Is there a generic name for the following group of redshift types? Are they sometimes all called Cosmological redshift, even though two of them are not?

• Doppler redshift
• Cosmological redshift (Hubble redshift)
• Gravitational redshift

Conversely, all other proposed redshift mechanisms seem to me lumped into a "non-cosmological" redshift group, or "tired light" redshifts, or "scattering" redshifts, and might include:

• Compton scattering
• Ramam scattering (and CREIL)
• Plasma redshift
• Wolf effect redshift

Are there any others common terms in use?

Regards,
Ian Tresman

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Ian Tresman
plasma-universe.com

It really depends on what you mean by Doppler redshift and gravitational redshift. Conventionally, a particular coordinate system is chosen, called the comoving frame, where the local coordinates move with the galaxies and time and space are measured in that frame, which keeps changing as the photons propagate. In that frame, Doppler redshifting refers to the special motion of objects relative to the comoving frame, and gravitational redshifting refers to gravity changes relative to the gravity of the homogeneous universe. In that nomenclature, cosmological redshift is something that you get even when these other two are zero, and since it keeps happening throughout, over long distances the cosmological redshift dwarfs the other two. However, what unifies these three redshifts is that they are all what might be called reference-frame redshifts, as they all have to do with changing perceptions when mapping from one local coordinate to another. The other redshifts might be described as "absolute" in the sense that they don't map into each other when you change coordinates, they are what they are.

Well, most of the "other redshifts" have yet to be demonstrated to be at all significant in the context of measuring the redshift of a cosmologically distant object. Beyond a couple billion lightyears, even the known redshifts due to gravitational escape and local doppler movement become more and more insignificant compared to the spectral shift effect from the cosmological distances.

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Everyone is entitled to his own opinion, but not his own facts.

The major difference between the two groups, from an observational POV, is that the former apply across the board (gammas to radio), the latter apply to only limited ranges within the EM spectrum (and the shift may even be frequency/wavelength dependent).

As Ken G already said, since the two groups arise from quite different types of physical causes, it is straight-forward (in principle!) to distinguish between them.

Yes, and what follows from Nereid's point is that it is difficult to categorically distinguish between redshift mechanisms that occur over the entire spectrum and are not mediated by the presence of any particular intervening species. The three mechanisms in the first subgroup are thus only pedagogically distinguishable over cosmological scales, in my (limited!) understanding, and only if one is careful to identify one's coordinate system. In practice, this caveat is rarely recognized.

The significance is irrelevant. The only common factor in the first group is their use and application in Big Bang cosmology. Do they have a collective name?

Regards,
Ian Tresman

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Ian Tresman
plasma-universe.com

Actually, the Wolf Effect can also produce a frequency independent, full-spectrum redshift. See the references here.

I've had a least three authors of peer-reviewed papers confirm it to me.

Regards,
Ian Tresman

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Ian Tresman
plasma-universe.com

I seem to have missed it Ian; none of these papers purports to show that 'the Wolf Effect can also produce a frequency independent, full-spectrum redshift', even in principle.

Specifically, none of these papers advances the summary of the current state of understanding, as outlined by Tim Thompson, back on 13 September (extracts):Perhaps there are more recent papers, reporting (for example) multi-line spectra, or non-Gaussian lines, or ...?

Correlation-induced spectral changes (1996) in Rep. Prog. Phys. 59 (1996) by Emil Wolf and Daniel F V James, write: "It has been found not long ago that spectral shifts produced by dynamic scattering may, in some cases, imitate the Doppler effect. This possibility was first pointed out by Wolf
(1989a, b) and was subsequently investigated more fully by James et al (1990) and by James and Wolf (1990, 1994)."

Wolf, 1989a. Phys. Rev. Lett. 63 2220
Wolf, 1989b Coherence and Quantum Optics vol VI ed J H Eberly, L Mandel and E Wolf (New York: Plenum) p 1235
James D F V, Savedoff M P and Wolf E 1990 Astrophys. J. 359 67
James & Wolf, 1990 Phys. Lett. 146A 167
James & Wolf, 1994 Phys. Lett. 188A 239

Sounds rather hasty? Don't you think that the impications should be considered first?

Regards,
Ian Tresman

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Ian Tresman
plasma-universe.com

Hmm, seems we're either in complete agreement, or we have a communication gap.

The 'implications' include whether or not the Wolf effect can produce a redshift greater than a line width for a multi-line spectrum, how the effect works for non-Gaussian lines, the extent to which the inevitable blurring can be restricted to unobservable parts of the spectrum, how anything other than a point source could show such an effect, what physical conditions are required to generate the effect in astronomical objects, and so on.

AFAIK, Tim's summary is up to date (and your references were all included in his post).

Is this a field you are actively working in?

