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The simplest of questions.

  1. Feb 13, 2009 #1
    Is it the case that when a galaxy and an observer move apart, that the absorbtion lines within the spectrum are shifted toward the red end, and that this is the case regardless of whether the galaxy is moving away from us, the observer, or we from it, or each from the other?
    As this red-shift can be seen exhibited by galaxies in all directions, at a rate that increases with distance, is it reasonable to suppose that the universe is expanding outward in all directions, and, if so, is this the current thinking? Silly perhaps, but I would like it answered please.
  2. jcsd
  3. Feb 13, 2009 #2


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    Yes to both questions, although "outward" is not quite the way it is looked at. The earth is not at the center of things. At any point in the universe, the expansion is observed in the same way, i.e. "outward".
  4. Feb 19, 2009 #3
    Thank you for your reply. Given that the earth is not at the centre of things, why did several different teams, over 3-4 decades, attempt to "rewind" the outward movement in order to calculate our age. This would only work if we were at the centre, wouldn't it? As you know, over this period several different figures were arrived at resulting in the "Hubble wars". N.A.S.A. called a halt to these "unseemly" disagreements in order not to alienate the public, (their paymasters). Who then, finally stated what the value of Hubble Constant is, and given that the rate of separation is increasing, isn't it now redundant?
  5. Feb 19, 2009 #4


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    You haven't listed all the possibilities, or even the most interesting of them, Peter :biggrin:

    In GR, distances between remote objects can increase without either of the objects moving.

    As far as we know, there is no "outward". There is no outside. There is only expansion.
    Mathman already pointed this out.

    !!!! :biggrin: You amaze me Peter. Why shouldn't they? Expansion has a finite history (governed by the Friedmann equations) so reconstructing the history is one good way to get a handle on the finite age. No centerpoint is required to do this analysis.

    Come on now! Of course not!

    CONSPIRACY!!! :surprised Dear Peter it would take more than a word from NASA to make Astronomers all over the world agree and shut up. Getting cosmologists to agree is like herding cats. And the top ones are all over the world. Lineweaver in Oz, Ellis in Capetown, White at the Max Planck Institute Garching, Maartens in the UK.

    You shouldn't call it the Hubble Constant because it is known to be decreasing. People now tend to call it the "Hubble parameter" or the "Hubble rate". I haven't called it the Hubble Constant for several years. If you wouldn't mind, how about joining me in calling it the Hubble rate?

    Wendy Freedman's team was given the job, using the Hubble Space Telescope. They published the current figure around 1998. The evidence was convincing so everybody adopted their figure of around 71. Since then they keep refining the estimate as new data comes in. Narrowing the errorbar and all that. A lot of new data has come in over the past 10 years, but the estimate has changed very little. Just the uncertainty reduced some.

    The Hubble rate is decreasing is expected to continue decreasing indefinitely, approaching an asymptotic value somewhere around 60. This is a consequence of the accelerated expansion model in general use, the LambdaCDM.

    The rate of decrease is very slow. Won't see a detectable change for thousands of years.
    Is the current value of around 71 redundant???? :uhh:
    By no means! it is an extremely useful parameter!
    Last edited: Feb 19, 2009
  6. Feb 22, 2009 #5
    Marcus, Is the Hubble rate the same in all directions, and, if the answer is yes, has this been tested?
    The rate of decrease is sufficient to be measurable in the 70 years between '29 and '99 although I know that you don't see the universe as I do.
  7. Feb 22, 2009 #6


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    How so? What has the decrease been?
    Compare the percentage decrease with the percentage uncertainty of measurement (which were quite large prior to 1998 and are still significant).
  8. Feb 23, 2009 #7
    This is where I differ from you, and probably all other cosmologists, in that I believe that the increase in the rate at which the galaxies are separating is due to a slowing of the rate of expansion, not an increase. I assumed, perhaps incautiously, that the increase was relative to that made when the "faster with distance" view was first measured. But whatever the increase was relevant to, this is the rate of decrease.
    Re answer #4, Mathman states in answer #2 that all views are outward.
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