What are the Three Sources of Red-Shift in Astronomical Observations?

[nomadreid]

There appears to be three sources of red-shift: or are there only two, and two of the following are equivalent?
(1) Classical Doppler shift
(2) Relativistic Doppler shift
(3) Red-shifting from the expansion of space
Otherwise put, which ones are calculated for retreating galaxy clusters and the like?
Thanks.

 

[bcrowell]

The classical Doppler shift is just the low-velocity approximation to the special-relativistic Doppler shift.

In SR, there is only one kind of Doppler shift, which is the kinematic Doppler shift due to the relative motion of the source and the observer.

In GR, you get both kinematic and gravitational Doppler shifts ( http://en.wikipedia.org/wiki/Pound-Rebka_experiment ).

When you apply GR to cosmology, there is no clear distinction between kinematic and gravitational Doppler shifts. This is because GR doesn't have an unambiguously defined way of defining the relative motion of two objects that are distant from one another. All cosmological redshifts can be interpreted as kinematic or gravitational, or as some combination of those.

 

[Ich]

There is only one type of redshift. You can split it approximately into different components if you have certain symmetries to exploit.
For example, if you can define reasonably well static coordinates (approximate time-translation symmetry), you speak of gravitational and doppler redshift. (The latter is always the relativistic one).
Or, if you have approximate spatial homogeneity and istotropy, you speak of redshift from the expansion of space and redshift from peculiar motion.
If you have both kind of symmetries, you can use whichever you want. The physics is not different, it's always GR.

 

[nomadreid]

Thank you, bcrowell and Ich, for your helpful and quick replies.

 

[Unite physics]

Increasingly visible space is necessarily balanced with increasingly transparent/invisible space in astronomical/telescopic observations.