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neophyte question about hubble's law
Thus spake Phillip Helbig---remove CLOTHES to reply
LOTHESvax.de In article , Oh No writes: Thus spake dfarr --at-- comcast --dot-- net The 'Hubble's law' Wikipedia article states '...that the velocity at which various galaxies ARE receding from the Earth IS proportional to their distance from us.' (emphasis added) Bear in mind that this applies only for small cosmological distances That depends on how one defines Hubble's Law. See my other post in this thread and the recent thread in sci.physics.research. Hubble's law is defined as a linear relationship between redshift and distance. This relationship only holds for small cosmological distances. For large cosmological distances such a relationship doesn't even make sense unless you first define what you mean by distance, and there certainly isn't a natural definition of distance which would give a linear relationship. You would only get a linear relationship if you defined distance from Hubble's law - which is tautology, and certainly an unhelpful measure of large scale structure. Perhaps. It's just a pity Harrison's ideas about the expansion of space time are somewhat inaccurate. Care to elabourate? I have only dipped into the book, so cannot comment on much of it, but the discussion of the Hubble sphere on p281 struck me as particularly misleading. It is absolutely not meaningful to talk about the recession velocity of a star in the early universe with respect to ourselves now. Think of the balloon analogy. Cosmological redshift is the consequence of cosmological expansion, not recession velocities. It just happens that, for small cosmological distances, cosmological expansion looks like recession velocity. You can't take that concept too far. Regards -- Charles Francis moderator sci.physics.foundations. charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces an= d braces) http://www.rqgravity.net |
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neophyte question about hubble's law
In article , Oh No
writes: Hubble's law is defined as a linear relationship between redshift and distance. Some people define it as the linear relationship between velocity and distance, i.e. between the present derivative, with respect to cosmic time, of the proper distance and the proper distance of the object. (Hubble OBSERVED a relation between redshift and luminosity, which can be taken as a proxy for distance. At small redshifts, none of these distinctions matters. Perhaps. It's just a pity Harrison's ideas about the expansion of spac= e time are somewhat inaccurate. Care to elabourate? I have only dipped into the book, so cannot comment on much of it, but the discussion of the Hubble sphere on p281 struck me as particularly misleading. It is absolutely not meaningful to talk about the recession velocity of a star in the early universe with respect to ourselves now. Think of the balloon analogy. Cosmological redshift is the consequence of cosmological expansion, not recession velocities. It just happens that, for small cosmological distances, cosmological expansion looks like recession velocity. You can't take that concept too far. Yes, but it does go this far. Assuming a homogeneous and isotropic expansion, the recession velocity is proportional to distance---otherwise the homogeneity and/or isotropy are destroyed. At large redshifts, though, these aren't observable distances and observable velocities. (Knowing the cosmological parameters, though, they can still be calculated.) |
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