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Universe expansion and hubble's law

  1. Apr 10, 2010 #1
    I've been reading about the red-shift, the universe expanding, ect... And about the observation that the further a galaxy is from us, the faster it is moving away from us, accounting for an ever expanding universe.

    I'm also curious about the fact that the further the galaxy, the further 'in the past' the observation of the state of the galaxy is.

    Say a galaxy cluster is 500 million light years away from us, it is only telling us what the rate of expansion of the universe (in a broad sense) was 500 million years ago.

    I'm sure that is accounted for when deducing the current rate of expansion of the universe, but I was wondering if the fact that galaxies far away move faster and accelerate faster is only because we observe them as they were closer to the origin of the universe.

    So in term of a universe where the expansion is slowing down, it would make sense to see a closer, more 'up to date' galaxy be moving and expanding slower than a distant, 'younger' galaxy.

    Or is that 'time-shift' effect due to observation and the speed of light insignificant compared to the actual 'red-shift' observed?
  2. jcsd
  3. Apr 10, 2010 #2


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    It is not just the fact that the further a galaxy is, the greater its recession, that leads us to the conclusion that the universe will continue to expand. We would see that even if the universe's expansion was slowing. That conclusion is based on the exact ratio of red-shift to distance and how it changes over distances.

    If we saw that this ratio did not change with distance, we could conclude that the rate at which the galaxy expands does not change over time. But we do not see this; what we see is that the red-shift/distance ratio varies with distance, which indicates that the universe expansion rate has sped up over time.
  4. Apr 10, 2010 #3
    thanks! :)

    But does it changes over time? The red-shift we observe was some time in the past. Would it be fair to say that the red-shift of that galaxy 'now' could be different? Obviously we cannot observe that, since the speed of light is finite.

    Maybe I need to read a bit more about the actual maths.
    Last edited: Apr 10, 2010
  5. Apr 11, 2010 #4
    I see what you are asking.
    A galaxy that is 2 billion light years away exhibits a velocity that was happening 2 billion years ago and one that is 13 billion light years away is exhibiting the velocity it was travelling 13 billion years ago. We won't know what their present velocity is for another 13 billion years.

    Has this been taken into account in the calculations and how?
  6. Apr 11, 2010 #5


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    for some hands-on experience with the conventional cosmo model, try this.

    google "cosmos calculator"

    You first need to prime it by typing in the correct values of 3 parameters
    Put in .27 for "matter fraction"
    Put in .73 for "cosmological constant"
    Put in 71 for Hubble.

    Then you can put in any redshift and find out the recession speed THEN and the recession speed NOW!

    this is the kind of thing I see you asking about. What the standard model says about the recession rate back then when it emitted the light, and the recession rate now on the day that we receive the light with our telescope.

    All the numbers come from a simple math model called the Friedman equations. You can google Friedman or Friedmann with two Ns and get that to look at but it can actually help more to play around hands-on. The friedman model cosmos is built into the calculator.

    there are a bunch of these calculators, all give you basically the same answers but some may give a lot more, with fancier terminology. Compare for example when you google
    "wright calculator" or "cosmo calculator" (cosmo without the final S). You get more frills.
    The first one I told you about is simpler to read because it is barebones. Doesn't give all the extras.

    So why don't you google "cosmos calculator" and give it a try? Ask questions if you run into any difficulty or want to discuss the results.
  7. Apr 11, 2010 #6
    Cool. So it is taken into account.

    Are galaxies really travelling that close to the speed of light from each other? That's amazing.

    Last edited: Apr 11, 2010
  8. Apr 11, 2010 #7
    There was another logic flaw that was troubling me on the same subject, but seems to stem from my lack of understanding of cosmology and relativity.

    I'm reading http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf" [Broken] first!
    Last edited by a moderator: May 4, 2017
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