Terminology for Doppler shifts

[bcrowell]

Here are some different descriptions of Doppler shifts:

1. The gamma rays in the Pound-Rebka experiment were Doppler shifted.
2a. We make a cosmological model using coordinates in which an object moving with the Hubble flow has a zero coordinate velocity, so all galaxies are "at rest." An observer in galaxy A observes a red-shifted photon from cosmologically distant galaxy B.
2b. This is exactly the same physical situation as in 2a, but we pick coordinates in which A is at rest but B is in motion, and furthermore we decide to pick these coordinates such that the special-relativistic equation for the Doppler shift gives the observed result when we plug in the coordinate velocity attributed to B.

By general covariance, 2a and 2b are physically indistinguishable. The difference between them is purely verbal.

Everyone would describe 1 using the term "gravitational Doppler shift."

If we have to make the purely verbal distinction between 2a and 2b, probably most people would describe 2b as a "kinematic Doppler shift." Personally I'm in the habit of describing 2a as a "gravitational Doppler shift," but I think my usage may be nonstandard, and it does have the disadvantage that 2a, unlike 1, can't be computed from a gravitational potential, so in some sense the same word is being used to describe dissimilar phenomena.

Is there a better or more standard term for 2a? The only thing I can think of is something cumbersome like "Doppler shift due to the expansion of space."

-Ben

 

[Dale]

How about "cosmological redshift"? Not a standard term afaik, but it is short and clear.

 

[bcrowell]

How about "cosmological redshift"? Not a standard term afaik, but it is short and clear.

But wouldn't that cover both 2a and 2b? What if you want to distinguish 2a from 2b?

 

[Dale]

I would call 1 "gravitational redshift", 2a "Doppler shift", and 2b "cosmological redshift"

 

[bcrowell]

I would call 1 "gravitational redshift", 2a "Doppler shift", and 2b "cosmological redshift"

Hmm...so to you, if someone says "cosmological redshift," it implies that they're *not* describing the distant galaxy as being in motion relative to us? It seems to me that most cosmologists actually have a tendency to refer casually to cosmological redshifts as if they were purely kinematical (although they surely know that GR doesn't allow such a distinction to be made in any absolute sense).

 

[Dale]

Well, I am certainly not a cosmologist, so take my comments with a grain of salt.

Oops, I just noticed that I got 2a and 2b backwards in my previous reply.

In my mind "Doppler shift" implies motion (which of course is coordinate dependent). So 2b qualifies as "Doppler shift". But I wouldn't call 1 a Doppler shift in standard Schwarzschild coordinates, I would call it "gravitational redshift". And then 2a doesn't qualify as a Doppler shift either, but I would call it "cosmological redshift" to distinguish it from the static spacetime case of 1.

 

[dm4b]

Hmm...so to you, if someone says "cosmological redshift," it implies that they're *not* describing the distant galaxy as being in motion relative to us? It seems to me that most cosmologists actually have a tendency to refer casually to cosmological redshifts as if they were purely kinematical (although they surely know that GR doesn't allow such a distinction to be made in any absolute sense).

I think you're right. It's kind of analogous to others talking casually about virtual particles, while not actually beleiving they exist.

I've always understood (1), (2) and (3) as DaleSpam mentioned.

If you're interested, Sean Carrol has a nice description of the difference between cosmological and Doppler shifts, in his GR book (in two different spots). It's not super sophisticated, but it's a nice and clear demonstration.