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Spacetime - infinite or expanding?

  1. Dec 17, 2008 #1
    Excuse my ignorance on this topic but it's not exactly my main topic of study, just a curiosity that came to mind. As I understand it:

    Big Bang happened, spewing space-time into existence (and if I remember correctly, according to Dr. Kaku, apparently tearing our 4-space out of 10-space). My uncertainty lies on the edge of the universe. Is space-time expanding? If you were to magically teleport to some very distant coordinate, would you be outside of space-time (ignore the paradox) or is it truly infinite? If it is infinite, then the big bang mustn't have "spewed" space-time, but rather... caused it to exist in an instant (since nothing can accelerate fast enough to achieve infinity)?

    Thanks for clearing this up for me.
  2. jcsd
  3. Dec 17, 2008 #2


    Staff: Mentor

    There is no indication that there is an "edge of the universe", but that is not the same question as whether or not the universe is finite or infinite. A crude example (embedding a 2D surface in a 3D space) is that there is no "edge" of a sphere, but the area of a sphere is finite.
  4. Dec 17, 2008 #3
    I suppose I could have more appropriately asked if there was an end to space-time. A sphere has no edge, but then again if you travel on it long enough, you'll go back to where you started. I guess my concept of the topology of the universe is a little foggy, but the real question is: if the big-bang made the fabric of space-time and space-time is, in fact, infinite (I think that's what you suggested in your post), then how did it get in place.
    Your response makes me think of a 2D version of space-time as a growing sphere, but then I run into the problem I mentioned above. thanks again.

    edit: by the way, if something has no edges, it's infinite, right? (not to say the converse is true)
  5. Dec 17, 2008 #4
    infinity plus x is still infinity...
    Dalespam's answer is good and is what I would have posted... nobody is even sure about the shape flat, concave or convex....but theory posits it one of the three...

    we don't even know if there is one universe, many or an infinite number!!!
  6. Dec 17, 2008 #5
    So... it would be correct to say that space-time, whatever shape it may have, is infinite and expanding. Then, inside space-time, is our universe (matter, to remove any ambiguity) which is currently expanding as well. ?

    If it isn't getting in too deep over my head, I'd also like to know how we know space-time is expanding without viewing it from the outside.
  7. Dec 17, 2008 #6
    Actually, nobody can be sure whether the domain of galaxies which we call our "universe" has a physical outer edge (if of course the domain of galaxies doesn't extend infinitely.) Standard cosmology *assumes* that a finite universe has no edge because no edge effects have been observed to date. Of course even if there were an edge, we wouldn't observe edge effects if the edge has always been beyond our particle horizon.

    I believe that the GR theory makes no specific assertion as to whether the domain of galaxies we call our universe encompasses all of the space and time that exists. On the contrary, it seems very possible that there could be a finite domain of galaxies with a very large or infinite domain of empty space "outside" of it. And there could be eras before our domain of galaxies existed and after it ends, if it has a finite lifespan.

    For reasons that aren't clear to me, novices are always instructed that the universe has no "outside", even though the possibility that such an "outside" exists has not been disproved or demonstrated to be highly unlikely, nor is it in any way ruled out by GR theory.
  8. Dec 17, 2008 #7
    As nutgeb said, the edge of the universe is an unobservable effect, so the question will never be answered by physics D:
  9. Dec 17, 2008 #8
    Thank you everyone for the replies. I see now that we couldn't possibly know if the universe has an edge of sorts or not, but my original question still stands. The shape and such was just sort of a tangent question. The original was basically: how'd it move into existence? I don't know if this is another unanswerable question, but did it expand at an... infinite rate? or did it just sort of plop itself down, declaring its presence with a bang?
  10. Dec 17, 2008 #9
    When I think of how our universe began, I think of an HQ of a highly-advanced civilization, and a hippy programmer in a research center just got this brand new supercomputer, so to test it out he entered in a few dimensionality equations and hit "Start"

