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Red shift dilemna

  1. Jun 16, 2004 #1
    HELP red shift dilemna

    is it at all possible that the apparent red shift of distant stellar light sources is not actually as a result of expansion?? For example, the whole distance is a mess of gravity wells that could cause red-shift, as well as the emitting characteristics of Hydrogen which is the most abundant element in the known universe and is absorbing/re-emitting photons throughout the distance?
    Last edited: Jun 17, 2004
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  3. Jun 17, 2004 #2


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    Quantitative studies have indicated that the gravitational effect would be nowhere near enough. Hydrogen gas clouds show up as discrete absorption lines, not as red shift.
  4. Jun 18, 2004 #3


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    Welcome to Physics Forums jfxjmn!

    Looking at the observed redshifts of just galaxies you can quickly see why it would be very unlikely to be purely gravitational.

    We can now observe the redshifts from different parts of distant galaxies, enough (in some cases) to make crude rotation curves (think of an edge-on spiral galaxy - the parts moving towards us will be blue-shifted (relative to the galaxy as a whole), and those away, red-shifted; the amount of blue and red-shifting, when plotted against distance from the centre of the galaxy gives a 'rotation curve'). These distant galaxies have rotation curves like those of galaxies closer to home; they also look like our neighbours (I'm oversimplifying, there are many interesting details).

    So, if a substantial part of the red-shift were due to something other than expansion (and local motions of galaxies, within their local cluster, for example), you'd have an even stranger universe (than one which is expandint)! How strange depends on what takes the place of red-shift = expansion; for example, if distances are now all completely different, there'd be super-giant galaxies that were, in every other respect, quite normal (or stars which emitted vastly more light than exactly the same sort of stars we have locally) ... or the reverse ("Mommy, I shrank the galaxies!").

    The great thing about modern physics is that it's a house of cards* ... a small push on one part and the whole lot collapses in confusion. Weird, you may be saying, how could that be 'a great thing'?!? Because it allows some quick checks of alternatives! As in your example of "the whole distance is ... the emitting characteristics of Hydrogen which is the most abundant element in the known universe and is absorbing/re-emitting photons throughout the distance" If that were the case, there'd be a whole lot of other things that'd likely be different too (for example, polarization, X-ray emission, 'fingerprints' on the CMBR, maybe even TeV gammas?), but since we don't see those other things, it's highly unlikely something odd about H is causing the universal red-shifts!

    *If you leave out the equations, there are ~25 numbers ('constants') which must be measured; everything else - from the wavelengths of the millions of atomic and molecular 'lines', to the mass and composition the core of a star must have to go supernova - follows from these.
  5. Jun 21, 2004 #4
    Thank you very much for the explanation. I woke up one morning and was confused, now I know. Thanks again, and thanks a lot for the quantity of answer that I got, that was awesome!!
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