be the only perceived effects.

The 4th dimension battle is between entropy and inertia.

This tendency towards entropy would be inflation's driving force. Gravity is thus a fictitious force as matter coalesces due to non-uniform expansion rates. Distortions in the rate of inflation at the quantum level will culminate away from space with higher inflation rates that are devoid of other matter and cause itself to spatially move through 3 dimensional space towards higher matter density regions.

As matter still expands and gathers, more inertia induced drag to inflation is created. Matter itself is setting the rate towards entropy (localized time). Culminations of matter will continue to slow entropy until a singularities is achieved.
Drakkith
#83
Jan28-13, 03:46 PM
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Quote Quote by wdevlin View Post
When trying to image an expanding timespace frame, let's not neglect that matter itself occupies space and this space too would be accelerating into the 4th dimension. Matter's volume would increase concurrently with surrounding space. It would be virtually undetectable in any give temporal frame of reference. Gravity and time would be the only perceived effects.

The 4th dimension battle is between entropy and inertia.

This tendency towards entropy would be inflation's driving force. Gravity is thus a fictitious force as matter coalesces due to non-uniform expansion rates. Distortions in the rate of inflation at the quantum level will culminate away from space with higher inflation rates that are devoid of other matter and cause itself to spatially move through 3 dimensional space towards higher matter density regions.

As matter still expands and gathers, more inertia induced drag to inflation is created. Matter itself is setting the rate towards entropy (localized time). Culminations of matter will continue to slow entropy until a singularities is achieved.
The entirety of your post sounds like nonsense to me. Do you have some sort of reference that supports your view on expanding matter and the rest of your post?
ConformalGrpOp
#84
Jan28-13, 05:40 PM
P: 40
Quote Quote by wdevlin View Post
Gravity is thus a fictitious force as matter coalesces due to non-uniform expansion rates.
I just sat down on a fictitious chair to look at my collection of fictitious Chrondites, Octahedrites, Howardites and Tektites, all of which prove that gravity is a fictitious force arising as matter coalesces due to non-uniform expansion rates.

When it gets to the point where it is permissible to label as a fiction a physical phenomenon which, using the analytical apparatus of scientific investigation, we are able to (a) measure as a force encountered in nature and (b) predict the effects of that force on matter, one might suppose that physics will cease to be a useful intellectual endeavor.
wdevlin
#85
Jan28-13, 07:28 PM
P: 4
Sounds like you need to begin here... http://www.lifesci.sussex.ac.uk/home....htm#Inflation

And try a different reference frame.
Tanelorn
#86
Jan28-13, 07:42 PM
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Thanks Devlin, great link. Roughly how many Cosmologists are there today worldwide working in the field?
wdevlin
#87
Jan28-13, 09:13 PM
P: 4
Quote Quote by Tanelorn View Post
Thanks Devlin, great link. Roughly how many Cosmologists are there today worldwide working in the field?
Professional Cosmologist?
Not including any resent Chinese involvement, I would say no more than 100. That's an estimation based on the amount of published work I've come across since my early education by Sagan:)
Mordred
#88
Jan28-13, 09:56 PM
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I looked at that site you posted. Looks to me that much of what he' s saying is outdated. Not to mention misleading.
Chalnoth
#89
Jan28-13, 10:57 PM
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Quote Quote by wdevlin View Post
Professional Cosmologist?
Not including any resent Chinese involvement, I would say no more than 100. That's an estimation based on the amount of published work I've come across since my early formal education by Sagan.
In all of cosmology? There are at least three times that working on the Planck satellite alone. I'd wager to guess there are in the range of 10,000 in total, perhaps more. Though I suppose the lines blur a bit between astronomers and cosmologists, and if you're only talking about theoretical cosmologists the number is much smaller (probably only a couple thousand).
wdevlin
#90
Jan29-13, 08:41 AM
P: 4
Quote Quote by Mordred View Post
I looked at that site you posted. Looks to me that much of what he' s saying is outdated. Not to mention misleading.
Being that books on the subject of Eternal Inflation by Andrei Linde and David H. Lyth can cost $100 (and which I can only previewed);
Do you have any up-to-date, more accurate material on the subject?


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