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Question about looking back in time

  1. May 30, 2003 #1
    Ok I was thinking about this the other day...

    When telescopes like Hubble look at distant galaxies, it is really seeing them how they were x amount of millions/billions of years ago because that is how long it took for the light from those galaxies to reach the telescope.. Right?

    Now if the universe is expanding, and said galaxies are moving farther away from us, then are we getting images of where the galaxy used to be? or where it is now? How does that work? Do we plot where these galaxies are in relation to where they are in the telescope shots we get? Or in relation to where we believe they are now (based on our estimated speeds of the expanding universe, and the speed the galaxy is moving away from us..

    Also, If the universe started with a big bang, and was tiny and has been since expanding... looking back in time should show a smaller universe (i know we haven't seen an "end" of the universe.. but theoretically, shouldn't everything from billions of years ago that we see how be a lot closer together??)

    oww my head hurts

    someone clear this up for me??
  2. jcsd
  3. May 30, 2003 #2


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    The clearest answers on the web to these questions (with easy to understand diagrams) are at the UCLA website of a guy named Ned Wright who teaches cosmology there---he is also a member of the team that put the MAP satellite up to take the most accurate to date look at the Cosmic Microwave Background.

    Wright is unusual in that he is highly qualified but also really gifted and motivated as an explainer at the beginner-level.


    Various ideas of distance are used. There is a "present moment" distance that takes into account the expansion of the universe since the light left. There is also a "light travel time" distance based on the estimated time it took to come here. And a couple of other main ones.

    The "CosmoCalculator" at one of those links will calculate the various distances for you if you give it the redshift of the object.

    Put in the redshift (z) and press "flat"----which assumes spatial flatness, the prevailing view. It will make adjustments for cosmological constant and so on.

    The FAQ at the site are interesting. Wright's students must ask a lot of questions.

    Have a look at the tutorial---it has four pages, several of which have good diagrams. It isnt perfect and it looks unfinished, but it is a lot more pictorial and accessible than other stuff I've found on the web.

    Have fun.

  4. May 30, 2003 #3


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    Where it used to be.
    They are normally plotted using the Hubble law distance which is their distance at the present moment as seen by observers at rest with respect to the expansion of the universe---also called the "comoving distance". It gives a snapshot of the universe as it is believed to be now---not at a mishmash of various times in the past.

    Plotting by "light travel time" distance would give you a plot that is a mishmash of various past epochs at various stages of expansion. And the light travel time distance is not compatible with the Hubble distance law---v = H0 D----which is basic to the subject. The Hubble law refers to the present moment.

    What was in a very small volume has been expanded out and is all around us.

    The light that came from that small volume is now coming from all directions.

    There are good diagrams of this in Wright's tutorial---especially page 3.

    The whole-sky picture of the CMB-----that blue and red mottled oval you often see----is a snapshot of the earliest light we can see and gives a baby picture of our portion of the universe.

    It is not from the very beginning but from some 300 thousand years after the bang----when things cooled down enough so the hot plasma turned transparent and didnt block the light. So that mottled oval is how it looked then.

    Of course expanded out so it is all around us.

    Look at the tutorial and enjoy!
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