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Quantum tunneling and the universe

  1. Aug 18, 2012 #1
    Alexander Vilenkin has a model of cosmic origins which describes the universe coming into being as a quantum tunneling event. The problem that I see with this, is that this requires the universe to be closed. However, we know the universe is probably flat with a margin of error of less than 1%. Does this mean, the model is a dud? Or, is there some way around it?
     
  2. jcsd
  3. Aug 18, 2012 #2
    Everyone on the planet entertains some model of the beginning. Why is his model more plausible than the other million or so?
     
  4. Aug 18, 2012 #3
    The shape of the universe is still an open debate:

    http://en.wikipedia.org/wiki/Closed_universe#Closed_Universe

    I suspect a majority believe the universe is close to Ω = 1......and that seems reasonable
    so far; but if history is any guide, that may not be very important.
     
  5. Aug 18, 2012 #4
    Well, of course science at this point is not absolute, anything can potentially be proven wrong. The point is, based on all available evidence we have, the universe is most likely flat.
     
  6. Aug 18, 2012 #5
    Well, the key is 'within one percent'. The mainstream view is that the early universe underwent an incredibly rapid expansion called inflation. One effect of inflation is make any region of the universe enormous. So, after inflation, any spatial curvature would become undetectable. Therefore, measuring the universe is close to flat may just mean that inflation made the universe large enough that no curvature can be detected with our current measurements. So, Vilenkin's model is perfectly valid.
     
  7. Aug 18, 2012 #6
    One percent is a pretty good margin of error I would say. Also, if any spatial curvature could not be detected anyway, then why does NASA and other websites claim it is flat wuth such accuracy? What is the point of this discovery which if one could just say "It's just as likely it is closed, but we just can't detect the evidence". Also, doesn't that assumption violate Occam's Razor? Doesn't it make more sense to say the universe is most likely flat based on all available evidence, until evidence contradicts it?

    Regardless, if the universe is really closed then it would have a net energy which is negative. This would mean the universe doesn't really have a net energy of zero, which means Alexander's model fails. Only if the universe has a net energy of zero could it have possibly come from "nothing". Alexander Vilenkin claims that the positive and negative energy balance out, but this is only true under the assumption of a flat universe. With a closed universe, isn't there a negative net energy?

    "The universe is neither spherical shaped, like the shape of a ball. That would be positive curvature, having net negative energy" - Neil deGrasse Tyson

    "The gravitational energy which is always negative exactly compensates the positive energy of matter. So the energy of a closed universe is always zero" - Alexander Vilenkin

    Can you explain how Alexander Vilenkin can get away with claiming a zero energy universe, if it is closed? Conventional wisdom is that a closed universe has a net negative energy based on what I have read, not a net energy of zero like in a flat universe.
     
  8. Aug 19, 2012 #7

    Chalnoth

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    It only requires that the universe be slightly closed. This is perfectly compatible with measuring near-flatness. It may even be compatible with measuring very slightly negative curvature, as we can only measure the curvature of our observable region, but the statement about a universe from a quantum tunneling event being closed is a global statement that doesn't necessarily rule out some sub-regions within the universe being open.
     
  9. Aug 19, 2012 #8
    How can the universe have a total energy of zero, if the universe is closed though? I thought that conventional wisdom told us that only provided the universe is flat, does the net energy equal zero. If it doesn't equal zero, then one is forced to ask where that little spark of energy came form. Thus, I still fail to see how Alexander's model works by assuming a closed universe. I'm not saying you are wrong, but would you please clarify your position? Thank you.
     
  10. Aug 19, 2012 #9
    Basically, Alexander Vilenkin can say the universe came from "nothing" if the total energy of the universe is zero (I guess this makes sense, because "nothing" would have a net energy of zero as well). This is what he does in fact claim. However, in a closed universe, there is a net negative energy based on what I have researched, and only in a flat universe do you get a net energy of zero.
     
  11. Aug 19, 2012 #10

    Chalnoth

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    It is only for a closed universe that it is possible (so far as we are aware) to define a total energy such that the total energy is zero. I don't know where you got the idea that this result is for a flat universe.
     
