
#55
Jan813, 08:34 AM

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#56
Jan813, 12:00 PM

P: 208

In the example in #43 you mentioned "observers with fixed spatial Schwarzschild coordinates". Instead one could think of observers falling freely one after another on a radial path. Wouldn't then the similarity with FOs in curved RW models be even closer? And if yes, wouldn't they similarly wonder whether their "space" expands and how to interpret the redshift, as they recognize their moving away from each other, the faster the farther? Sorry, this may be quite silly, thanks for your patience. 



#57
Jan813, 12:14 PM

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#58
Jan813, 12:58 PM

P: 208





#59
Jan813, 05:38 PM

P: 87

thread this claim is false; with a few exceptions said interpretation is in general simply inconsistent with the geometry of the RWmodels. So in my opinion the B&H paper is an extremely lousy paper; just about everything in that paper is wrong or misleading and it should never have been published (the reputation of the American Journal of Physics has been tainted by accepting it). The paper is a prime example of how bad things may turn out when trying to do physics by gut feeling. Furthermore, based on the reception of the paper, B&H have not only succeded in fooling themselves, but have apparently done a good job of fooling some other professionals as well. This is what I find the most remarkable about the paper. 



#60
Jan913, 05:01 AM

P: 208





#61
Jan913, 04:43 PM

P: 87

The cited statement should not be taken as a criticism of B&H, since the paralleltransport procedure Chodorowski performs is different from that referred to in B&H. That is, Chodorowski defines a "recession velocity" as the 3velocity obtained by paralleltransporting the 4velocity of the emitting FO along a spacelike geodesic rather than along a null geodesic. This is a perfectly valid procedure to do mathematically. But then he defines the redshift obtained from this "recession velocity" as the "kinematic" part of the redshift and the remainder part of the redshift is defined as "gravitational". These definitions are very misleading, since (for small distances) the definition of "kinematic redshift" does not correspond to the redshift obtained by using SR, and the definition of "gravitational redshift" does not correspond to the effects of spacetime curvature. Chodorowski should have named his spectral shift splitup in some other way, reflecting the procedure on which it is based. 



#62
Jan913, 05:48 PM

Mentor
P: 6,044

Chodorowski writes Consider an analogy: a portion of a great circle is geodesic on the 2dimensional surface of the Earth, but it is not geodesic in 3dimensional space. Similarly, the path chosen is a geodesic for the 3dimensional spatial hypersurface that represents all of space for one instant of cosmic time, but it is not a geodesic for 4dimensional spacetime. 



#63
Jan1013, 05:31 PM

P: 5,634

Two issues of interest:
I just came across the following in my notes..and had forgotten about the concept: GeorgeJones: I suggested the great circle on a balloon was not the same path as that used in coordinate and proper distance measurements....and consequently there were severe weakness in the balloon analogy regarding cosmological distances.... I like to use the Wikipedia illustration to 'picture' this for myself: http://en.wikipedia.org/wiki/Metric_...s_expanding.3F I've heard it described as a 'straight line...as when laying rulers end to end' [for a proper distance 'measure' ] and I believe also as a spacelike geodesic and a spacetime geodesic.... Seems like the proper description is 'a geodesic in three dimensional space'...... edit: Found this in my notes: Wallace: {commenting on weakness in balloon and raisin bread analogy} The rate of expansion [velocity] is unimportant; It is the rate of acceleration of the expansion [a’[t] that tells you what happens. So in a contracting universe a distant particle could move away, or in an expanding universe a distant particle could come toward you. You don't intuitively expect this behavior if you think of the universe as the model loaf of rising bread filled with raisins! [or the balloon analogy] Source not recorded: A curve of constant cosmological time [along which we would like to measure a proper distance’ ] connecting two points in a FRW [model] universe is not a "straight line", i.e. it is not a geodesic. But it IS the Hubble ‘distance’ calculated distance. So what, then is the 'Hubble curve' over which distance is calculated called? 



#64
Jan1213, 12:07 PM

P: 208

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 spacetime curvature. I hope, that's correct so far. 



#65
Jan1413, 02:24 PM

P: 5,634

timmdeeg
This issue is not one easy to describe in a few sentences. And summarizing long discussions about this issue as understandings and explanations evolve is also not so easy. Timmdeeg The issue is what does the observation [measurement] mean? How do we interpret observed redshift, exactly as posted by Chalnoth, post #2. Further,if you conclude the effect is 'physically not meaningful', how do you explain that CMBR radiation emitted at almost 3,000 K is today observed at about 2.7K? If this loss of energy had NOT occurred, there would have been no evolution of the universe as we observe it....no stars, no galaxies, no us....also recall I quoted Chalnoth in post #33 integrated Sachs=Wolf effect as experimental evidence. For example as bapowell posted in #3 with the FLRW model the wavelength of a cosmological photon [or a mass particle] λ(t) varies according to the scale factor a[t]. In this model, they go together. I don't even prefer the wording of the quote I posted..... "... the density and pressures of cosmological fluids must change over cosmic time, and it is this change that represents the basic property of an expanding (or contracting) universe...." I would have said something like "...it is this change [from general relativity] that causes [or 'powers' or 'determines']...the basic expanding universe....that is, Einstein's equations relate the evolution the scale factor to the changes in pressure and energy density of the matter in the universe. 