The terminology is simple enough. Cosmological redshift is the observed redshift which correlates with distance. In this respect, there are only two redshifts to consider: cosmological and non-cosmological. The non-cosmological redshift is any redshift, no matter how it is generated, that is due to processes in the local rest frame of the galaxy. The cosmological redshift is everything else, regardless of how it is generated.

In our current cosmological models, the cosmological redshift is due to the expansion of space, and to no other cause. So, "cosmological redshift" is the redshift caused by the expansion of spacetime. When cosmologists talk about "cosmological" redshift, there is no other meaning.

But now we have left the original question behind, and have reverted to a discussion of the Wolf effect. We are re-hashing things that have already been hashed several times in ATM threads, getting nowhere, making no progress, and learning nothing new.

Indeed, there are a few people investigating the Wolf effect as a possible source of galactic redshifts, including Wolf himself. However, at this time the state of their research is still rather primitive. The studies all involve single Gaussian profiles. What about full spectra? What about other line shapes (a Voigt profile, perhaps)? There simply is not enough theoretical evidence to make any conclusion other than that it is an interesting area of study, even as regards cosmological redshifts.

But there are several observational reasons for not thinking that the Wolf effect has much to do with cosmological redshifts. For one thing, all of the papers clearly show that both red & blue shifts are to be expected, yet blueshifts are extremely rare. Where are all the Wolf blueshifts that should be there? Furthermore, there is a definite and well established redshift - distance relationship. How can the Wolf effect do that? After all, the Wolf effect has nothing to do with distance, and everything to do with the specific properties of the scattering medium in the galaxies. There are quite a lot of galaxies large enough to measure the redshift across the image of the galaxy. In all cases that I know of, the resulting measurement is consistent with galactic rotation, once side having a slightly different redshift than the other. But why would the Wolf effect do that? Only if the Wolf redshift is the same all across the galaxy, and we really are seeing rotation superimposed on the Wolf redshift. But that requires that the scattering medium in each galaxy to be consistent across the galaxy, which one would certainly not expect to be the case.

So, as far as I can see, there are good, observational reasons, for rejecting the Wolf effect as a significant component of the observed, cosmological redshift. But the Wolf effect is an interesting optical phenomenon, worthy of study. And so people study it. As they do we may learn more, even as regards cosmology, though I think it unlikely.

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

Agreed.
Scientists must be very careful of using pedagogical statements as though they were absolute. I'll give you another cause right now: the shrinking of matter. How can it be distinguished from expanding space? I don't think it can, the difference is pedagogical.

Sounds like a reasonable conclusion to me.

Ken, you're going to have to explain this one. We've measured Cephids out to 108 million LY (NGC 4603), that's about 33 Mega parsecs (I'm using this distance as, with the cephids, the distance/recession relationship is pretty well corroborated) . With the Hubble value at approximately 70 km/sec per megaparsec, matter, at the distances of the furthest measured Cephids, would have to be shrinking at around 2300 km/sec (actual studies of the cluster indicate some recession velocities, within the cluster, at 3000 km/sec) . Your going to have to come up with some fancy explanations for matter shinking at that speed (or causing some effect to make it appear that it's shrinking that fast) since a sun sized object would shink to nothing in about 608 seconds (about 10 minutes).

Well, we don't see stars winking out after 10 minutes in NGC 4603. Unless you have more of an explanation than "shrinking matter".

Not without your explanation, it isn't.

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

Excuse me, but I said "In our current cosmological models, the cosmological redshift is due to the expansion of space, and to no other cause." I certainly did no such thing, as to say it as if it were absolute. I have always made it crystal clear that the interpretation of the cause of the cosmological redshift is model dependent. Of course one could argue for "shrinking matter" as the cause of the redshift, but that would be a different model.

So, exactly how does this "shrinking matter" model work? Would we not see changes in quantum mechanics? The energy levels of atoms & molecules are very sensitive to inter- & intra-atomic distances. And likewise, chemical bond length is important. Silicon can only single bond, unlike carbon, even though they have the same outer shell structure, because silicon bonds are longer. So if matter is shrinking, what happens to chemistry? Just off hand, it seems the two should be distinguishable.

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

How are you guys proposing that you measure the increasing distance to these Cepheids? No matter how you do it, it will be tantamount to laying rulers end to end, perhaps done at the same universal age done by a bunch of comoving observers. And as the age increases, the number of rulers it takes increases, yes? Now please prove to me that this cannot be due to shrinking rulers. The sole observable is that it takes more rulers! This is the point of relativity, to me-- all answers must be couched in terms of an observable, not absolute reality. And no, stars would not be "winking out", they would just be an amount smaller that you would say space has expanded. We already do this all the time, it's called Lorentz contraction, why limit this kind of thinking to special relativity? I admit I haven't worked out all the ramifications of this alternate pedagogy, but my intuition tells me there will never be any way you can distinguish yours from mine. Except that I don't have to explain where the space is coming from...

