The redshift of light (or any other electromagnetic radiation) arriving from very distant galaxies (some billion lightyears) is mainly due to the expansion of space.
While I do not question this I have problems to understand how light “knows” about the expansion. For me, space is merely a “concept” (like distance between objects) without any physical attributes. Therefore, I wonder how light can physically interact with a “concept” in a way which results in stretching of its wavelength. Unless space has some physical properties.
Which are the physical attributes of space that interact with light? Aether is dead! Vacuum fluctuations? Zero point energy?

Most probably, this has been discussed before here, but I can´t find it. So, if it has been discussed, a link would be appreciated as well.

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If everyone had even a basic grasp of scientific principles, this planet would be a better place (Phil Plait)

Die Lücke, die wir hinterlassen, ersetzt uns vollkommen (Carl Heinz Schroth)

As far as I know, we have not been able to reproduce this cosmological redshift from space expanding in the laboratory. The description is rather a hand-waving one, as far as I know. As the photon traverses space, tiny amounts of space show up in the middle of the photon, which increases its wavelength... And you may then ask, where does the energy go? to increase the wavelength... To this I have to say that it depends on the model you have for how space is expanding. We only know that it is expanding.

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Forming opinions as we speak

I came across a sentence in a book on General
Relativity, "a photon loses energy in an
expanding Universe but it loses it to nowhere
in particular". So your valued belief in
conservation of energy must be thought of as
a pokey, parochial concept

I have always felt you cannot distinguish
between doppler and cosmological redshift, it
is a matter of faith. Perhaps it helps to
think of the photon being unchanged until it
gets to our telescopes when it finds they are
receding dammed fast.

But in the history of this concept wasn't the proposed expansion the explanation for the redshifted light? Zwicky, Hubble, and not Hoyle.

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily avaiable to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

I know you are a person who takes his physics seriously, but isn't it said that most great discoveries aren't discovered with "Eureka!" but with, "Hmmm, that's funny." Big Don

Sure, in the ancient history of the idea, that was the case. Eventually observations, and other factors, made this idea passe, end universal expansion took over.

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Forming opinions as we speak

This is not an area that I actively study, but what other factors, for instance?

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily avaiable to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

I know you are a person who takes his physics seriously, but isn't it said that most great discoveries aren't discovered with "Eureka!" but with, "Hmmm, that's funny." Big Don

jlh. In a quantum sense, the loss of energy by the photon must be accompanied by the gain elsewhere. When the General relativists get involved they point out that conservation of energy....inviolate in a particle physics lab...can be considered not conserved locally, but is eventually conserved globally (somewhere else...unspecified). It addresses the problem that a free photon traveling in space that is "empty", needs a victim to deliver it's momentum and energy to....hence voiding many arguments that the light is merely tired from it's lengthy journey.
I have pointed out that the representation of a vacuum as "empty" and free of potential "victims" for the photon is itself a false model, based on grade school representations of Bell jars and ringing doorbells. Space is not empty, and while bosons may pass freely through each other, fermions will interact. Nuff said. pete

As Antoniseb said there's "lots of hand waving out there"...I prefer a collision with particles involved, conservation laws upheld, and all species accounted for with numerical algorithms....that's how the LHC is set up.

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A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov

OK, Thanks...unspecified? OK. Now where is that rock...Ahh yes it is definitely specified, it's right here...OK

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily avaiable to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

I know you are a person who takes his physics seriously, but isn't it said that most great discoveries aren't discovered with "Eureka!" but with, "Hmmm, that's funny." Big Don

The (hand waving!) way I understand it, redshifts of z>0.1 are considered to be cosmological in nature and although there is no way to distinguish between (relativistic) doppler redshift and cosmological redshift for measurements up to around z=1.4 (which equates to an apparent recession speed of c), once you are looking at larger redshifts they cannot be due to relativistic doppler effect due to the limit of c on any differences in apparent velocity due to classical momentum and energy transfer.

Surely, once we are considering high redshifts and apparent superluminal recession speeds, where relative inertial motion is a negligible factor, the cosmological component of that redshift would be absolute? i.e. however fast you were travelling through space, you would observe that distant galaxy to be apparently receding from you at a speed faster than light.

