Could Redshift not result from the Doppler effect?

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Could "Redshift" not result from the Doppler effect?

Redshift is the astronomical phenomenon where galaxies and other far-away objects appear shifted to the "red" side of the visible light spectrum. The farther an object is from us, the more pronounced this shift is. This phenomenon is most commonly attributed to the "Doppler" effect -- objects are moving away from us and the light they emit therefore appears shifted to lower frequencies. Therefore, we think that the farther an object is from is, the faster it is moving away from us! This is often cited as evidence that the universe is expanding.

My question is: Is it possible that redshift is not due to the Doppler effect?

In general, waves seem to be able to travel longer at lower frequencies. The most obvious example is sound -- we can hear thunder from far away, but not high-pitched noises. This also seems to be true for the electromagnetic spectrum (in which visible light resides). We can see light from far away stars, but it is very hard to detect the energy in the form of gamma rays and X-rays (which we can detect from the sun). We detect microwave radiation from places in the universe that are at much greater distances from where we can detect any visible light.

Is it possible that redshift is simply a result of light at higher frequencies being absorbed, before it reaches us from very far objects?

Thanks for humoring me!
 

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  • #2
jbriggs444
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What we see when observing cosmic red-shift with a spectrograph is not so much a broad shifting of the peak of the spectrum from high frequencies down to low. It is a shifting of individual precise emission lines from high frequencies down to low.

A theory that high frequencies are attenuated more than low could explain the receipt of more red photons rather than blue (e.g. in a sunrise or sunset). But it cannot explain the shifting of the frequency of individual photons.
 
  • #3
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I don't know that observations support this idea. This mechanism would likely not maintain blackbody spectrums, and I think we do see plenty of x-ray and gamma ray activity in the universe, but the processes that create this radiation are a bit rarer. Even the sun does not give off a ton of x rays and gamma rays.

Interestingly enough, astronomical red-shift is NOT most accurately attributed to doppler shift. Recall that objects cannot move faster than the speed of light, but that the space between them CAN expand at rates much greater than light. What really causes redshift in the universe is expansion, which, given enough time to act on light from far away galaxies, will cause an increase in the light's wavelength to a distant observer. It is not, strictly speaking, to do with relative motion, though it's easy to think about it that way. The main difference being the idea that the Doppler shift predicts simultaneous frames where light is red shifted according to one retreating observer, and blue shifted to one approaching observer. In astronomical redshift, that does not happen, but rather, light is red shifted in all directions according to all observers based only on distance from the light source.

You might ask how energy is conserved in such a universe, and the answer is, it does not appear to be. General Relativity does not enforce global energy conservation, only local. Perhaps the expansion adds a balancing amount of "Dark Energy", that's not for me to say, but perhaps an interesting topic.
 
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This is an interesting thought, although I think that when electromagnetic energy is travelling through a vacuum, it does not lose energy.
 
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phinds
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Redshift is the astronomical phenomenon where galaxies and other far-away objects appear shifted to the "red" side of the visible light spectrum. The farther an object is from us, the more pronounced this shift is. This phenomenon is most commonly attributed to the "Doppler" effect -- objects are moving away from us and the light they emit therefore appears shifted to lower frequencies. Therefore, we think that the farther an object is from is, the faster it is moving away from us! This is often cited as evidence that the universe is expanding.

My question is: Is it possible that redshift is not due to the Doppler effect?

In general, waves seem to be able to travel longer at lower frequencies. The most obvious example is sound -- we can hear thunder from far away, but not high-pitched noises. This also seems to be true for the electromagnetic spectrum (in which visible light resides). We can see light from far away stars, but it is very hard to detect the energy in the form of gamma rays and X-rays (which we can detect from the sun). We detect microwave radiation from places in the universe that are at much greater distances from where we can detect any visible light.

Is it possible that redshift is simply a result of light at higher frequencies being absorbed, before it reaches us from very far objects?

Thanks for humoring me!
Thinking outside the box can be a very productive thing to do, but ONLY after you have learned what's in the box. Do you really think that all the thousands of physicists over many years who have worked in the area of cosmology have not thought of this?

When you come up against something that flies so utterly in the face of established science, it is not a good idea to start off asking if maybe it COULD work the way you think, but rather to start off with the assumption that you have made a mistake somewhere and try to find out where it is by asking why it does NOT work the way you think If you have NOT made a mistake you will find the flaw in the established science, but that is very unlikely to happen. If you start off thinking that you have overturned established science you are likely to just end up embarrassed.
 
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Many thanks for the informative posts above.

To clarify -- I was not attempting to "overturn established science"! This was simply a question on an interesting topic which I know little about. "Embarrassment" aside, the above posts were very helpful.
 
