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Another newbie question: paradox: size vs. age

  1. Jan 14, 2016 #1
    I've never understood this: we measure the edge of the observable universe to be around 13 billion LY away, so we are seeing the state of things 13 billion years ago. But 13 billion years ago, wasn't the universe much, much smaller? Did distant quasars and galaxies look like they do "now" so soon after the Big Bang?

    Simple analogies would be very helpful for me here...
  2. jcsd
  3. Jan 14, 2016 #2


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    The edge of the observable universe is actually about 46 billion light years away. I hope that doesn't hopelessly confuse things...

    13 billion years ago the *observable* universe was much, much smaller. That's an important distinction.

    If we could see all the way out to the edge of the observable universe, we'd see no galaxies there. The first galaxies didn't form until several hundred million years after the big bang.
  4. Jan 14, 2016 #3


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    The CMB was emitted about 13.7 billion years ago. The matter that emitted the CMB we see was only about 42 million light years away at the time. Today, that matter is about 46 billion light years away.
  5. Jan 14, 2016 #4
    Thank you both for your responses. I am still trying to wrap my brain around it and will probably have more questions soon. I think I'm missing something critical.
  6. Jan 14, 2016 #5
    Ok, so I'm looking at an image of the furthest known galaxy, 13.1 billion LY away. So a few hundred million years after the Big Bang, our universe was at least 13.1 billion LY in radius? Probably even much bigger, right? If that's the case, it all makes sense. My mistake was in assuming the universe was still very small. I guess I didn't realize just how huge inflation was. Thanks again.
  7. Jan 14, 2016 #6


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    I find it easier to imagine like this:
    take our position as the center of the observable universe (isotropy and homogeneity allow this). Nothing is really moving, but the spacetime is expanding.
    Big bang happened everywhere and Universe started expanding. As a result everything "around us" started to redshift.
    The CMB as a result is obvious from every direction, and what we are able to see is what happened right after the recombination period (or around that time), when the universe became transparent and light was able to reach us.
    Galaxies that were formed after that, can still appear pretty far away from you.
  8. Jan 15, 2016 #7


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    The universe was much smaller 13 billion years ago. Despite the enormous rate of expansion of the early universe light was a real trooper pushing outward and eventually gaining ground as expansion slowed down finally reaching us today; 13 billion years later. Despite the fact the source of CMB photons has receeded more than 40 billion light years during that long journey
  9. Jan 15, 2016 #8


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    That farthest galaxy is seen as it was 13.1 billion years (i.e. Giga-years, or Gy) ago. It is not 13.1 billion light years (Gly) away in any sense. It was at around 3 Gly distant when it emitted that light we observe now, but due to the continuous expansion of space in the path of the light, two things happened:
    - it took much longer than just 3 billion years for the light to get to us (there was more and more distance to cover as the universe grew), i.e. 13.1 Gy
    - by the time the light emitted 13.1 Gy ago finally got to us, the galaxy has been carried away by the expansion, and is now at about 30 Gly.

    It's important to keep track of the different kinds of distances in cosmology. You may often see the same word used for:
    the distance of the source when the light was emitted
    the distance the source is now, when it is observed
    the naive distance the light would travel in a static universe in the time it took it to reach us (i.e. time x velocity) - this one has no real meaning in our universe outside the relatively close neighbourhood of our galaxy, where expansion has no effect due to local gravity holding everything together.
    Last edited: Jan 15, 2016
  10. Jan 15, 2016 #9
    Great explanation. Thank you. But I wonder why, in all the popular media, they say that objects are x light years away, as in the example of "the furthest known galaxy?" Seems like most people are as misinformed as I am (was), even the quasi-scientific writers. But I guess that's another thread.
  11. Jan 15, 2016 #10


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    Usually when they say something is X light years away, they're using light travel time as the distance measure. So when something is 6 billion light years away, they really mean it took 6 billion years for the light to reach us. Usually other distance measures are denoted in units of either parsecs or centimeters (yes, centimeters for measuring intergalactic distances are a thing, though parsecs are more common).
  12. Jan 29, 2016 #11
    Sorry to resurrect an old thread, but this line of discussion has me wondering, how do you apply the concept of "now" to something 30 billion light-years away?
  13. Jan 29, 2016 #12
    Essentially just extrapolation and accounting for the doppler shift on energies, due to the measurements of specific stars and supertnovae that are used as 'standasrd candles', the relstive motions (proper and perceived due to expansion) can be calculated and the gradient of measured expansion used to provide a reasonable prediction of where the object likely will be found.

    Of course 'now' only has meaning per reference frame, and there is no agreement between them.
    Now for the HST receiving the Deep Field photons is pretty much the same as the 'now' for those on earth, adjusted somewhat fort relative motion, acceleration/altitude and of course, how far from the telescope they are.
    This 'now' is increasingly divergent with relative motion and the lightlike distance, but I think what is meant is if it were possible to take a 'snapshot' image of the entire universe at the point in time of 'now', then from the telescope to the distant gaslaxy would be a spatial distance of 30 ly
    This is not possible, though and no way to measure all points at an agreed simultaneous time.
    However, from the reference frame of the galaxy, which emitted light to the 3Gly distance, 13.1Gyears ago that was essentially the same 'now', then :)

    This is not even considering the time dilation due to relative motions!
  14. Jan 29, 2016 #13


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    Typically what's done is to divide the universe into equal-time slices. One way of creating such a slice is saying that it comprises all points that see the CMB at a specific temperature. For example, we observe the CMB as 2.725K, so "now" on a galaxy 30 billion light years away would be the time where it also observes the CMB at 2.725K.
  15. Jan 29, 2016 #14
    OK, that's pretty cool, thanks. (And thank you, PJ)
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