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B Was the Big Bang a quantum mechanical vacuum fluctuation?

  1. Jul 30, 2018 #81
    Deepblu has brought up three heterodox ideas, I thought I would mention my own responses.

    First, what if there's no dark matter? I certainly think the success of MOND, a type of modified gravity, is very important; but it is not relativistic. There is a kind of superfluid dark matter theory, due to Khoury and Berezhiani, which can reproduce the features of MOND. One might also look for MOND to be a quantum gravity effect, perhaps involving extra degrees of freedom beyond the classical metric.

    Second, what if there's no dark energy? This is more problematic in that QFT has vacuum energy and you expect vacuum energy to gravitate, so you actually expect there to be something like dark energy. Although then one has the problem that the expected magnitude of dark energy is vastly greater than what is actually observed. The anthropic answer is that there are positive and negative contributions to vacuum energy, and they happen to almost cancel because if they didn't, there wouldn't be galaxies, atoms, or life. More interesting is the idea that the QFT vacuum energy does cancel or almost cancel due to some symmetry, like a crypto supersymmetry. If it's only an almost cancellation, the observed dark energy can then be the residual vacuum energy. If the cancellation is exact, then the accelerating expansion has to come from somewhere else, such as a quintessence field.

    Third is the heterodox idea that interests me the most precisely because I haven't thought about it: what if energy is conserved after all, in the true theory of quantum gravity? I am used to the usual line of thought, which is that you can get local conservation of energy in GR if you use pseudotensors. But how does the issue look in quantum gravity? This seems to be very little discussed. First of all, Noether's theorem already works a little differently in quantum field theory, compared to classical physics, because of the peculiarities of the quantum framework. And here is a perspective that is new for me: energy conservation is due to time translation, but the Hamiltonian of quantum gravity has no time evolution! How does that affect the attempt to reason about energy conservation? I have no idea.
  2. Jul 30, 2018 #82
    "what if there's no dark matter?" and "what if there's no dark energy?" are not invalid, or suppressed questions, as it is sometimes portrayed. People are looking at all possible ways to develop new theories and explain observations.
  3. Jul 30, 2018 #83


    Staff: Mentor

    This is a big issue - you know what the vacuum energy really is - a big fat zero by a process called normal ordering. That it is not zero comes from popularizations - but your post suggests you are beyond that so I am surprised you do not know it. The only popularizer that seems to discuss this and other issues correctly is Penrose - but he includes that dreaded thing - equations. I am reading one of his books right now and he explains the 1+2+3+........... = -1/12 counter intuitive equation by its real reason - analytic continuation and explains correctly why that is - as well as its issues. He also explains the Casmir effect correctly. I know to explain this stuff to lay audiences is a big issue and things of dubious correctness will get through popularizations.. But Penrose shows it can be reduced considerably.

    Please, please take with a grain of salt what people say in popularizations - with the possible exception of Penrose.

    Again - I think you are beyond popularizations - the above was not meant for people that know the technicalities. Were you aware of normal ordering? I know some QFT books don't mention it and just say its infinity so you need to by definition set it to zero - to which I go groan - its like some textbook explanations of re-normalization I have read. I just want to cry. Then you read the BPH method and say - why did you bother with the other rot - that makes perfect sense.

    Last edited: Jul 30, 2018
  4. Jul 30, 2018 #84


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  5. Jul 30, 2018 #85
    Bill - I don't know what Penrose says, but the usual lore is that normal ordering becomes problematic once you are in curved space, because the difference between negative and positive frequency (and thus between annihilation and creation) is now frame-dependent. Also that in nongravitational QFT, you only care about energy differences; but gravity couples to the absolute value of energy, and vacuum energy functions as a cosmological constant.
  6. Jul 31, 2018 #86


    Staff: Mentor

    Thanks for the post.

    Penrose doesn't mention it ie the issue with gravity and normal ordering. Normal ordering is what I have gleaned from studying QFT. I have studied a few books - the ones I mentioned that I was not happy with plus some I am eg:

    I wanted to move onto Weinberg, I have the books, but wanted a bridge, so I got Zee. I found some parts good other parts - well not so good - but I felt at the end of the day it wasn't the book to get me to Weinberg. I now have - Srednicki - which a regular poster/science adviser here (Vanhees) who teaches it thinks could be a good bridge. Like all things - especially as you get older - it just takes time.

    I wasn't aware of the issue you mentioned with GR, but my view of Quantum Gravity is a little unusual as you may have gleaned from another post I made in this thread. Need to look into what you said - sounds both complicating and interesting. Another issue with vacuum energy is they often do an arbitrary cutoff at the plank scale. Yes - I know effective field theory and all that makes it a reasonable thing - but does it really resolve the issue. For me normal ordering was the only one that did.

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