  • #7
phinds
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Many thanks for the informative posts above.

To clarify -- I was not attempting to "overturn established science"! This was simply a question on an interesting topic which I know little about. "Embarrassment" aside, the above posts were very helpful.
You might also find it helpful to look up "cosmological red-shift" (as opposed to doppler red-shift), which is what Soothsayer was talking about.
 
  • #8
Bobbywhy
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lockEF, Welcome to Physics Forums!

Here is some material concerning the cosmological redshift mechanism and the Doppler effect. I am unsure how to answer your question: (Is it possible that redshift is not due to the Doppler effect?). I respectfully request qualified astronomers, cosmologists, and astrophysicists here to enlighten all of us.

In his textbook, “Cosmology, The Science of The Universe” Harrison states on p. 314: “In cosmology we have two basic and beautiful laws: the velocity-distance law (Eq. 15.17) and the expansion redshift law (Eq. 15.18). These basic laws cannot be combined to give recession velocities and distances in terms of redshifts except when redshifts are small. When the redshifts are not small we must use a particular model, such as the Einstein-de Sitter universe, in which we know how the scaling factor changes with time. The custom of referring to the velocity-distance law as the Hubble law and the expansion-redshift as a Doppler redshift rank among the most curious aspects of modern cosmology.”

And on p. 318: “The Doppler redshift depends on motion through space at the instants of emission and reception, whereas the expansion redshift depends on the expansion of space between the instants of emission and reception. The Doppler and expansion redshifts are not identical.”
E.R. Harrison 2000, Cosmology, The Science of The Universe, 2nd edition (CUP, Cambridge)
http://books.google.com/books/about/Cosmology.html?id=kNxeHD2cbLYC

And, in an apparent rebuttal:

“Harrison’s interpretation of the cosmological redshift revisited
Valerio Faraoni
Physics Department, Bishop’s University
2600 College St., Sherbrooke, Qu´ebec, Canada J1M 1Z7
Email vfaraoni@ubishops.ca
Abstract
Harrison’s argument against the interpretation of the cosmological redshift as
a Doppler effect is revisited, exaggerated, and discussed. The context, purpose, and limitations of the interpretations of this phenomenon are clarified.”
http://arxiv.org/abs/0908.3431

Then this:

“The kinematic origin of the cosmological redshift
Emory F. Bunn_
Department of Physics, University of Richmond, Richmond, Virginia 23173
David W. Hogg
Center for Cosmology and Particle Physics and Department of Physics,
New York University, 4 Washington Place, New York, New York 10003

A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts.”
http://arxiv.org/abs/0808.1081

And these authors seem to say sometimes redshifts are a kind of Doppler effect:

“A physical interpretation of Hubble’s law and the cosmological redshift from the
perspective of a static observer
Simen Braeck_
Oslo and Akershus University College of Applied Sciences,
Faculty of Engineering, P.O. Box 4 St. Olavs Plass, N-0130 Oslo, Norway
Øystein Elgarøy†
Institute of Theoretical Astrophysics , University of Oslo, Box 1029 Blindern, 0315 Oslo, Norway

We derive explicit and exact expressions for the physical velocity of a free particle comoving with the Hubble flow as measured by a static observer, and for the frequency shift of light emitted by a comoving source and received, again, by a static observer. The expressions make it clear that an interpretation of the redshift as a kind of Doppler effect only makes sense when the distance between the observer and the source vanishes exactly.”
http://arxiv.org/abs/1206.0927

Lastly, this workshop on the topic provides a great variety of proposals.

“International Workshop on Redshift Mechanisms in Astrophysics
and Cosmology
An extraordinary event took place recently in Ireland. A group of independent and professional researchers met to discuss an old heterodox topic with important consequences in astrophysics and, especially, in cosmology: possible causes of the redshifts in the spectra of astrophysical objects other than a Doppler or expanding universe mechanism. Many decades of work have been devoted to this kind of research, most of it forgotten by the greater part of the astrophysical community nowadays. But the question is still open; the debate is still alive, as was shown by the participants in the present Workshop. There is no smoke without fire, and the existence of many facts and theories on alternative origins of redshifts may point to some new pathways in physics that deserve further attention. This was precisely the aim of this meeting.”
http://arxiv.org/abs/astro-ph/0701061
 
  • #9
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Really, I wonder if Doppler shift vs. cosmological redshift might just be semantics, but I also wonder if there's a bit of physics to be learned by distinguishing the two (or finding out that one is not able to!)
 
