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Observable universe

  1. May 4, 2015 #1
    Dear PF Forum,

    After searching many links in Google and threads in PF, I can't find the farthest object in the universe. I have some questions here, perhaps someone can give me quick and simple answer.

    A. What is the farthest object in the universe? How far away?
    B. The radius of the (observable?) universe is 46 gly, so space must be expanding.
    - Where do the expansions occur?
    - Does space only expand in intergalactic space?
    - Can space expand inside a galaxy? Or inside our solar system?
    C. Suppose, there are objects beyond 46 gly. Do the objects still belong to our observable universe?
    D. "Where" is the "nearest" multiverse?
    - Is "where" the correct question? or "when"?
    - Is Multiverse in the other dimension?
    - If it is, is it spatial? Temporal? or something else?
    E. I once read Xenoverse, omniverse, metaverse.
    - Are they speculative?

    Thanks

    Steven
     
  2. jcsd
  3. May 4, 2015 #2

    bapowell

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    A. The cosmic microwave background is the oldest light we can see and so, though not an "object" in the traditional sense, is the farthest thing we can see. As far as galaxies and so forth, here's a list: http://en.wikipedia.org/wiki/List_of_the_most_distant_astronomical_objects

    B. The expansion occurs uniformly throughout the space, but is not operative locally in strongly inhomogeneous space (like the solar system). Expansion is best thought of as occurring on large length scales over which the universe is approximately homogeneous.

    C. No, because we cannot see them.

    D. Which multiverse? I wasn't aware we'd discovered any!!

    E. I've never heard of any of those terms. They are not used by any self-respecting cosmologist.
     
  4. May 4, 2015 #3

    phinds

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    I recommend the link in my signature for some of your confusion.
     
  5. May 4, 2015 #4
    Thanks Bapowell for the answer.
    Okay, I'm dismissing xenoverse and omniverse. But I still hold "multiverse" term.

    at 9:30 it describes omniverse, so, I think this video is speculative.
    Perhaps you might want to take a look at it in your spare time. I believe you. There's no "xenoverse"

    Thanks Phinds for the answer. What a great article you have in Balloon analogy.

    Okay, I understand that it does not make any sense to have any objects beyond CMB. But...
    Can the universe, physically, has any objects beyond CMB?

    Are there any objects between 13.5 gly to 46 gly?

    Perhaps I should consult www.dictionary.com. There are stars accross SMBH in Milky Way, which we cannot see them
    What is the definition of the "observable universe"?
    Is it the region of space inside CMB sphere?

    Thanks for any answers.

    Steven
     
    Last edited: May 4, 2015
  6. May 4, 2015 #5

    Chalnoth

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    As for D), there are multiple multiverse ideas, and the answer would depend upon which idea. In nearly all cases the answer would be highly speculative and not at all fixed by the model. The only one that wouldn't have a speculative distance is the quantum multiverse. There is no physical separation between different worlds in the quantum multiverse. They all occupy the same space, so there is no distance between them.
     
  7. May 4, 2015 #6

    phinds

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    That was perhaps a poor choice of phrasing since you are right that there are things in the observable universe that we can't see simply because they are occluded by object in the forground. The OU IS generally described as the sphere within which objects can be detected by us. There is generally believed to be a MUCH larger universe outside that but we will never know via direct observation. The CMB is inside the OU and it is believed that if we had really good neutrino detectors or gravity wave detectors we could probably see a little farther than that (something less than 400,000 light years)

    EDIT: another, more helpful, definition of the the OU is that sphere inside which objects are in casual contact with us and always have been.

    Just FYI, there are objects that are "now" (I put that it quotes since definitions of "now" on cosmological scales get tricky) just outside the OU that will, over time, be overtaken by the expansion of the OU's sphere. The sphere will expand by a pretty small amount over cosmological time but will not keep expanding forever.
     
  8. May 4, 2015 #7

    Chronos

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    The observable universe is defined in terms of the time light has had to travel since the Big Bang [~13.8 billion years] with the caveat that light was unable to escape the hot plasma created by the Big Bang until it cooled sufficiently to permit a structure known as the 'surface of last scattering' [source of the CMB photons] to form. This corresponds to a proper distance of about 46.5 billion light years, or the distance to the CMB you would measure with a gimongous tape measure if you could persuade the universe to stop expanding long enough to unroll the tape and make the measurement. We could, however, detect structures more distant than the CMB with neutrino or gravity wave detectors because they are unimpeded by the hot primordial plasma of the Big Bang. In theory there should be two more distant structures hiding behind the CMB. We could call them the 'surface of neutrino decoupling' and surface of gravity decoupling'.
     
  9. May 4, 2015 #8

    bapowell

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    Sure, the term "multiverse" is widely used in modern cosmology.

    The CMB is not in a fixed location, so it doesn't make sense to talk about objects being "beyond" the CMB. The CMB is uniformly spread throughout the universe: the CMB photons that we receive on Earth at this moment can be thought to have originated on the surface of a giant sphere with radius ~ 46 bly with Earth at the center (this is called the "surface of last scattering"). So what you are really asking is: are there objects beyond the surface of last scattering? Yes -- more CMB photons!

    What you are really getting at is how much more universe is out there. Of course, we don't know about anything beyond the observable universe, but some theories of the early universe, in particular, the inflationary universe scenario, posit that there is more -- perhaps much more -- universe out there beyond what we can observe. If conditions out there are similar to those here in the observable universe, we should well expect there to be galaxies, planets, and so on out there beyond the last scattering surface.

    Sure, although there is nothing special about the distance 13.5 gly.


    The observable universe is bounded by the distance that light has traveled since the big bang. The edge of the observable universe is called the particle horizon. Since the CMB was created a few hundred thousand years after the big bang, the particle horizon is a little bit further out than the last scattering sphere.
     
  10. May 4, 2015 #9
    Thanks Bapowell for your detailed explanations, and to you too, Phinds, Chronos and Calnoth.

    Dear PF Forum,
    Can anyone give me a simple and quick answer.

    1. Is our universe...
    A. Open?
    B. Closed?
    C. Flat?
    D. Still debating?

    2. What would our universe characteristic be if the universe is:
    A. Open.
    B. Closed.
    C. Flat
    Surely we can't drag three gigantic rulers as Chronos sugested to determine if our universe is Eucledian or not.

    3. But, suppose, just suppose, that we can measure the universe with 3 very long rulers. Is it true that open universe has less than 1800 and closed universe has more than 1800?

    4. Whether it is true or false, how can we be sure if those rulers are not curved by the mass inside the universe?
    Does mass determine the shape of the universe, or it's the space itself which is open/closed/flat regardless how much mass in there?

    Thanks

    Steven
     
  11. May 4, 2015 #10

    Chalnoth

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    It's very close to flat, within less than a percent. Its future fate is to become more flat.
     
    Last edited: May 4, 2015
  12. May 4, 2015 #11

    Chronos

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    The CMB is the most practical 'ruler' at our disposal. The tiny temperature differences [anistropy] in the CMB provide us valuable clues about the size, geometry and expansion history of th observable universe. We still do not, and may never know if it is open or closed.
     
    Last edited: May 5, 2015
  13. May 4, 2015 #12
    Last edited: May 4, 2015
  14. May 5, 2015 #13

    Chalnoth

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    That's not quite correct. The spatial geometry is determined by the relationship between the rate of expansion and the energy density. When the two are equally-balanced, we have a flat universe. When the expansion is slower, that's a closed universe. When the expansion is faster, it's open.
     
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