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B What is vacuum energy?

  1. Aug 24, 2016 #1
    I often wonder about how little I understand vacuum, and only recently I've been paying attention to this "vacuum energy" hypothetical.

    I see it being associated with things as small as spontaneous emissions to things as large as the expansion of the universe. This is a huge range of length scales. I know of no other force (or energy or whatever) with that kind of reach.

    What could this potentially mean?

    As far as we can observe, is it reasonable to hypothesize that "vacuum energy" is scale invariant (does it make sense to say that energy or energy potential is scale invariant?), hereby enabling the coupling of physical phenomena from different length scales, and maybe even holding the key to the unification of QFT with GR?

    Or am I smoking crack?

    Have to say that it looks like a promising concept, even though I don't really understand what/how it is yet. Hence, I defer the title question to the experts.
    Last edited: Aug 24, 2016
  2. jcsd
  3. Aug 24, 2016 #2
    I also need to learn a lot about vacuum energy and QFT in general. But from what I know vacuum energy is associated with the vacuum diagrams of a given theory. Usually one sets the vacuum energy to zero, which is equivalent to normal ordering quantum fields. We do this because we only care about energy differences.

    All this is fine until we consider gravity. Gravity couples to energy, so the vacuum energy will have a gravitational effect. Supposedly the vacuum energy of all fields in the universe should add up to give a value for the cosmological constant, and this calculaton was done by S. Weinberg (he may have been the first, not too sure on that). Yet, the experimental value of the cosmological constant is order of magnetudes smaller than the predicted value accoding to QFT. This calculation, which may be dubbed the worst prediction in physics, is the cosmological constant problem.
  4. Aug 24, 2016 #3
    Yes, I've read about it.

    Here's an article: http://math.ucr.edu/home/baez/vacuum.html

    Have the feeling that no one really knows what (and if) vacuum energy really is.
  5. Aug 25, 2016 #4


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  6. Aug 25, 2016 #5
    You can only care about energy difference as long as you ignore gravity and work in flat Minkovski space. With GR, energy tensor itself, not the difference, affects curvature.
  7. Aug 25, 2016 #6
    Yes, of course. This is why I said "All this is fine until we consider gravity" ...
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