If the expansion of the universe causes the wavelength of light to be "stretched" (by introducing extra space into the proton during its journey, for instance) then if that light had been travelling for billions of years it would be "stretched" by an absolute amount and any difference in the relative velocities of emitter and observer that are caused by their relative inertial movements would add or subtract only a small and apparent value to that absolute amount?

I learned a lot about this question from this thread, which is already back on Q&A page 5. It's a rather long thread. If you don't have much time, you might start on page 4 (of 5), where Spaceman Spiff points out...

Tim Thompson, above, already linked to this article, but I think it's worth pointing to again: "Expanding Space: The Root of All Evil?" (download the full pdf file, either from that link or directly, here). Try it - you'll like it! (or rather at least learn something new)

I loved this line from the paper....

The balloon-with-dots or bread-with-raisins analogies, like any analogies, are useful so long as we are aware of what they successfully illustrate and what constitutes pushing the analogy too far.... They correctly demonstrate the effects of the expansion of the universe, but not the mechanism. That they fail at some level is hardly surprising: we’re representing 4-dimensional pseudo-Riemannian manifolds with party supplies.

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

Thank you all for your responses.
My question in the OP is linked to "expanding space". However, I was not planning to discuss whether space is expanding or not. My main concern was (is) how a physical entity like light can interact with a concept (like space).
When trinitree88 says

then we wouldn´t need "expanding space" as an explanation for the cosmological redshift

__________________
If everyone had even a basic grasp of scientific principles, this planet would be a better place (Phil Plait)

Die Lücke, die wir hinterlassen, ersetzt uns vollkommen (Carl Heinz Schroth)

Cougar,
Thank you for the link to the pdf-file. I downloaded it, but I will have to wait for the weekend to read it. Hopefully, I will understand at least 40% of it.

(I wonder if "I downloaded it" is correct grammar)

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If everyone had even a basic grasp of scientific principles, this planet would be a better place (Phil Plait)

Die Lücke, die wir hinterlassen, ersetzt uns vollkommen (Carl Heinz Schroth)

Looks good to me!

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

A very very faint possibility of Trinitrees
suggestion being testable. If good colour
measurements of stars surrounding the 1987a
supernova were made in the months afterwards,
any reddening due to passing through the shell
of neutrinos could in principle be seen. This
would be in space yet to have the expanding
debris.

But then I have made this suggestion before in
relation to gravitational waves and I do not
think you thought much of the chances.

I posted a similar question to the one I posted above, on page 4 of that thread and I had already downloaded that pdf file but unfortunately I had forgotten to read it yet!

Now I have read it and I see that the authors state the following:

"The key is to make it clear that cosmological redshift is not, as is often implied, a gradual process caused by the stretching of the space a photon is travelling through. Rather cosmological redshift is caused by the photon being observed in a different frame to that which it is emitted. In this way it is not as dissimilar to a Doppler shift as is often implied. The difference between frames relates to a changing background metric rather than a differing velocity. Page 367 of Hobson, Efstathiou, & Lasenby (2005) as well as innumerable other texts shows how redshift can be derived very simply by considering the change in the orthonormal basis of observers with different scale factors in their background metrics. This process is discreet, occurring at the point of reception of the photon, rather than being continuous, which would require an integral. If we consider a series of comoving observers, then they effectively see the wave as being stretched with the scale factor."

and

"In this paper, we have shown how a consistent description of cosmological dynamics emerges from the idea that the expansion of space is neither more nor less than the increase over time of the distance between observers at rest with respect to the cosmic fluid. This description of the cosmic expansion should be considered a teaching and conceptual aid, rather than a physical theory with an attendant clutch of physical predictions. We have demonstrated the power of this pragmatic conceptualisation in guiding understanding of the universe, particularly in avoiding the traps into which we can be lead without rigorous recourse to general relativity."

The authors are saying that, rather like Doppler shift, it is the difference between the frames of reference of the emitter and the receiver that causes an apparent redshift, but the difference is in the scale factor of the background metric, not in their relative velocities which are apparent effects of that changing metric as they can both be considered to be "at rest" relative to the universe as a whole.

To me, this means that both our galaxy and the most distant galaxy we have observed (at just under z=7) are at rest relative to the universe, but the universe has expanded to be almost 7 times larger than it was when the light from that galaxy was emitted, which causes us to receive that light with a wavelength that is apparently almost 7 times larger than it was when it was emitted.

