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B The known universe is expanding?

  1. Feb 26, 2017 #1
    Is our known universe expanding?
    Hypothetically we should we be seeing a light year further every year as the light hits us from distant stars.

    However once we take into account the expansion of the universe, are we really seeing any further?
    I am confused by the while topic of light and being able to detect stars only when light hits us.
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  3. Feb 26, 2017 #2


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  4. Feb 26, 2017 #3


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  5. Feb 26, 2017 #4


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    @Lewis123 I recommend the link in my signature
  6. Feb 26, 2017 #5
    At a certain distance, which I will loosely call the edge of the known universe, the objects (galaxies) are receding away from us faster than the speed of light, so in order to be able to see these objects you will have to wait the initial amount of time that it would take light to reach you and add that to the extra time caused by the recession of those distant galaxies.
  7. Feb 26, 2017 #6


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    Strictly, it depends how far those stars are. The very faintest galaxies (z ~10) presently recede from us at about 2 lyr/yr, due to the expansion during the time the light was in transit. So I suppose one can say that we can potentially see galaxies that are 3 lyr farther every year, simply because there was an additional year of light travel time.
  8. Feb 27, 2017 #7


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    That distance is called the Hubble radius, where the recession rate is 1 lyr/yr. With the present accelerating expansion, if a star crosses the Hubble radius, we will never receive any light emitted from then on.

    In the past, when the expansion was decelerating, the Hubble radius was increasing rapidly and light from beyond the Hubble radius could eventually cross it towards us. Most of the galaxies that we observe today falls into this category, but most have also already crossed the Hubble radius again and present emissions will not reach us.
    Last edited: Feb 27, 2017
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