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I How on Earth This is possible? (distance to a far galaxy)

  1. Jun 26, 2018 #1

    Read the Section in this link:
    Cosmic alignment
    The galaxy we investigated has the catchy name ESO 325-G004 – let’s call it E325. Located some 450m light years away, it is one of the closest examples of a rare cosmic alignment – sitting directly between us and a second, more distant, galaxy. The background galaxy in this case is some 17 billion light years further behind. The centres of these two galaxies are aligned to better than one ten-thousandth of a degree.

    17 billion light years???? Isn't the universe 13.7 billion years old?
  2. jcsd
  3. Jun 26, 2018 #2


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    At 17 Gly is where it is now, not at the time when it emitted the light that is captured in the picture.
  4. Jun 26, 2018 #3
    Need a lil elaboration :eek: ... as I am imagining in this way that the light from that Galaxy literally took 17 billion years to travel to us in with velocity 'c'.
  5. Jun 26, 2018 #4


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  6. Jun 26, 2018 #5


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    Let's say the light is emitted at a distance D from the observer. In a non-expanding universe, that initial distance would remain constant in time, and it'd take t=D/c to cover the distance.
    In an expanding universe, as the light travels, the distance D grows - both in the part that the light still has to cover, and the part already covered.
    This means that the time needed to reach the observer is higher than t=D/c, and the distance at which the source galaxy ends up being by the time of reception is higher than the initial distance D.

    edit: typo
    Last edited: Jun 27, 2018
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