# Red shift

1. Aug 7, 2008

### wolram

arXiv:0808.1081 (cross-list from physics.pop-ph) [ps, pdf, other]
Title: The kinematic origin of the cosmological redshift
Authors: Emory F. Bunn, David W. Hogg
Comments: 14 pages. Submitted to Am. J. Phys
Subjects: Popular Physics (physics.pop-ph); Astrophysics (astro-ph); General Relativity and Quantum Cosmology (gr-qc); Physics Education (physics.ed-ph)
A common belief among cosmologists is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must instead be viewed in terms of the stretching of space. We argue that the most natural interpretation of the redshift is in fact as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that of coordinate freedom, specifically the idea that it is always valid to choose a coordinate system that is locally Minkowski. We show that, in any spacetime, an observed frequency shift can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a family of observers along the photon's path. In the context of the expanding Universe, the kinematic interpretation corresponds to a family of comoving observers and hence seems to be the more natural one.

2. Aug 7, 2008

### marcus

Wolram, I was just coming to the forum to post the same link!
This paper has an interesting interpretation of the redshift as the cumulative effect of an infinite sequence of infinitesimal Doppler shifts!

One imagines an infinite number of observers distributed along the path of the light, each observer with his own reference frame.

As the light goes from one observer to the next, it experiences a Doppler shift.

This produces the same effect exactly---one gets the same answer. I think it is an excellent paper. Thanks for posting it!

3. Aug 7, 2008

### jonmtkisco

Hi Wolram,

I agree it is an interesting paper. It is broadly supportive of the ideas we have been discussing in the Superluminal Recession and Cosmological Redshift thread. I note the following excerpt:

"We should not compute velocities at a fixed instant of cosmic time (either t = te or t = t0); rather, we should compute the velocity of the galaxy at the time of light emission relative to the observer at the present time. After all, if a distant galaxy’s redshift is measured today, we wouldn’t expect the result to depend on what the galaxy is doing today, and certainly not on what the observer was doing long before the age of the dinosaurs."

The authors pursue a complicated analysis of 4-vectors to illustrate this point. But I think the same point is illustrated more simply by the diagram I attached to an earlier post in the Superluminal Recession thread. It shows that Doppler recession velocity should be calculated as the relative difference between the emitter's recession velocity at the time of emission, and the receiver's recession velocity at the time of reception, or:

$$v_{doppler} = \frac{v_{e} + v_{r}}{2}$$

One normally thinks of Doppler redshift as being derived from the discrete velocity difference between the emitter and receiver. This paper treats it as an accumulation of infintesimal velocity changes along the route, but I think it amounts to the same thing.

It's interesting that these authors consider Doppler redshift and gravitational redshift to be alternative concepts rather than elements of a combined solution. I need to think more about that. I wish the paper were more clear about exactly what aspect of gravitational redshift they are referring to.

Also, rather strangely, although the paper claims that the accumulated Doppler redshifts must calculate the same result as the traditional cosmological redshift formula, I don't see where they directly prove that to be true. They seem to rely on an indirect string of logic.

Jon

Last edited: Aug 7, 2008
4. Aug 8, 2008

### Sundance

G'day from the land of ozzzzzz

This is quite interesting paper.

http://arxiv.org/abs/0807.4177

Evolution of the Intergalactic Opacity: Implications for the Ionizing Background, Cosmic Star Formation, and Quasar Activity

Authors: C.-A. Faucher-Giguere, A. Lidz, L. Hernquist, M. Zaldarriaga (Harvard University)
(Submitted on 26 Jul 2008)

and

http://arxiv.org/abs/0807.2641
Evidence against non-cosmological redshifts of QSOs in SDSS data

Authors: Sumin Tang, Shuang Nan Zhang
(Submitted on 16 Jul 2008 (v1), last revised 18 Jul 2008 (this version, v2))

Abstract: In the unusual intrinsic QSO redshift models, QSOs are ejected by active galaxies with periodic non-cosmological reshifts, thus QSOs are generally associated with active galaxies, and certain structures will be revealed in the QSO redshift distribution. As the largest homogeneous sample of QSOs and galaxies, SDSS data provide the best opportunity to examine this issue. We review the debates on this issue, focused on those based on SDSS and 2dF data, and conclude that there is no strong connection between foreground active galaxies and high-redshift QSOs. The existence of two dips in the SDSS QSO redshift distribution at z=2.7 and 3.5 has recently re-ignited those controversial debates on the origin of QSO redshift. It also turned out that both dips are totally caused by selection effects and after selection effects have been corrected, the two dips disappear and no structure in the redshift distribution of SDSS DR5 sample. These results support that the reshifts of QSOs are cosmological.

Last edited: Aug 8, 2008