But anyone in the universe who is looking at an object with a redshift of z=7 must be in a universe 7 times larger than it was when the light was emitted, so why should that light be considered to be unchanged in itself? If nobody in the universe that is 7 times larger than it was when the light was emitted can see the light as anything but stretched, then can't light be considered to be stretched? I mean, it is not as if relative velocities have anything to do with it. If that light cannot be measured as unchanged in redshift by any observer in that comoving frame of reference (within a factor of around z=0.1 due to their peculiar velocity), why is it better to consider it as an apparent redshift? If a series of comoving observers will always see the light stretched to their scale factor, then the only observers who could ever measure that light as unchanged are observers close to the original frame of reference of emission - when the scale factor was unchanged.

This is very different from Doppler shift, where it is theoretically possible to be in a comoving frame that can show the light as unchanged.

It does seem to make more sense to think of cosmic expansion and redshift in the way the authors of that paper propose, but I am unsure how to take their statement that "cosmological redshift is not, as is often implied, a gradual process caused by the stretching of the space" when they state later on that "This description of the cosmic expansion should be considered a teaching and conceptual aid, rather than a physical theory".

Just because we can conceptualise it this way and it might help stop certain misconceptions from arising, does that mean it is the correct way to conceptualise it?

(I guess by "unchanged" you mean unstretched)

Being in a universe "7 times larger than it was .." . Do you mean by this that galaxies, stars, planets, men (if they existed at z=7), etc. must have expanded "sevenfold"?? I don´t think so. I understand that the expansion of the universe does NOT result in an expansion of its matter-constituents. If they don´t expand, why should light "expand"?

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If everyone had even a basic grasp of scientific principles, this planet would be a better place (Phil Plait)

Die Lücke, die wir hinterlassen, ersetzt uns vollkommen (Carl Heinz Schroth)

Yes, I meant unstretched.

No, I didn't mean all matter-constituents expand, only the distance between non-bound matter-constituents (i.e. the gaps in between the galactic clusters).

In the standard model (Lambda-CDM) we theorise that the observable universe was only around 40 million light years in radius when the CMBR was emitted, and what was the edge of the observable universe then is known as the "surface of last scattering" and is now considered to be around 46 billion light years away. Our observable universe is now around 1090 times the size it was when the CMBR was emitted, and the surface of last scattering is estimated to have a redshift of around z=1089.

The z factor equates to the difference in the scale factor of the background metric between emission and observation, so when we look at a galaxy with a redshift of z=1 we are looking back to a time when the observable universe was half the size it is currently. To put it another way, the scale factor is usually written as 1+z, so the universe is now twice the size it was when the light was emitted from a galaxy with a redshift of z=1, is now 3 times the size it was when the light was emitted from a galaxy with a redshift of z=2, is now 4 times the size it was at redshift z=3 and so on... so I shouldn't really have said "seven times larger" for z=7, it should have been 8 times the size, and it is easy to get a little confused with the semantics here! (This is why people don't like "word salads"). But that is what the "z" in cosmological redshifts represents, in a nutshell.

So if we imagine the light leaving a galaxy that we measure as having a redshift of z=7, when that light passed a galaxy with a redshift we measure as z=6, the universe had doubled in size since that light was emitted. As it passed a galaxy at z=5, the universe was 3 times the size it was when that light was emitted. So as it passed what we measure as z=4 it had grown to 4 times the size, at z=3 it was 5 times the size, at z=2 it was six times the size and as that light passed a galaxy we measure at z=1 the universe was then 7 times the size it was when that light was emitted. As it reaches us (we are at z=0!), we find the universe is 8 times the size (1+z) it was when the light was emitted.

You are correct to ask why light should expand with the cosmic expansion, and I have no answer except to say that however we conceptualise the mechanism behind it, we measure the wavelength of light as having been stretched by the same factor that we think the universe has expanded by since that light was emitted.

Isn't the question of redshift also connected with true relative motion? And actual relative distance? Cosmological redshift seems to be a theoretical interpretation of observations. I have not heard of or seen observational data that proves it is possible.

If you assume that space cannot physically expand or that space is no