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How Does the Expansion of the Universe look?

  1. Jun 25, 2015 #1
    Hey all,
    I was reading a webpage which I found rather confusing:
    The problem I found with this is that the guy (who is a famous theoretical physicist) is stating that because the objects will be travelling FTL we will lose sight of them, forever.
    However this would only make sense if the universe expanded intuitively, if it did, we should not be able to see the CMB!
    To my understanding, because light has such incredibly long travel times on scales as large the visible universe the pictures we look at are not things, say, twelve billion ly away, they're pictures of EM waves that have travelled for twelve billion ly across an expanding universe, because the actual objects are receding away, the light is red-shifted. To add to this, matter actually gets more packed together as you look back in cosmic time.

    How then, can the entire universe dissipate away and leave no trace of itself after such a short time in cosmic terms? Am I missing out on something? Did I misinterpret the statement? Or am I right, if this is the case; why would a theoretical physicist tell a bit of a lie just to push an idea that we live in a special time?
    Last edited: Jun 26, 2015
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  3. Jun 26, 2015 #2


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    First of all, it is not clear what you mean by "expanding intuitively". Second, it is not clear how you think this would infer that the CMB could not be seen. Rest assured that everything is self consistent.

    Cosmological red shift is more accurately due to the expansion of space leading to longer wavelengths. Both source and receiver can be at rest in comoving coordinates.
  4. Jun 26, 2015 #3


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    Larry Krauss writes popular books and he naturally wants them to sell, and reporters in media can quote him out of context and get things wrong, and stuff gets sensationalized.

    Here is an essay that Krauss and a colleague submitted to judges in an annual Gravity Research Essay Contest. It says something like what you were wondering about but with more careful conditions, reservations, qualifications. It is free. We don't have to buy some popular-written book that oversimplifies. We can all look at the same firsthand writing and try to make sense of it.
    It is still slightly sensationalist but it is more careful because going to a professional audience:
    The Return of a Static Universe and the End of Cosmology
    Lawrence M. Krauss (1,2), Robert J. Scherrer (2) ((1) Case Western Reserve University, (2) Vanderbilt University)
    (Submitted on 2 Apr 2007 (v1), last revised 27 Jun 2007 (this version, v3))
    We demonstrate that as we extrapolate the current ΛCDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our "island universe" will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented.
    5th prize 2007 Gravity Research Foundation Essay Competition, to appear, GRG October 2007;
  5. Jun 26, 2015 #4

    Simon Bridge

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    The light can never reach us - or it is infinitely red-shifted.

    Why not? All it means is that the CMB we see around us comes from closer than the FTL edge.

    Not all light has very long travel times - the light from your computer screen takes very little time to reach you.
    The red-shift is due to the expansion during the intervening time - not so much the speed of the emitting object. i.e. it is not a doppler effect.
    Certainly the current models have a more dense distribution of matter in the earlier Universe.

    If that's a short time then I shudder to think what you'd call a long time!
    It did not "dissipate away", it expanded. As space expands, the clumps of matter get further apart and so the mas-density decreases. That's just geometry.

    You seem to have got the statement about FTL OK - it's the nature of the CMB you don't understand properly.

    The bit about being in a special time is a bit poetic - don't take it too much to heart. Physicists can get a bit carried away.

    [edit] the others beat me to it.
  6. Jun 26, 2015 #5
    "First of all, it is not clear what you mean by "expanding intuitively". Second, it is not clear how you think this would infer that the CMB could not be seen. Rest assured that everything is self consistent."
    Just because of the light travel time having a large effect, I read somewhere that the calculation is that objects are receding ftl beyond 11b ly..
    According to this post: https://www.physicsforums.com/threads/why-can-we-still-detect-the-cmb.801895/,
    The expansion of the universe should not result in the CMB becoming undetectable.

    EDIT: sorry for my mis-clarification, I wasn't saying that CMB radiation comes from beyond the FTL edge, I was saying that it must have been emitted before this time, and that we are not seeing the distance, only the product of the light travel time.
    Last edited: Jun 26, 2015
  7. Jun 26, 2015 #6

    Simon Bridge

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