  • #10
phinds
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Really, I wonder if Doppler shift vs. cosmological redshift might just be semantics, but I also wonder if there's a bit of physics to be learned by distinguishing the two (or finding out that one is not able to!)
No, the difference is NOT semantics. If you were to insist that all of the red-shift seen for very distant galaxies were due to Doppler red-shift, you would be stating that the universe is NOT expanding, and you would be flying in the face of all known physics regarding the expansion. Do you really want to take that position? For one thing it leads to the statement that objects are traveling faster than light. Not RECEDING FTL, but actually traveling FTL
 
  • #11
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Thinking outside the box can be a very productive thing to do, but ONLY after you have learned what's in the box. Do you really think that all the thousands of physicists over many years who have worked in the area of cosmology have not thought of this?
Lighter side: Our local Honda dealer is running a commercial,
"We have so many Honda the Universe is expanding to make room for them".

Seriously: My wild thought is many of Cosmological Mathmatical theories may turn out wrong when we are able to positively (like landing on the moon) verify how things behave outside our solar system.
 
  • #12
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No, the difference is NOT semantics. If you were to insist that all of the red-shift seen for very distant galaxies were due to Doppler red-shift, you would be stating that the universe is NOT expanding, and you would be flying in the face of all known physics regarding the expansion. Do you really want to take that position? For one thing it leads to the statement that objects are traveling faster than light. Not RECEDING FTL, but actually traveling FTL
I understand all of that, but really what is the fundamental difference between two objects increasing the amount of space between them and two objects receding away from each other due to an expansion of the space between them? Of course, I get that space IS expanding and that very distant galaxies are receding quicker than the speed of light, as a result, but I don't see the difference between traveling a distance Δx in a time Δt from another object, and having the space between those objects increase by Δx over a period of time Δt--It sorta seems like they should be indistinguishable from a relativistic perspective. I'm sure there IS a difference, but it would be nice if you could clarify for me.
 
  • #13
phinds
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I understand all of that, but really what is the fundamental difference between two objects increasing the amount of space between them and two objects receding away from each other due to an expansion of the space between them? Of course, I get that space IS expanding and that very distant galaxies are receding quicker than the speed of light, as a result, but I don't see the difference between traveling a distance Δx in a time Δt from another object, and having the space between those objects increase by Δx over a period of time Δt--It sorta seems like they should be indistinguishable from a relativistic perspective. I'm sure there IS a difference, but it would be nice if you could clarify for me.
I really don't know any way to express it other than exactly what I said in the previous post.
Getting things to travel FTL is NOT OK from a relativistic perspective. It's as simple as that.

Perhaps another way to help you understand it would be this: Recession velocities don't trouble relativity becuase the frames of reference are moving apart. If you ignore the recession, then you are saying that distant galaxies are in the same frame of refernce as Earth, so their velocity truely IS FTL.
 
  • #14
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Perhaps another way to help you understand it would be this: Recession velocities don't trouble relativity becuase the frames of reference are moving apart. If you ignore the recession, then you are saying that distant galaxies are in the same frame of refernce as Earth, so their velocity truely IS FTL.
Ok, that makes sense. So basically, we wouldn't have to do a Lorentz transformation between the two distant galaxies, no relative time dilation or anything like that?
 
  • #15
phinds
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Ok, that makes sense. So basically, we wouldn't have to do a Lorentz transformation between the two distant galaxies, no relative time dilation or anything like that?
Exactly.

In actuality, it is my understanding that there is a very small proper motion between distant galaxies, aside from expansion, so "exactly" is a bit of a simplification, but it would only be noticeable down at quite a few orders of magnitude and would result is a very small Doppler red-shift.
 
  • #16
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Ah, I see. Thanks for that clarification, that's actually quite interesting.
 
  • #17
phinds
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Ah, I see. Thanks for that clarification, that's actually quite interesting.
Yeah, I find this stuff fascinating. I never went beyond undergrad physics but I've read 15 or so books and tons of internet articles in the last year+ and have been very active on this forum and learned a lot from all the great minds here.
 
  • #18
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I'm doing an astrophysics degree, so I was able to take a course in cosmology and also am taking GR this next quarter. Really awesome subjects. Difficult as all get out, though.
 
  • #19
phinds
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I'm doing an astrophysics degree, so I was able to take a course in cosmology and also am taking GR this next quarter. Really awesome subjects. Difficult as all get out, though.
Very cool. Yeah, I've heard that you really can only fully understand GR (or even get close) by getting down to the nitty gritty math, and that doing so makes your head hurt.
 
  • #20


I've read with interest the posts on this subject, and as an old engineer (and not a physicist) the question of red shift has bothered me for some months. If the speed of light is truly a universal constant, then it seems to me that doppler-type shifting of light isn't possible when dealing with light in vacuum.

