Re: Galaxies without dark matter halos?

In article <[EMAIL PROTECTED]>,
Dag Oestvang  <[EMAIL PROTECTED]> wrote:
>[EMAIL PROTECTED] wrote:

> I will try to clarify: Unless V to a good approximation appears 
> from a cleverly arranged field of 4-velocities in flat space-time 
> (this is exact in the Milne model), there is no natural way to 
> interpret V in this way if the effects on V of curved space-time 
> on parallel transport is not negligible.

That's true.  But in the low-redshift limit, the effects on V of
curved spacetime on parallel transport *are* negligible.

Proof: Try it and see!  Neglect spacetime curvature and
parallel-transport V.  In the low-redshift limit, you get the right
answer.  Therefore, neglecting spacetime curvature didn't
do any harm.  Spacetime curvature was negligible.


> In other words; insofar as the effects on V coming from the
> curvature of space-time can be neglected it is perfectly all
> right to interpret V as coming from a field of 4-velocities in
> flat space-time. But it is not if the size of V depends crucially
> on space-time curvature.

I agree.  But again, the last "if" clause is false in the limit
I'm considering.

Do you think that "the size of V depends crucially on space-time 
curvature" in an Omega = 0 FRW Universe?  (I certainly hope not,
since there is no curvature in this spacetime.)  How about
in an Omega = 10^(-50) FRW Universe?  

>   It seems that we mostly agree. This is my position: If the effects 
>   of curved space-time on V are negligible then interpretations of 
>   spectral shifts as Doppler effects in flat space-time are OK; 
>   otherwise not. Do you agree?

Yes.  But again, the "If" clause is false in the low-redshift
limit of an FRW Universe.

-Ted

-- 
[E-mail me at [EMAIL PROTECTED], as opposed to [EMAIL PROTECTED]