Re: post for sci.astro.research

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

> >> 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.
> 
> Let me ask you this.  Suppose you lived in an open FRW Universe with
> Omega = 10^(-50).  You observe a (very low-mass!) galaxy at a redshift
> z = 0.1.  Would you say it's wrong to call that redshift a Doppler
> shift?  Remember that in this spacetime, the geometry is Minkowski,
> with errors of only one part in 10^50 all the way out to the horizon
> scale!  
> 
> In particular, the geometry of this spacetime is much closer
> to Minkowski than the geometry of spacetime near Earth's surface,
> yet the cops use radar guns to measure speeds (interpreting frequency
> shifts as Doppler shifts) all the time.

In this case, I would say it's OK.  In our univese, if the redshift is 
small enough, it's OK.  Still, I don't see what advantage it brings, 
since at larger redshift in our universe it is not correct.  My 
experience has been that it is more trouble than it is worth, in 
practice.  (I've even seen folks try to explain the cosmological 
redshift to a popular audience and mention the Doppler effect---but they 
had to explain the Doppler effect first!  That's certainly going 
overboard.  IF one already knows what the Doppler effect is, then it 
might be OK to mention it, but one should point out that it is only 
correct in the limiting cases.)

This might be a good time to direct readers of this thread to
astro-ph/0310808.