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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. |
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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. |
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In article , Ted Bunn
writes: On the other hand, let me urge everyone not to think that this justifies saying that the cosmological redshift "really is" a Doppler shift rather than a gravitational redshift. Right, but cosmological redshifts are different than conventional gravitational redshifts. Of course, in practice objects have all three, but the typical object at a cosmological distance has a cosmological redshift much greater than (and independent of) any Doppler or gravitational redshift. In article , Ted Bunn writes: If you use this recipe to calculate the cosmological redshift of a distant galaxy, the v that you get by this procedure does not correspond to the recession speed of the galaxy in any of the usual senses. For instance, you might reasonably define the present recession speed of a galaxy to be dr/dt, where t is cosmic time and r is the distance to the galaxy measured at constant t. (This is the thing people most often mean when they talk about the recession speed of a cosmological object.) That quantity dr/dt is not the same as the v that goes into the above formula. That (I think) is what Phillip Helbig meant by the comment that you can't use the special-relativistic formula to calculate cosmological redshifts, and he's quite right about that. Yes, this is the important point. If someone mentions the normal Doppler formula (relativistic version or not) in the context of cosmological redshifts, it's just wrong. 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. |
#4
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post for sci.astro.research
In article , Ted Bunn
writes: On the other hand, let me urge everyone not to think that this justifies saying that the cosmological redshift "really is" a Doppler shift rather than a gravitational redshift. Right, but cosmological redshifts are different than conventional gravitational redshifts. Of course, in practice objects have all three, but the typical object at a cosmological distance has a cosmological redshift much greater than (and independent of) any Doppler or gravitational redshift. In article , Ted Bunn writes: If you use this recipe to calculate the cosmological redshift of a distant galaxy, the v that you get by this procedure does not correspond to the recession speed of the galaxy in any of the usual senses. For instance, you might reasonably define the present recession speed of a galaxy to be dr/dt, where t is cosmic time and r is the distance to the galaxy measured at constant t. (This is the thing people most often mean when they talk about the recession speed of a cosmological object.) That quantity dr/dt is not the same as the v that goes into the above formula. That (I think) is what Phillip Helbig meant by the comment that you can't use the special-relativistic formula to calculate cosmological redshifts, and he's quite right about that. Yes, this is the important point. If someone mentions the normal Doppler formula (relativistic version or not) in the context of cosmological redshifts, it's just wrong. 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. |
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