From: Michael Wood-Vasey (wmwood-vasey@lbl.gov)
Date: Wed Apr 16 2003 - 09:25:39 PDT
On Wed, Apr 16, 2003 at 08:55:36AM -0700, Don Groom wrote:
> He argues that it is perfectly valid to threat the cosmological redshift
> as a relativistic Doppler shift in "adding" velocities. Several of us had
> an extended discussion of this at teatime a week or two ago.
You just have to do it right.
> While it is easy to construct examples (with pathological R(t)) in which
> the redshift cannot be construed as an expansion velocity, we still behave
> schizophrenically and talk about "the expanding Universe." Eric can object
> when he reads this, but he argues that this is OK when combining proper
> motion with cosmological expansion.
Because it's really about correcting your _observations_ to some
preferred frame. The argument for going to the CMB frame is really
that we believe (for good reasons) that the CMB frame represents a
more fundamental (co-moving) frame. We can always transform our
observations from our frame into what they would be if we had
observered them in the CMB.
BTW, when doing this it's a lot easier to start from the beginning and
figure out the z from spectra shifted to the CMB frame rather than
apply a correction later. It's the discussion of how to correct
already-derived redshifts that makes things confusing.
- Michael
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