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I The observable Universe and its shape

  1. Apr 15, 2016 #1
    Laymen here.

    1. From my understanding the universe is like the surface of a balloon. The universe is expanding as a balloon grows when air in being placed inside of it. Just like a surface of a balloon if you go in any direction in a straight line you will come back to the original point.
    Is this description correct, if no please elaborate.

    2. When we look at other galaxies we are looking at how they were in the past as light takes a long time to travel to us. This would mean that we are looking at what that galaxy was like when the surface of the balloon was smaller. We can't observe the entire surface of the balloon in its present form because by the time light reaches us the balloon is bigger.
    Is this description correct, if no please elaborate.

    3. How thick is the surface of the balloon?
     
  2. jcsd
  3. Apr 15, 2016 #2

    Chalnoth

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    That's one possibility. There are other potential geometries.

    Note that in practice the rate of expansion is too fast, so that you'd have to go faster than the speed of light to actually traverse the distance, which naturally isn't possible.

    Sure

    It has no thickness at all. It's an analogy, and an imperfect one.
     
  4. Apr 15, 2016 #3
    The universe being a surface of a balloon is only useful as an analogy when referencing how space expands. So your understanding of why we can see things that are currently much further away than we should be able to see, they were closer in the past.

    The shape of the observable universe is a sphere, but that says nothing about the actual universe. That's just what happens when you are an observer, no one has any idea what it looks like on the grandest scale.
     
  5. Apr 15, 2016 #4

    bapowell

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    The curvature of the observable universe is consistent with flatness, not a sphere.
     
  6. Apr 15, 2016 #5

    Chalnoth

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    The available evidence is consistent with either shape, but in any event the radius of curvature is much larger than the size of the observable universe. As long as the dark energy continues to behave like a cosmological constant in the future, the curvature will never have any practical impact in the future.
     
  7. Apr 15, 2016 #6

    bapowell

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    Sure, but if you take flatness as your null hypothesis, there is insufficient evidence to prefer nonzero curvature.
     
  8. Apr 15, 2016 #7

    Chalnoth

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    It's not clear that flatness is the right null hypothesis, though.

    Either way, our universe is very close to flat and there's a reasonable chance we'll never be able to measure any spatial curvature.
     
  9. Apr 15, 2016 #8

    bapowell

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    We should start with the most parsimonious model. There is not sufficient evidence to warrant introducing curvature as a free parameter.
     
  10. Apr 15, 2016 #9

    Chalnoth

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    The parameter isn't introduced, though. It's a degree of freedom that stems directly from General Relativity and minimal assumptions of homogeneity and isotropy.

    It may appear to be an added free parameter if you look at the first Friedmann equation:
    [tex]H^2 = {8 \pi G \over 3}\rho - {k c^2 \over a^2}[/tex]

    After all, without that parameter the equation is simpler:
    [tex]H^2 = {8 \pi G \over 3}\rho[/tex]

    But this begs the question: why should the matter/energy density be directly proportional to the square of the expansion rate? There's no fundamental reason for this to be the case. It's perfectly plausible to have a universe which is, at a given point in time, expanding much faster or much slower compared to the matter/energy density. General Relativity only predicts how this expansion will change from that initial configuration.

    That's what the curvature parameter is there for: it's basically a way of describing the initial relationship between expansion and density. Because of the way GR works, that initial relationship translates into a geometric shape for the universe.
     
  11. Apr 15, 2016 #10

    bapowell

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    But can't this be said of any "free" parameter? For example, apply your argument to the running of the spectral index: Should we not consider it a free parameter because its absence would beg the question: why should the spectrum of perturbations be a power law?
     
  12. Apr 15, 2016 #11

    Chalnoth

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    I think that's a somewhat different situation. In that case, there are a lot of competing models for what the spectrum of primordial perturbations should be. It's really difficult to do a comparison between two models that use entirely different functions for the spectrum of primordial perturbations. So the convention of using the spectral index was born: it's a practical measure to allow experimental teams to produce data that doesn't depend upon any specific model for laying down the primordial perturbations.

    It was chosen largely because it's a decent first approximation to the way a lot of inflation models behave. In general I don't expect anybody really believes that the spectral index is likely to be completely accurate, but it's a fairly good way to at least get the model comparisons started.

    The spatial curvature is a very different sort of thing. It's a degree of freedom that remains after making some minimal assumptions about the universe. Because it's only a single, constant parameter, there's no need to do complicated things like series expansions.
     
  13. Apr 16, 2016 #12
    If the universe is expanding in all directions from one central point then how could the shape of the universe not be spherical?
     
  14. Apr 16, 2016 #13

    PeterDonis

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    It isn't.
     
  15. Apr 16, 2016 #14

    phinds

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    I recommend the link in my signature
     
  16. Apr 17, 2016 #15
    The Universe is, in theory, "everywhere", to imply a sphere suggests that there is a measurable "shape" or "size". It is an everywhere that has no shape, size, edge or center, these are all measurable things. The Universe theories(whether finite or infinite, thanks @) are more conceptual representations of something that is immeasurable, an unquantifiable quantity on either side the coin. I'm newly discovering that both theories work(and probably more that I know nothing of yet). They can both(probably all) be seen in a manner that seems to give the same answers.

    I can understand what you mean by a "central" point though, given that Big Bang "should" have a point of origin. That's the enigmatic nature of the Universe though, with no known limit in any direction, how can there be any kind of center or edge?

    The concept of an expanding Universe has nothing to do with movement or an increasing in size(whether that is what is actually happening or not), it is a relation of distant objects(relatively speaking) accelerating at an accelerated rate compared to non distant objects(relatively speaking).

    Everyone feel free to correct or clarify any fine points that grab their attention here.
     
    Last edited: Apr 17, 2016
  17. Apr 17, 2016 #16

    Chalnoth

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    This isn't known. It is clear that our universe is significantly larger than the part of it which is observable, but there's no way to know (currently) just how big it is, or whether or not it is infinite in space.
     
  18. Apr 17, 2016 #17
    Is that to say that if the Universe is not infinite then it is finite in an infinite vacuum of "empty" space that it can or may expand into at some point? Or is that another point that gets debated either way?

    I get you, I just mean that we have to accept infinity as representing an unknown, immeasurable thing because we can't observe a limit. We can't say there is a definitive limit to its size(though a limit may or may not exist), nor can we say that there is a center. More a theoretical thing due to our lack of observability or measurability.

    I'll edit my post to reflect it as being a general theory instead of a known definite.
     
    Last edited: Apr 17, 2016
  19. Apr 17, 2016 #18

    timmdeeg

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    In case the universe is infinite in space, there are still degrees of freedom regarding its curvature and its shape (means its topology). It can be flat or negatively curved. In the letter case, the topology can be somehow, it isn't determined.

    Note that even being finite in space, the universe has no edge and no center.
     
  20. Apr 17, 2016 #19
    Yes, a wraparound that translates to no observable, measurable or reachable "edge" or "center"
     
  21. Apr 17, 2016 #20

    timmdeeg

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    As an edge or center doesn't exist, "reachability" makes no sense in this context. If you talk about the northern pole of the earth it would, if you talk about its surface however, it wouldn't.
     
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