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Message: <blockquote data-quote="briggart" data-source="post: 9817187" data-attributes="member: 6805135">It is mainstream in the sense both authors are working cosmologists and know what they are talking about. The paper was part of a genuine discussion between members of the community, which I'm not sure it found a definite resolution or simply faded away. I wouldn't call it mainstream in the sense that "this is how it is explained in every cosmology textbook" today, partly because I feel some of this is the cosmological version of the "interpretation of quantum mechanics". People generally agree on the math, but have different opinions on the interpretation of the results for things so far removed from our everyday experience. Yes, that is correct. In curved spaces there is no way to uniquely relate quantities measured at vastly different places. Assume that you are on the Equator and have measured a vector quantity and found that it points straight North. Then assume that you want to translate this vector all the way to the North pole in a way that the vector always remains parallel to itself at any point of the translation. The direction of the vector at North pole will be different if you go straight North from the Equator, or if you first move along the Equator by 90 degrees and then move North. This doesn't happen on a flat surface.In cosmology, if you want to relate a quantity as measured by a far away observer to what you would measure, you face a similar issue. Bunn & Hogg choose to transport the velocity along the path a photon would travel from the distant source to us, and they find that redshift can be fully described by incremental Doppler shifts. They claim this is the "only natural path". I agree that is "natural", not sure about "only" (see now where subjective opinions are entering the discussion?).This paper (<a href="https://arxiv.org/abs/0911.3536" target="_blank">The kinematic component of the cosmological redshift</a>) instead chooses a different path and finds that redshift is a combination of a kinetic contribution (they call it Dopplerian) due to the part of the path connecting the position of the emitter with the one of the observer, and a gravitational contribution due to the change to the local inertial frame at the observer position. For a somewhat different viewpoint, Nick Kaiser (<a href="https://arxiv.org/abs/1312.1190" target="_blank">Astronomical Redshifts and the Expansion of Space</a>) argues that in general redshift cannot be fully reduced to kinematic effects. I think that the introduction of this paper is also a very balanced summary of the previous discussion.Overall, I think that all papers agree that cosmological redshift is not a unique effect but it can be expressed in terms of the usual Doppler and gravitational redshifts, so the argument that the existence of a third form of redshift proves that expansion of space is a real physical effect distinct from movement of galaxies is not valid. Similarly for the issue of superluminal recession velocities.Haven't read either of these, but here is a public version of the second one: <a href="https://arxiv.org/abs/1901.03602" target="_blank">Velocities of distant objects in General Relativity revisited</a>My understanding is that Bunn & Hogg are saying that there is no continuous expansion, so that is not in contradiction with other possible local effects. But I haven't read the paper you linked, so maybe I'm missing something.</blockquote>

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