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The nature of nothingness/space, and potential multiverse

  1. Aug 15, 2012 #1
    Is the current mainstream thinking that spacetime was created with or by the big bang? I've read many posts about a "wrap-around" universe in which there is no border, and traveling to the "edge" of the universe would pop you out on the other side. I've done some brain-wracking, and I haven't yet been able to conceive of how that's possible, though it seems like there's much evidence for it.

    If wrap-around universe is what's actually out there, and the answer to, "what is outside of the universe?" is "nothing" (not "nothing" as in space, but nothing as in complete non-being), then wouldn't that rule out a level I multiverse? If there is literally nothing outside of our known universe, then how could there be any other cluster of energy (i.e. a universe) very far away from our known universe?

    Basically I'm wondering what the mainstream theory is about the nature of spacetime its boundaries, and how travelling to, or past, the boundaries our universe would work. Why couldn't we just go past the boundary of the known universe into space unoccupied by any energy at all? Isn't void infinite?

    As always, thank you for your time and energy.
  2. jcsd
  3. Aug 15, 2012 #2

    Simon Bridge

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    Strictly speaking the thinking is that the very early history of the universe includes a very dense state where everywhere was close to everywhere else, which underwent a period of rapid expansion.

    You understand that the word "created" tends to be avoided for it's supernatural connotations.
    It's not much different from thinking that if you travel far enough on the surface of the Earth you'll "pop back over" the opposite horizon.

    Accumulated evidence has been that the bit of the Universe we can see is extremely flat. But continue...
    It can be separate without being far away "outside" ... it just needs to be further than we can see or possibly interact with.

    You can imagine, for instance, someone in "our" universe who is on a planet by a star so far away that when they look out, none of their observable universe includes us. With a big enough universe, their bit may never include us. In which case - they are "in a different Universe".

    Anyway - the type I multiverse comes from chaotic inflation theory - the standard big bang model is just "cosmological inflation".

    "Void" is a property of the inside of the Universe I'm afraid. There is no such thing as "outside" to the Universe.
    We cannot travel to a space with no energy since, being material, we are made of energy ... everywhere we go has energy: us.
    The very concepts "travel" and "space" require energy. "Boundary" requires geometry requires space - there is no space and no geometry "outside" the Universe - and there is no "boundary" to "cross".

    If you can get "there" from "here" then "there" is part of this Universe.

    Talking about this stuff is really hard ... wait till you try talking about a "beginning" for time!
  4. Aug 15, 2012 #3
    Thank you so much!

    I have many thoughts and questions, but I'll limit it to two for now:

    In a level I multiverse, are all universes supposedly the result of their own big bang?

    Is there a good link that will help me grasp how the wrap-around is possible? I just can't visualize it.
    Last edited: Aug 15, 2012
  5. Aug 15, 2012 #4
    No, a level one multiverse is the simplest, and it's one that we know exists. In fact, I certainly wouldn't even call it a multiverse, I don't know why Tegmark chooses to. The idea is that some regions of the universe so far apart, that the expansion of the universe ensures that they are causally disconnected, since no light could ever get from one to the other. So, you may call them separate 'universes' since they can never contact each other.

    However, the second part of the idea is based off of the possibility that the universe is infinite - which we certainly do not know is true. If the universe is infinite, then somewhere there would be an exact copy of the solar system. So, if we were to consider the above causally disconnected regions 'universes' (which I would dispute), then we have a 'parallel universe'. I would just call it a region of the universe that looks that same as ours, not another universe.

    Right, because you can't. The universe is three-dimensional. When we visualize shapes, we do so by putting them into an extra dimension - you can only visualize three dimensional space, so you can only visualize two dimensional surfaces (think about it - you can't see an entire 3D cube at once without turning it somehow). So, visualizing a 3D curved universe would force you to mentally place it into a fourth dimension, which the human brain cannot do.

    Cheesy mathematician joke: Q: How do you visualize a four dimensional space? A: Easy! You visualize an ##N## dimensional space, and let ##N## go to four.
  6. Aug 15, 2012 #5

    So is it fair to say that multiple universes, each with their own big bang or big bounce or whatever, and far enough apart that they don't influence each other, do NOT fit into the current mainstream thinking?

    As I understand it, this scenario is impossible (according to the mainstream) because it implies that something can exist outside of the universe, which in this case would be another universe.

    Maybe I've veered way off track here. Thanks for following me.
    Last edited: Aug 15, 2012
  7. Aug 15, 2012 #6
    Well, the big bang isn't something that can 'happen'. Remember that the big bang is the term referring to the moment in time at which the entire universe was filled with a hot dense plasma. So, saying that multiples big bangs happened is a meaningless statement, since it refers to one moment in the history of the universe.

    So yes, the scenario is impossible because you need to specify what you mean by several big bangs happening. There is one very viable scenario called 'eternal inflation'. I'll explain firstly what normal inflation is.