  12. Aug 19, 2012 #11
    "Alan Guth's term of inflation, which causes our universe to be flat, like inflating a ball decreases its surface curvature and makes it flat. Another factor proposed by Guth which should be considered is that in such a flat universe, the total gravitational energy equals zero." - http://news.softpedia.com/news/Zero-Energy-and-the-Flat-Universe-98861.shtml

    "A gravitational field has negative energy. Matter has positive energy. The two values cancel out provided the universe is completely flat. In that case the universe has zero energy and can theoretically last forever." - http://en.wikipedia.org/wiki/Zero-energy_universe

    Everything I have researched, concludes that if the universe is flat it has a net energy of zero, and if it is closed, it has a net negative energy.

    "The universe is neither spherical shaped, like the shape of a ball. That would be positive curvature, having net negative energy." - Neil deGrasse Tyson ()
     
    Last edited by a moderator: Sep 25, 2014
  13. Aug 19, 2012 #12

    Chalnoth

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    The statement that the universe has zero total energy comes from what is known as the Hamiltonian formalism of General Relativity. See here:
    http://arxiv.org/PS_cache/gr-qc/pdf/0605/0605063v3.pdf

    Looks like I was part-wrong, in that it is also possible to define zero total energy for a flat universe.

    That said, one of the main problems here is that the pseudo-tensor approach which leads to the zero-energy result is somewhat arbitrary: it depends explicitly upon the coordinates used. There isn't any absolute sense in which it is possible to state that any universe is or is not zero-energy, because there isn't any absolute way to define the total energy of a universe. To me, the zero-energy universe is just an interesting conceptual idea to understand how it is perfectly sensible for a universe to appear as a vacuum fluctuation, or for the apparent energy in matter fields to increase or decrease as the universe expands. It should not be taken as truth, or even as necessary. Because the simple fact of the matter is that in General Relativity, energy is not conserved.
     
    Last edited by a moderator: Sep 25, 2014
  14. Aug 19, 2012 #13
    (i) A closed universe means positively curved, hence a negative net energy (no?). How can a closed universe have net energy zero? I am not saying you are wrong, but could you provide a source stating that a zero energy universe is also possible with with a closed universe? From what I have read, it is possible with a flat universe, but a closed universe would be negative (once more, I could be wrong)

    (ii) I never said the zero-energy universe was absolute truth. However, with all the evidence we have it is entirely plausible, and an attractive idea on top of that.

    (iii) How is energy not conserved? Isn't that the First Law of Thermodynamics?
     
  15. Aug 19, 2012 #14
    “If the shape of the universe were anything else [besides flat], positively curved or negatively curved, it would not have zero energy.” – Neil DeGrasse Tyson

    "The gravitational energy which is always negative exactly compensates the positive energy of matter. So the energy of a closed universe is always zero" - Alexander Vilenkin

    Basically, both of the above statements cannot be true. Neil says that a closed universe could not have a net energy of zero, but Alex says the energy of a closed universe could always be zero. Who do you agree with?
     
  16. Aug 19, 2012 #15

    Chalnoth

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    Not at all. I think you're mistaking the critical density for the density required for zero net energy. This isn't at all what that means.

    I did, in the very post you're quoting. You can see the mentions in the introduction, and the calculation in section 3C, on pages 8-9.
     
  17. Aug 19, 2012 #16
    “If the shape of the universe were anything else [besides flat], positively curved or negatively curved, it would not have zero energy.” – Neil DeGrasse Tyson

    So basically, what you are saying, is Neil DeGrasse Tyson is mistaken?

    Also, yes you did provide a link, I see it now. Thank you.
     
  18. Aug 19, 2012 #17

    Chalnoth

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    Yes. Which is perfectly understandable. It's not quite his field. He's an astronomer. This is about cosmology. And while there is some overlap, it isn't as much as you'd think.
     
  19. Aug 19, 2012 #18
    This is false. The total energy of a closed universe is zero. A flat universe may have zero energy, if and only if it is finite in size. See MS Berman's derivation:

    http://arxiv.org/pdf/gr-qc/0605063.pdf
     
  20. Aug 19, 2012 #19
    By finite in size, do you mean, not expanding forever? If so, how can this be? Doesn't a flat universe necessitate the expansion of the universe slowing down, but never quite stopping?
     
  21. Aug 19, 2012 #20
    I find it weird this video was released then, without it being peer-reviewed:



    However, what you are saying makes sense. Therefore, I thank you for answering my questions, you have been a big help!
     
    Last edited by a moderator: Sep 25, 2014
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