#66
Jan1513, 04:46 AM

P: 208

Naty1,
I welcome you criticism. Would you agree with that: Truely physical: the cosmological redshift, increasing distances between FOs. Measurable: The redshift. It yields information about the increase of the scalefactor between emission and absorption and thus about increasing distances. Interpretation : the redshift can be interpreted as due to the stretching of space or as due to the motion of galaxies, #2. Furthermore, the interpretation of the redshift depends "on the spatial geometrie", #39. Stretching of space not measurable: a thought experiment may result in increasing distances, but these again can be interptreted in this or that way, #16. So, the experiment doesn't prove the stretching of space (or the creation of space, resp.). If I claim that I have measured the stretching of space, you could say, no, you have measured just motion. Especially here I ask for any differing opinions. Correct description: the interpretations are "correct descriptions" of a "real physical phenomenon", #31. And "as space expands, the wavelenth must increase", #3, is to my understanding also covered under correct description. So, "correct description" and "real phenomenon" don't have the same meaning. I agree, without further explanation the wording "physically not meaningful" gives rise to misunderstanding. Perhaps "not truely physical" or "not a real physical phenomenon", would make more sense. 



#67
Jan2413, 02:24 PM

P: 40

I appreciate the contributions that have been made to this thread. I found it helpful and stimulating, especially in confirming my view that a large percentage of cosmologists misconceive the concept of expanding space, space stretching, etc.
One question I do have, which I believe is subsumed within the subject matter of this thread, (and perhaps many others), is the following: What is the experimental basis for the assumption that the frequency/wavelength of light remains constant when traveling over cosmologically relevant distances? Are there any plans for conducting an experiment to verify the behavior of light at such distances? My sense is that there really is no experimental data on this issue and that astronomers, astrophysicists and cosmologists all rely on evidence produced from local experiments for the interpretation of data obtained from light received from distant sources. It just seems to me that until such an experiment is conducted, much of the interpretation of what we observe from sources throughout the universe remains open to controversy. Is there any reason why the scientific community would not be interested in the results of such an experiment? [if this deserves its own thread, I trust someone will pursue that] 



#68
Jan2513, 02:48 AM

P: 208

Nobody assumes that. Since Hubble we know about the redshift (i.e. the non constancy of the wavelength) of distant galaxies. 



#69
Jan2513, 06:16 AM

P: 40

Tim, I believe it is assumed that a fundamental property of EM is that the wavelength of light emitted from a distant source propagating along an unobstructed, field free path of fixed distance to an observer with no relative motion with respect to the source will exhibit no spectral shift.
In other words, it is assumed that the "doppler" and "cosmological" red shifts are not the result of the "non constancy of the wavelength", but a result of the relative motion of the observer with respect to the source (doppler), and the additional distance the light traveled from the time it was emitted to the time it was observed due to the expansion of the universe (cosmological), which looks like the same thing insofar as the expansion imparts a de facto relative motion between the source and the observer. In both cases, it is assumed that the wavelength of the light remains unchanged from the point of emission to the point of observation, and it is only the relative motion of the observer with respect to the source that results in the observer detecting a shift in the wavelength. At least, that is my understanding of the current state of knowledge with respect to the behavior of light. [Note: Some theorists contend that "space stretching", a term I believe is a misnomer, imparts a stretching of the wavelength analogous to the stretching of a dot on the surface of an inflating balloon. But, most knowledgeable scientists would view this conception as little more than a second rate, and certainly misleading pedagogical device. The more accurate explanation is that as the universe expands, the successive photons being emitted from the source travel further than the preceding ones, and this, like the doppler effect, accounts for the observed spectral shift.] I am inclined to view the problem slightly differently; that Maxwell's equations admit a solution that permits light to "travel" within a metric that is not locally Minkowskian. Therefore, not inconsistent with Milne's view, the observed spectral shift, or perhaps a component of it, is quite likely to be the result of an intrinsic characteristic of light which is not velocity dependent, but rather is time/distance dependent. Verification of the behavior of light over distances where Hubble's law becomes relevant is within our ability to ascertain by experiment, a circumstance wasnt even a fathomable possibility at the time when Eddington and others announced that the relation between red shift and distance codified in Hubble's law provided compelling evidence that the universe was expanding as predicted by the de Sitter and LaMaitre models. I don't know of any experiments such as those hypothesized by Poincare, Milne and Whitrow (doing away with rigid rods and synched clocks, and using calibrated light signals), that have been proposed or carried out to specifically test the behavior of light over cosmologically relevant distances. One might ponder the idea of a massively scaled up version of the MichelsonMorley experiment, (or its modern analog), and wonder would the experiment still yield a null result? From a historical standpoint, its a bit intriguing to think about what might have happened if the late 19th Century experimentalists had access to the technology necessary to perform such a scaled up version of the MM experiment and the experiment had yielded a positive result! Would Einstein have been able to successfully convince all those who were held to the theory that light could only propagate if there existed an omnipresent aether that: "NO! The positive results demonstrate a previously unknown property of EM allowed by Maxwell's equations, not proof of our motion through the aether!" ? (Rendering the theory of aether obsolete is, in my view, one of the most significant advances resulting from Einstein's theory of Special Relativity; which is a bit ironic for Einstein's SR was developed out of, and informed by the work of Lorentz and Poincare in attempting to explain the results of the MM experiment with respect to the aether). Anyway, just a thought. Thanks for taking the time to respond. 



#70
Jan2513, 10:19 AM

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#71
Jan2513, 11:07 AM

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#72
Jan2513, 02:05 PM

P: 40

Hi Chalnoth, I'm not one to quibble about such things, and certainly not with the attribution of my comment to " a large percentage" cosmologists (as opposed to "most"). The comment might have been better taken if I made no such particular attribution and simple noted that the misconception is prominent in the field. But, here too, I was speaking to the idea that empty space has physical properties (a new "forcelike" characteristic), which is implied in the notion that "space" stretches. This is distinct from the concept of an "expanding" universe, which is perfectly consistent with the physical implications of the standard model.
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." But then again, I could be "completely wrong." Cheers! 


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