    There are a few problems with the vanilla big bang model - first is the flatness problem. We observe that the universe is very, very close to being spatially flat. Since expansion would cause the universe to deviate away from flatness, it must have been even flatter at the time of the big bang. Ridiculously flat. How did it get this way? Second is the horizon problem. We observe that the universe is homogeneous on large scales, that is, it's pretty much the same everywhere. This means that primordial plasma must also have been perfectly homogeneous, which is confirmed by observations of the cosmic microwave background. However, if the expansion of the universe was extremely rapid from time zero onward, how did this plasma come to equilibrium? It certainly wouldn't have the time to do this. And third is the monopole problem. Grand Unified Theories, or GUTs, are theories that unify the electroweak interaction with the strong nuclear force. They have the unfortunate feature of predicting that hot temperatures of the early universe should have produced an abundance of heavy magnetic monopoles, which we certainly don't observe. Fourth is the homogeneity problem - why are there no inhomogeneities besides galaxies? What made the early plasma so 'smooth'?

    Inflation fixes all of these problems. Inflation proposes that the very early universe underwent an enormous expansion, growing the universe by at least 60 e-folds. This expansion would be driven by the inflaton field. This field would reach an undesirable energy value, called a false vacuum. When it's in this false vacuum, it has the property that it exerts an enormous negative pressure (somewhat similar to dark energy). This drives inflation. After a very short period of time, the inflaton field reaches it's true vacuum (through normal quantum effects such as tunneling). When this happens, it decays into a bath of radiation, heating the universe so that the big bang model can go from there.

    So, how does this solve the problems of the big bang model? Well, the enormous expansion would eliminate any curvature, making the universe extremely flat. This solves the flatness problem. Second, it would allow the universe to expand very slowly before inflation, allowing it to come to equilibrium. This solves the horizon problem. Any monopoles produced in the early universe would have been spread out so that we would only see about one in the entire observable universe, so the monopole problem is solved. And finally, inflation would 'iron out' any large scale inhomogeneities with the rapid expansion.

    So, what is eternal inflation? The idea is that inflation ends only in certain places, so that these slowly expanding regions are separated by the eternally inflating region in between them. So, each of these slowly-expanding would develop independently, forever seperated by the continually inflating region.

    That is the closest thing I can think of to what you are asking about. Since the big bang is the time when the universe is filled with a hot, dense, plasma, these separate slowly expanding regions would all be undergoing this process separately, and so we could see that separate big bangs are occurring. But it's a bit of an abuse of terminology.
  8. Aug 21, 2012 #7

    there is no mainstream model/explanation of space-time.
    maybe discrete, maybe continous.

    Last edited: Aug 21, 2012
  9. Aug 21, 2012 #8
    a topology of T2x R1 (a torus) a flat finite universe with
    two dimensions compactified, and one infinite.

    The Topology and Size of the Universe from the Cosmic Microwave Background

    We study the possibility that the universe is flat, but with one or more space directions compactified. Using the seven-year WMAP data, we constrain the size of the compact dimension to be L/L0 ≥ 1.27,0.97,0.57 at 95% confidence for the case of three, two and one compactified dimension, respectively, where L0 = 14.4 Gpc is the distance to the last scattering surface. We find a statistically significant signal for a compact universe, and the best-fit spacetime is a universe with two compact directions of size L/L0 = 1.9, with the non-compact direction pointing in a direction close to the velocity of the Local Group.

    Creation of a Compact Topologically Nontrivial Inflationary Universe

    If inflation can occur only at the energy density V much smaller than the Planck density, which is the case for many inflationary models based on string theory, then the probability of quantum creation of a closed or an infinitely large open inflationary universe is exponentially suppressed for all known choices of the wavefunction of the universe. Meanwhile under certain conditions there is no exponential suppression for creation of topologically nontrivial compact flat or open inflationary universes. This suggests, contrary to the standard textbook lore, that compact flat or open universes with nontrivial topology should be considered a rule rather than an exception.

    torus is a nontrivial topology.

    Last edited: Aug 21, 2012
  10. Aug 21, 2012 #9


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    You're mixing up a whole bunch of different things here.

    (1) The answer to the "with or by" issue is basically "with." We have a FAQ about this: https://www.physicsforums.com/showthread.php?t=506991 [Broken]

    (2) There is no mainstream cosmological theory in which the universe has a boundary. There is no empirical evidence for a boundary. General relativity is incapable of discussing boundaries.

    (3) It is possible that the universe has a nontrivial topology -- the "wrap-around" idea: http://en.wikipedia.org/wiki/Topology_of_the_universe#Global_geometry But even if it doesn't, that doesn't mean it has a boundary.

    (4) We don't know whether the universe has positive, negative, or zero spatial curvature, but we know that it's very close to zero. This is separate from 1 and 2, and although it can be linked to 3 in certain ways, the link is not direct or obvious.

    A nice popular-level book that covers this sort of thing is Relativity Simply Explained, by Hardner. (It's somewhat out of date, though.)
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  11. Aug 21, 2012 #10


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    Actually, that's Gardner, not Hardner, and even though it IS dated, it's an excellent intro with next to no math.
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