Re: Cosmological redshift and Doppler shift

Dag Oestvang <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>...
> [EMAIL PROTECTED] wrote:
> 
> > Dag Oestvang   wrote:
> > >[EMAIL PROTECTED] wrote:
> > >
> > >> Dag Oestvang  wrote:
> > >>
>  
[Lots of complicated stuff]

Well, gentlemen, I don't want to get involved in the technicalities
of who is right and who wrong. But this discussion has definitely
established *one* incontrovertible fact: namely that Ted Bunn's
contention that there is something to be gained *pedagogically*
from pretending that cosmic redshift is somehow a Doppler shift
*is* wrong! If this idea is a cause of such confusion even 
among experts, does Ted really see it helping students? 

The way this stuff was explained to me was as follows. Gravitation
is essentially geodesic deviation. Geodesic deviation can work in
spatial directions [eg tearing you apart inside a black hole]
but it can also involve time. Geodesic deviation of that kind
makes distances between inertial observers increase. That is 
called the expansion of the universe, and it has nothing to do
with "motion". [Clearly there is no geodesic deviation in the 
[really rather ridiculous] Milne "cosmology", which indeed isn't
really expanding.] Of course, galaxies can move, and have doppler
shifts. That is called peculiar motion. To me all this seems
very clear indeed, and just why Ted wants to confound geodesic
deviation with peculiar motions, two *utterly different* things,
is frankly a mystery to me. Indeed, it looks like the most
perfect recipe for total confusion. Give it up, Ted! :)