
#1
Dec1612, 03:58 AM

P: 203

Hi,
according to several scientists, among them J.A.Peacock: A diatribe on expanding space M.J.Chodorowski: The kinematic component of the cosmological redshift E.F.Bunn&D.W.Hogg: The kinematic origin of the cosmological redshift space doesn't expand, instead the cosmological redshift is due to a combined kinematic/gravitational effect. Peacock is author of the book "Cosmological Physics". An interesting consequence is e.g. "Summing up, the expansion of the universe is never superluminal", Chodorowski. The MaxwellEquations don't have a "stretching" term, but it seems a plausible assumption that the streching of space goes along with the stretching of a photon's wavelength, though  at least to my knowledge  there is no fundamental physical explanation for that. Or am I wrong? This leads to my question, if and how the stretching of space could in principle be proved experimentally. One can imaging longliving physicists in the center of a large void (they don't see galaxies), which supposedly expands because of its subcritical energy density, equipped with all necessary tools like watches, metersticks, ropes, lightpulsegenerators, redshift analysis, whatsoever. With which kind of experiment could the physicists prove the stretching of space? 



#2
Dec1712, 05:35 PM

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P: 4,721

There are multiple ways of describing the universe that are mutuallycompatible. The Bunn & Hogg paper that you cite makes this explicit:
"We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon’s path." In other words, it is perfectlyvalid to talk about the redshift either as coming from the motions of galaxies, or as coming from the stretching of space. The stretching of space interpretation is, however, mathematically simpler in many respects, and doesn't have weird effects at the edge of the visible universe. But it is always useful to realize that there are other ways of describing the universe. 



#3
Dec1712, 07:37 PM

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#4
Dec1712, 08:21 PM

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PF Gold
P: 9,182

How to prove the stretching of space
The integrated SachsWolf effect is the clearest, independent [of supernova], evidence of dark energy [the strange antigravity effect that powers expansion]. The sloan digital sky survey [SDSS] confirmed this effect to better than 4 sigma  which is almost good enough to be considered conclusive even by particle physics standards. Photons entering a large gravitational well [like a galactic supercluster] get a gravitional energy boost upon entering the region causing a small gravitational blue shift. Upon exiting, they lose this free energy and redshift back to their original energy state upon exiting  almost. If the universe were flat and static, the net effect would be zero. In an expanding universe, the photon takes so long to pass through the gravity well that it gets to keep a small amount of the blue shift it acquired on the way in due to expansion and the resulting dilution of gravity. This extra energy shows up as a slight anisotropy in the CMB photons passing through a supercluster or supervoid [the effect is just the opposite for CMB photons passing through a supervoid]. See http://arxiv.org/abs/0805.3695 for discussion.




#5
Dec1812, 08:17 AM

P: 203

In contrast, Bunn & Hogg criticize this notion (page 8): "The common belief that the cosmological redshift can only be explained in terms of the stretching of space is based on conflating the properties of a specific coordinate system with properties of space itself. This confusion is precisely the opposite of the correct frame in mind in which to understand relativity." Supposing that stretching belongs to the "properties of space itself", shouldn't this be measurable? 



#6
Dec1812, 08:43 AM

P: 203





#7
Dec1812, 08:46 AM

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#8
Dec1812, 09:30 AM

P: 203

But I suspect the authors I named would argue that during the parallel transport of the velocity fourvectors along the path of the photon any changes of the rate of the expansion of the universe are included. Thus, it seems still possible to argue that the cosmological redshift is due to kinematic/gravitational effects. However I am not sure myself at all. 



#9
Dec1812, 10:46 AM

P: 203





#10
Dec1812, 01:53 PM

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P: 4,721

My main point is that the "stretching of space" picture is mathematically simple for many calculations, and so most people working in the field will naturally default to this view out of simple pragmatism. The Bunn & Hogg paper is, to me, mostly useful in terms of showing that we should be careful as to whether some apparent fact of the universe is a real fact, or simply a result of a particular interpretation that disappears if we look at the universe a bit differently. The recession velocity of galaxies is one such fact: what the recession velocity of a given galaxy is depends entirely upon your coordinate choice. You'll get very different answers for the "stretching of space" and "things moving apart" views. 



#11
Dec1912, 04:33 AM

P: 203

Thank you for answering, Chalnoth,
I agree with most of that, but have still a problen with your statement: The result  the photon's stretched wavelength  is the same. But the physical reasons, climbing in a gravitational field versus travelling through stretching space is much different and hence not subject of personal preference. I must have missed something, where am I wrong? 



#12
Dec1912, 07:31 AM

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#13
Dec1912, 11:36 AM

P: 203

All this in mind I still miss the meaning of your remark "whether the redshift is a result of gravitation or velocity is not a physical question at all, as the answer depends upon your coordinates." You seem to say that the physical result, the stretched wavelength, originates from mathematics, not from physics. 



#14
Dec1912, 12:38 PM

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#15
Dec2012, 04:25 AM

P: 203

At least in that sense, the stretching of space should be measurable. When a gravitational wave passes by, the change of spacing of two testmasses relative to coordinates provided by metersticks is measurable using interferometry techniqes. An analogue measurement should be possible in a void. What happens to two testmasses, which initially are at rest relative to each other and then are allowed to move freely. They don't get a kick. Will their distance measured with metersticks increase over time? In the case of a receding galaxie we have nothing but a redshift which we can interpret. In the void we have distances additionally. Is the moving apart from each other (provided that happens) still a matter of interpretation? Thanks for your valuable comments. 



#16
Dec2012, 06:23 AM

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#17
Dec2012, 10:07 AM

P: 203





#18
Dec2112, 02:54 AM

P: 87

question about a choice of coordinates. The reason for this is simple: in the RWmodels there is a set of "preferred" observers (the socalled "fundamental observers" (FOs)) defining the cosmic redshift; i.e., the high symmetry of the RWmanifolds implies that they can be foliated in a "preferred" way such that the spatial hypersurfaces are homogeneous and isotropic. The FOs are those observers always moving orthogonally to the "preferred" hypersurfaces. The cosmic redshift is then defined as that obtained by exchanging pulses of electromagnetic radiation between the FOs. This means that the cosmic redshift is in principle an observational result defined via specific observers, and that cannot be dependent on a choice of coordinates. Moreover, it is possible (at least for sufficiently small regions) to change the geometry of the RWmodels from curved to flat but holding the world lines of the FOs and the coordinate system fixed. One may then compare the cosmic redshift calculated in the two cases, and in general the two results will differ (these calculated results are of course independent of the choice of coordinate system). In particular, it is possible that the redshift may vanish in the flat spacetime case (this happens for all RWmodels with flat or spherical space sections). In these cases it is obvious that the cosmic redshift is entirely due to spacetime curvature so that any interpretation in terms of a Doppler shift in flat spacetime is mathematically inconsistent with the RWmodel. In sum, the question of interpretations of cosmic redshifts as described by the RWmodels is not a subject of personal preference, but rather depends on the geometrical properties of the particular RWmanifold under consideration. This is a mathematical fact, and no arguments based on personal gutfeelings can change that. 


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