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Expansion of space - Red shift

  1. Apr 8, 2012 #1
    I have always been puzzled by how we see an object from the past and can tell what it is doing now if it is billions of light years away.

    Given that;

    1. the shift in the measurement of the light spectrum indicates the speed at which an object is retreating or getting closer.

    2. that the furthest objects are retreating quicker than the nearest.

    3. that when we look far away we are looking into the past.

    Q1> How do we distinguish that an observable shift in the light spectrum is greater or less now than at any other time in a distant object's time line?

    Q2> If the universe expanded quicker in the early stages of its existence and then progressively slower, would we not be observing the light that is reaching us now from that period as red shifted more than from objects closer to us?
  2. jcsd
  3. Apr 8, 2012 #2


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    Staff: Mentor

    When we measure redshift, we are only measuring a single moment in time. Since the redshift tells us that the object is moving away, we infer that it was closer to us in the past. Since closer objects have less redshift, we infer that the closer something was to us the slower it was receding. Does that make sense?

    Actually, I believe something like this has happened. Our measurements of supernova redshift has told us that the expansion, which WAS slowing down, has started to accelerate. I think that when we look out we can cut the universe into "slices" of time the further away they are. IE looking at an object 1 billion years in the past is x distance away. (It is not 1 billion light years away, but far more due to expansion) This lets us observe the redshift and look for differences between all these slices, which lets us see how the universe was at different periods in the past. I believe this is how we discovered that the expansion was accelerating.
  4. Apr 8, 2012 #3


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    Gold Member

    Yep, that's what I've read in several places.
  5. Apr 8, 2012 #4
    Thanks for taking the time to reply Drakkith

    I guess my confusion comes from focusing on one single element of the argument for expansion, measurements of red shift. Alone it doesn't make the case. It is only when you add it to other elements that the total argument becomes convincing. For instance, the fact that expansion is proportional to distance and that observation is consistent with expansion solutions that have been derived from Einstein's equations.

    You mention:
    Can you point me towards a source for my reading please?
  6. Apr 8, 2012 #5


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    Staff: Mentor

  7. Apr 9, 2012 #6
    Thank you
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