
#73
Jan2513, 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.




#74
Jan2513, 05:50 PM

Sci Advisor
P: 4,721





#75
Jan2513, 09:45 PM

P: 40

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. 



#76
Jan2613, 04:02 AM

P: 203

One can moreover imagine a large nonexpanding (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. 



#77
Jan2613, 10:06 AM

P: 40

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. 



#78
Jan2613, 10:15 AM

P: 5,634

Conformal:
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!! 



#79
Jan2613, 11:47 AM

Sci Advisor
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#80
Jan2613, 12:29 PM

P: 40





#81
Jan2813, 02:58 AM

P: 40

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") 



#82
Jan2813, 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 nonuniform 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. 



#83
Jan2813, 03:46 PM

PF Gold
P: 10,991





#84
Jan2813, 05:40 PM

P: 40

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. 



#85
Jan2813, 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. 



#86
Jan2813, 07:42 PM

P: 695

Thanks Devlin, great link. Roughly how many Cosmologists are there today worldwide working in the field?




#87
Jan2813, 09:13 PM

P: 4

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:) 



#88
Jan2813, 09:56 PM

PF Gold
P: 1,493

I looked at that site you posted. Looks to me that much of what he' s saying is outdated. Not to mention misleading.




#89
Jan2813, 10:57 PM

Sci Advisor
P: 4,721





#90
Jan2913, 08:41 AM

P: 4

Do you have any uptodate, more accurate material on the subject? 


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