Re: post for sci.astro.research

In article <[EMAIL PROTECTED]>, Ted Bunn
<[EMAIL PROTECTED]> writes: 

> On the other hand, let me urge everyone not to think that this
> justifies saying that the cosmological redshift "really is" a Doppler
> shift rather than a gravitational redshift. 

Right, but cosmological redshifts are different than conventional 
gravitational redshifts.  Of course, in practice objects have all three, 
but the typical object at a cosmological distance has a cosmological 
redshift much greater than (and independent of) any Doppler or 
gravitational redshift.

In article <[EMAIL PROTECTED]>, Ted Bunn
<[EMAIL PROTECTED]> writes: 

> If you use this recipe to calculate the cosmological redshift of a
> distant galaxy, the v that you get by this procedure does not
> correspond to the recession speed of the galaxy in any of the usual
> senses.  For instance, you might reasonably define the present
> recession speed of a galaxy to be dr/dt, where t is cosmic time and r
> is the distance to the galaxy measured at constant t.  (This is the
> thing people most often mean when they talk about the recession speed
> of a cosmological object.)  That quantity dr/dt is not the same as the
> v that goes into the above formula.  That (I think) is what Phillip
> Helbig meant by the comment that you can't use the
> special-relativistic formula to calculate cosmological redshifts, and
> he's quite right about that.

Yes, this is the important point.  If someone mentions the normal 
Doppler formula (relativistic version or not) in the context of 
cosmological redshifts, it's just wrong.

> Personally, I think that Phillip tends to oversell the notion that you
> can never (not even at low redshifts) think of the cosmological
> redshift as a Doppler shift. 

You CAN, I just say that you SHOULDN'T.  :-)

I think the disadvantages of overselling it are far outweighed by the 
advantages of avoiding the confusion created by mentioning Doppler 
shifts in the context of cosmological redshifts.