No, why would you expect to see differences? Does quantum mechanics work less well when you look at an atom from a greater distance? The point is, there is degree of freedom embedded into reality that you can never extract, since all meaningful quantities are ratios.

Well, since all the forces between atoms vary depending on distance, if the atoms are shrinking, the force between them would change also.

Ok, forget about the distance. How exactly are the atoms shrinking at 2300 km/sec in NGC 4603? Remember, that is the meausred red shift. If they are not shrinking that fast, how is the shrinking matter causing the measured red shift? And, if the atoms are shrinking, you are going to have to explain why the nuclear forces haven't changed much in two billion years between the Oklo natural reactor and the present.

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

Let me clarify. In the shrinking-matter picture, nothing is happening to the light. There is no need for any 2300 km/sec anywhere. All that is happening is the measured wavelengths are getting longer. This is because they are measured relative to something-- matter!

Handwaving. Exactly how does shrinking matter cause us to measure a redshift consistant with a recession velocity of 2300 km/sec. Your claim is that shrinking matter can somehow give us the same measurements, and is pedagogicly the same, as expanding space.

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

You are asking me how could it do this, but I am asking you, how could it not? Perhaps this would be best addressed on the other thread that just opened up in the "Astronomy" section, about what is the difference between expanding space and shrinking matter. On that thread I have argued that there is no difference, and I maintain that the onus lies on those who claim there is a difference to explain what experiment can be done to detect the difference. Maybe there is one, I don't know.

Oh no you don't, this is your claim, not mine. I have no idea how this would work, your the one claims it can. So I ask again, exactly how does shrinking matter cause us to measure a redshift consistant with a recession velocity of 2300 km/sec.

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

On the contrary, I am claiming that any experimental result you quote as evidence that space is expanding, I will quote as evidence that the matter you used to conduct the experiment contracted. How is this not obviously going to give the same experimental result? I already answered the redshift question. Just ask yourself, why do you infer a recessional velocity of 2300 km/sec? Follow the steps in your own argument, and you will see where I can inject shrinking matter.

Lets see, a spectrum,taken here on earth from a Hydrogen discharge tube, showing the the Balmer series, is compared to the spectrum of a galaxy, again taken here on earth, shows the same line shifted enought to indicate a recession velocity of 2300km/sec. You are comparing two objects,( the images of the spectra) prodcuce here on earth. Supposedly one shrank, the other didn't. So I ask again, for the third time, exactly how does shrinking matter cause us to measure a redshift consistent with a recession velocity of 2300 km/sec. In other words, how does shrinking matter explanation cause the shift in the line, here on Earth, in one image, but not the other image?

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

Wavelengths are not measured at all. The frequency is measured against a standard clock, and the wavelength is inferred from the measured frequency, and the previously measured, but now defined, speed of light. So we are measuring against a clock, not a ruler. How does that fit into the "shrinking matter" paradigm?

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

The detector is the same, but not the source. In one case you used "today's" hydrogen to make the Balmer photon, but in the other case, you used "yesterday's". Yesterdays was a much larger hydrogen atom, so it made a higher-wavelength photon (after all, there is a direct relation between the size of the atom and the wavelength of the photon, I'm not changing any physics here, or any ratios in the local frame). Haven't you ever seen Lorentz contraction used as part of the Doppler effect you see in normal special relativity? This is just the same idea, applied to GR.

Easy, I never said shrinking matter was the only thing going on. In fact, I'm being careful not to change any physics, so the speed of light must still be measured the same in all reference frames. So as our matter shrinks, our sense of time must also speed up. The basic equation c = lambda*nu is of course still preserved, and this is how we blithely go back and forth between frequency and wavelength. Besides, I'm not sure that spectrograms use "standard clocks" anyway, but it doesn't matter, since the answer is the same whether you are fundamentally measuring a wavelength or a frequency.

Ah, well, you also never said that anything else was going on, just "shrinking matter". So, now we have "shrinking matter" and "accelerating time". Is there anything else going on that we should be aware of? As far as I can see, the "model" you are talking about is no "model" at all, just an arithmetic trick. If you change everything, subject to the constraint that all dimensionless ratios remain unchanged, then of course, you have actually changed nothing. So the entire discussion devolves to one of those quaint arguments that philosophers like to indulge in, but which are in reality, devoid of interest.

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

Why yes, you might call the difference... purely pedagogical? Don't you recall that is how I prefaced this entire discussion?
So it seems we have come to agreement at last. My point is simply that, if there is any lesson of relativity, it is that we must avoid treating pedagogical statements about how things "are" as if they were absolute descriptions of reality.

I surely never disagreed with that, and never would. Indeed, it is true not just of relativity, but all empirical science.

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

Good, and it is in that spirit that I think that Doppler and gravitational redshifts are valid descriptions of cosmological redshfits, in the proper pedagogical context (read, coordinate system). If this is not true, then the reasons are quite subtle and beyond anything I know about GR, which sadly, is not as much as I'd like.