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Is there any evidence for universe still expanding?

  1. Jan 29, 2014 #1

    Lok

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    Considering that when we look at distant red-shifted structures of the universe we also look at their respective speed at that respective time of emission, is there any observational evidence that the acceleration is still happening today?
     
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  3. Jan 29, 2014 #2
    All observations of expansion are in the past, due to light speed limits. Locally we cannot observe expansion due to being in a gravitationally bound region. However we can and do observe expansion still occurring at the nearest systems not gravitationally bound. However that is still a past event observation.

    With all observations their is no reason to suspect of even consider that expansion has stopped accelerating as all our observations still supports expansion as still occurring
     
  4. Jan 29, 2014 #3

    Lok

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    Thanks for your reply.

    Is it not possible to see if expansion is still happening by time lapse? Has this been done?
    Would comparing the redshift of a Gravitationally lensed system be possible? I mean where in one path the light travels a few hundred years more.

    I ask these as evidence besides redshift for an expanding universe is scarce on wiki and the internet. And redshift is just the speed at a past point. There is nothing in it to say that this acceleration did not just dim away and all we see is the past image of it.
     
  5. Jan 29, 2014 #4

    phinds

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    There is also, in exactly the same way, no evidence whatsoever that the sun did not disappear 1 minute ago and we just haven't noticed yet and won't for another 7 minutes.
     
  6. Jan 29, 2014 #5

    Lok

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    Nice analogy. Yet if the Suns disappearance is gradual over an hour or so. With the difference in light intensity of one minute we can statistically predict it's demise.

    In the same way if we have a distant strongly red-shifted light-source I expect a slightly bigger redshift the next year (maybe a few years more).
     
  7. Jan 29, 2014 #6

    Chronos

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    Our redshift measurement uncertainty is no less than 1%, at best. Using a naïve approach, the CMB [z~1089] as our test subject, and Jorrie's cosmological calculator, http://www.einsteins-theory-of-relativity-4engineers.com/cosmocalc_2013.htm, the time required for a 1% change in distance at z~1089 is 6361 years. It would take a VERY long time to detect any measurable change in the CMB redshift.
     
  8. Jan 29, 2014 #7

    Bandersnatch

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    Hey, Chronos. I thought the rate of expansion was 1/144th of a percent per million years? Shouldn't it then take 144 million years to obseve a 1% increase in distance?
     
  9. Jan 29, 2014 #8

    phinds

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    I have no idea what you are talking about. If the sun magically disappears the loss of light at the Earth would be instantaneous after 8 minutes with no prior indication at all.
     
    Last edited by a moderator: Jan 29, 2014
  10. Jan 29, 2014 #9

    Chronos

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    That applies to the current Hubble flow [H0]. I used the CMB at z~1089 to illustrate how long it takes to notice any difference even when the Hubble flow was enormous compared to the present. If you plug in z=0 to Jorrie's calculator you will note the time required for a 1% growth in cosmic distance is ~140 million years.
     
    Last edited: Jan 29, 2014
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