  11. Dec 17, 2008 #10
    haha. sounds good to me. open source ftw!
  12. Dec 17, 2008 #11


    Staff: Mentor

    I already answered this. A sphere is a finite 2D space with no edges. So, no, if something has no edges it is not necessarily infinite. There is no reason to assume that the universe has an edge, there is no reason to assume that the universe is infinite now, and there is even less reason to assume that it was infinite at some point in time shortly after the big bang.
  13. Dec 17, 2008 #12
    Oh. I see. The sphere analogy threw me off, due to what I mentioned above (walking far enough in one direction brings you back to the beginning).
    By space-time, I mean the fabric on which matter lives, not the universe (the matter). My mistake was in assuming that we've accepted space-time (not the universe) to be infinite. I couldn't figure out how the big bang released an "infinite" amount of space-time in a finite... amount of time. Apparently it's non-nonsensical to say space-time has edges, is finite, or infinite, or... whatever. thanks for the help.
  14. Dec 18, 2008 #13


    Staff: Mentor

    Hi daytripper,

    Sorry about my last, it was written fairly late at night. Anyway, it is not nonsensical to talk about spacetime having edges or being infinite, it is just that they are two completely different questions. You can have an infinite space with edges, an infinite space without edges, a finite space without edges, and a finite space with edges. If you think about 2D spaces you can easily come up with examples of all 4 kinds. It is not nonsensical to ask about the topology of our universe, but the "edginess" and "finiteness" are two completely independent questions.
  15. Dec 18, 2008 #14
    no worries, I think I understand now. I guess it still kills me that we can't observe the topology of spacetime from, say, 5-space. I'm gunna go chill in my freezer; wake me up when we've figured out this question. Do you think you could point me in the direction of some theories on it? thanks.
  16. Dec 18, 2008 #15


    Staff: Mentor

    You might ask in the cosmology forum. I am sure there are some good theories on the topology of the universe (finite or infinite, edges or no edges) with some supporting observations, but I don't know the details.
  17. Dec 18, 2008 #16
    Yea, good point. Perhaps I should've posted there originally (seems now to be more appropriate). you've been very helpful. I appreciate your tenacity.
  18. Dec 18, 2008 #17


    User Avatar
    Science Advisor

    Edges have never been observed, so the basic assumption of modern cosmology still holds: space looks everywhere the same, there are no edges.
    It cannot be decided today whether the universe is finite or infinite. If the current standard model ist true, it can't be decided by observation, because space is assumed to be exactly flat, therefore infinite. This corresponds to a certain density parameter being exactly 1, where a parameter of 1.0000000000001 would make space finite.
  19. Dec 18, 2008 #18
    Could be either...nobody really knows. There are various inflation theories, for example, which may explain a lot..Alan Guth (from MIT) may have the best known...

    There are several ways things can "move faster than light" in creating or expanding a universe but not in the sense of relativity from Einstein. In our universe distant galaxies appear to be moving at such superluminal speeds because space itself is expanding in between...so such galaxies are not moving through space in the Einstein relativity sense...space is being created, is expanding between them!! Dark energy, whatever it is, may be the "powering agent". At the instant of the big bang and during the brief inflationary period that followed, it may have been that space did not yet exist...so inflation could have caused expansion at speeds greater than that of light...and space formed as that period ended. Forces as we know them today likely did not yet exist either...they were all the "same", unified.

    Oddly (to me at least) the mathematics of inflation also indicates the establishment of the HIGGS ocean as symmetry was broken and a phase transition took place from high energy instability (where all forces were unified) to a lower energy, more stable universe (with separate forces) we inhabit today..the Higgs ocean is the theoretical source of mass.

    The Large Hadron Collider (LHC) which they are trying to get working (again) in Europe may be able to detect Higgs bosons...the theoretical quantum carriers of mass. Last I heard some of their big electromagnets burned up...if they find evidence of Higgs particles it will confirm some theory; if not, it could be even more exciting as a lot of theorists race back to the drawing boards!!
  20. Dec 18, 2008 #19
    haha. My Higgs! I hope they find it. I want answers and I want them now...
    I didn't know the can of worms I was opening with this question. I've condemned myself to another couple weeks of study. I'm not too into particle physics, but are you say the big pool of matter and anti-matter that existed near the beginning of the universe was (in theory) created by a big ocean of higgs particles?
    About the LHC, if I remember correctly, apparently one of the electromagnets lost its superconductivity because it got a little warm.
  21. Dec 18, 2008 #20


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    Science Advisor

    I don't know what you mean by "get in place".

    No, that's not true- the suface of a sphere, as said before, has no edges but is not infinite. Nor is the converse true: A half plane is infinite but has an edge.
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