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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.
     
  22. Apr 17, 2016 #21
    If the universe was the size of an atom at one time (a fraction after the Big Bang) and then began to expand rapidly (inflation), but at a finite speed. Then how could the universe possibly be infinite? Something of finite size that expands/grows at a finite speed cannot possible be infinite. Please explain how my reasoning can be wrong.
     
  23. Apr 17, 2016 #22

    timmdeeg

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    The universe as a whole is either finite or infinite at all times, including the time, when it came into existence.
     
  24. Apr 17, 2016 #23

    phinds

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    The universe was never the size of an atom. The OBSERVABLE universe was somewhere around that order of magnitude in size but estimates of the total universe range from 10E25 (or thereabouts and that is a very low end estimate) bigger than the observable universe up to infinite so in any case, the OU is utterly trivial compared to the total universe.

    Also, the universe did not start as a point or in a single place. I recommend the link in my signature.
     
    Last edited: Apr 17, 2016
  25. Apr 17, 2016 #24
    Clarify that please. How is only the observable universe believed to have been the size if an atom but the whole universe wasn't. Big Bang only occurred within the observable universe but not the whole universe? Or Big Bang included the whole universe but rather than the whole universe expanding from something the size of an atom, is Big Bang just a term to say the universe went from the size it was to the size it is now?

    If only the observable was the size of an atom then why is it generally presented that the Universe, its dimensions, time and everything we call the laws of physics was birthed from a finite point at Big Bang? Is it an archaic disproven concept that is mistakingly still presented, or is it that the concept is misinterpreted?

    Is it that the "space" of the Universe is finite or infinite and always has been but the matter of the Universe(would the total of matter in the Universe be what you're calling the observable universe, or do you mean the extent of what we can see from where we are is the observable universe) did not occupy all of that space at one point?
     
    Last edited: Apr 17, 2016
  26. Apr 17, 2016 #25

    phinds

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    Uh, because, as I said, the total universe was huge number of orders of magnitude larger than the observable universe.

    That is EXACTLY what the "big bang theory" is ... a theory of how the universe expanded from a hot dense plasma of indeterminate size to what it is now.

    finite point at big bang is pop science nonsense.

    It is, was, and always has been a mistaken idea and it is still vigorously promoted in pop science nonsense.

    exactly. It's one or the other but we don't know which.

    The observable universe has never been the whole universe.

    You really should do some basic reading in cosmology and I do NOT mean pop-science nonsense.
     
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