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?