Re: post for sci.astro.research

Ted Bunn wrote:

>
> 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.  At low redshifts (distances much smaller
> than the horizon or the curvature scale), spacetime is Minkowski to an
> excellent approximation, and in that approximation galaxies are flying
> away from us.  By all means go ahead and think of the redshifts of
> those nearby galaxies as ordinary Doppler shifts if you like.

    The attractiveness of the unified approach to spectral shifts is
    that this approach clearly shows how spectral shifts are related
    to the geometry of space-time.

    In particular, spectral shifts due to curved space-time geometry
    should never be thought of as ordinary Doppler shifts in flat space-time.
    That is, it is not meaningful to approximate curved space-time with
    flat space-time and at the same time keeping spectral shifts due
    to curved space-time; such a scheme would be inconsistent.

    In particular this is true for a weak gravitational field, and also for
    the Friedmann models.