However, since it is possible in a laboratory setting to slow the speed of light (photons, at least) to mere inches/hour, what would such light appear, spectral-wise, using in some special spectrometer made for this job?
 
  • #21
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If the speed of light is truly a universal constant, then it seems to me that doppler-type shifting of light isn't possible when dealing with light in vacuum.
Light is ALWAYS measured at speed 'c' locally. Doppler shifting has NOTHING to do with the speed of light.
 
  • #22
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I've read with interest the posts on this subject, and as an old engineer (and not a physicist) the question of red shift has bothered me for some months. If the speed of light is truly a universal constant, then it seems to me that doppler-type shifting of light isn't possible when dealing with light in vacuum.

However, since it is possible in a laboratory setting to slow the speed of light (photons, at least) to mere inches/hour, what would such light appear, spectral-wise, using in some special spectrometer made for this job?
Hi Corndodger! Welcome to Physics Forums!

The Doppler shift does not alter the speed of the wave, only the wavelength. This is true in any constant medium as well as in a vacuum.

You're asking, in the second half of your paragraph, how slowing light affects its spectra? That's a good question. Most of the time, when scientists work with slowed light, they use laser pulses, so the light is entirely of one wavelength, in which case, there is no spectrum of which to speak. I don't know how feasible it would be to try and slow white light to those speeds, but if you managed it, so long as there wasn't any absorption of any particular constituent wavelengths, the spectrum should be unaffected. There shouldn't be any sort of Doppler shifting, if that's what you were trying to tie into. At least, in my mind, this would be the case.
 
  • #23
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A few descriptions from experts in these forums I saved to clarify several perspectives:

PALLEN: The Doppler effect is completely determined by the relative velocity of source and target. Both of these velocities are frame dependent, but the relative velocity between emitter at event of emission and receiver at event of detection is frame invariant. Thus all observers agree on the Doppler measured by a given detector from a given source.

Mathematically, relative speed is defined by parallel transport of 4-velocity from one event to another (in SR, this is path independent, thus unique), then dot product of transported source 4-velocity with unit 4-vector 4-orthogonal to target 4-velocity. Dot products are invariant - period. (In a standard inertial frame in SR, parallel transport leaves a vector unchanged).
PAllen:
Cosmological redshift is typically considered distinct from Doppler redshift because it is a relation between distance and redshift rather than speed and redshift, under the assumption that both source and target are motionless relative to center of mass of the local matter (here, local is quite large - galaxy or galaxy cluster).
Marcus:
Don’t think of the redshift as a Doppler [relative velocity] effect. It is not the result of some particular speed. The formula involves the entire [varying] factor by which distances have been expanded during the whole time the light has been traveling.
PeterDonis:
The law governing the relationship of emitted to observed photon energies (or frequencies) is general and applies in any spacetime. The 4-momentum of the photon gets determined at the emitter; then it gets parallel transported along the photon's worldline from emitter to observer; then you contract that 4-momentum with the observer's 4-velocity to get the observed energy (or frequency if you throw in a factor of Planck's constant). That "parallel transport" process is actually where the "redshift" occurs in an expanding universe; the expansion alters the 4-momentum of the photon as it travels (or at least that's one way of looking at it), whereas in a static universe the photon's 4-momentum would "stay the same" as it traveled.
 
  • #24
sophiecentaur
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All this is a lesson in how one has to ignore the waffle that you can hear on TV and in many classrooms! Doppler shift of sound (along with recordings of ambulances going past) is always used to illustrate the Red Shift and, each time it's used, it gives the serious Science teacher more to 'undo'.
 
  • #25


Hi Corndodger! Welcome to Physics Forums!

The Doppler shift does not alter the speed of the wave, only the wavelength. This is true in any constant medium as well as in a vacuum.

You're asking, in the second half of your paragraph, how slowing light affects its spectra? That's a good question. Most of the time, when scientists work with slowed light, they use laser pulses, so the light is entirely of one wavelength, in which case, there is no spectrum of which to speak. I don't know how feasible it would be to try and slow white light to those speeds, but if you managed it, so long as there wasn't any absorption of any particular constituent wavelengths, the spectrum should be unaffected. There shouldn't be any sort of Doppler shifting, if that's what you were trying to tie into. At least, in my mind, this would be the case.
I appreciate seeing and getting quality responses to my post. But I'm still in the dark regarding red shift - how does the wavelength get changed without a corresponding change in velocity? Isn't wavelength essentially defined with velocity as a factor? Maybe I'm just too grounded in the Newtonian era.

C
 

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