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Cosmological constant and Dark energy

  1. Apr 29, 2005 #1

    wolram

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    http://arxiv.org/abs/astro-ph/0504416

    Authors: Mustapha Ishak (Princeton University)
    Comments: 10 pages, 5 figures

    Associated with the cosmic acceleration are the old and new cosmological constant problems, recently put into the more general context of the dark energy problem. In broad terms, the old problem is related to an unexpected order of magnitude of this component while the new problem is related to this magnitude being of the same order of the matter energy density during the present epoch of cosmic evolution. Current plans to measure the equation of state certainly constitute an important approach; however, as we discuss, this approach is faced with serious feasibility challenges and is limited in the type of conclusive answers it could provide. Therefore, is it really too early to seek actively for new tests and approaches to these problems? In view of the difficulty of this endeavor, we argue in this work that a good place to start is by questioning some of the assumptions underlying the formulation of these problems and finding new ways to put this questioning to the test. Motivated by some theorems, we discuss if the full identification of the cosmological constant with vacuum energy is unquestionable. Next, we evaluate how much fine tuning the cosmic coincidence problem represents. We discuss some implications of the simplest solution for the principles of General Relativity. We stress the potential of some cosmological probes such as weak gravitational lensing to identify novel tests to probe dark energy questions and assumptions. Also, we discuss the relevance of experiments at the interface of astrophysics and quantum field theory, focusing on the Casimir effect in gravitational and cosmological contexts. We conclude that challenging some of the assumptions underlying the cosmological constant problems and putting them to the test may prove useful and necessary to make progress on these questions
     
  2. jcsd
  3. Apr 30, 2005 #2
    What do you think dark energy is?
     
  4. Apr 30, 2005 #3
    I wonder, if (and I don't know for sure) the expansion rate of the universe is a function of the emount of matter in the observable universe, and wherever there is a cosmological event horizon matter disappears behind it and leaves the obsevable universe, then is the universe now accelerating because so much matter has already disappeared behind the cosmologcial event horizon that there's not enough matter to slow the expansion?
     
  5. May 1, 2005 #4

    wolram

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    To be honest lucid385, no body knows, it is supposed to be something
    "energy", that opposes gravity, and accelerates the expansion of the
    universe, it has been called the cosmological constant, AFAIK all research
    to date has failed to identify DE.
     
  6. May 1, 2005 #5

    turbo

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    Here's a thought experiment: How can the vacuum of space exert pressure capable of resisting gravitation?

    Possible answer: The fermionic particle/antiparticle pairs of the quantum vacuum must obey the Pauli exclusion principle. No two fermions of the same state and spin can occupy the same location.

    Assume that the particle/antiparticle pairs can be polarized by the presence of matter, and their distribution can be densified by this attractive force that we perceive as gravitation. How can the "gravitational force" of the vacuum fields be so finely balanced? The attractive force is in dynamical equilibrium with the quantum force described by the Pauli exclusion principle. The tighter gravitation packs the virtual fermions of the quantum vacuum, the more they resist occupying smaller and smaller spaces with similar fermions.

    In "empty" space (no nearby masses) the vacuum fields are relaxed, the Pauli exclusion principle dominates, and the vacuum field density is very low. In the presence of large masses, the vacuum fields are polarized and densified, but the densification is limited by the dynamical balance between gravitational attraction and the quantum forces exerted in accordance with the Pauli exclusion principle.

    In his 1920 talk at Leyden, Einstein affirmed the need for a dynamical gravitational ether to mediate gravitational forces, AND he affirmed the need for an EM ether through which EM waves can propagate, but he denied that the EM ether could have any sensible properties in GR. I think that the fields of the quantum vacuum serve both purposes, and the densification of the vacuum fields by mass is responsible for the optical effects presently attributed to "gravitationally-curved space-time".

    When the Athena Project at CERN manages to produce experimentally-useful antihydrogen, one of their most important experiments will be to test the equivalence of the gravitational infall rates of matter vs antimatter. If that equivalence is broken, GR is in for a shake-up, because at last there will be a mechanical cause for vacuum polarization and gravitation.
     
  7. May 1, 2005 #6

    wolram

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    Still flogging that horse Turbo? well fair play to you :smile: It is a testable
    theory, and one i can not wait to see tested.
    I do feel that our U is not as mystical as some would have us believe, what
    with strings,loops, extra dimentions, singulaities, etc etc, it would be a job
    well done if any of these could be trashed, but i bet they will survive into
    2050s :confused:
     
  8. May 1, 2005 #7

    wolram

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    By Turbo.

    In "empty" space (no nearby masses) the vacuum fields are relaxed, the Pauli exclusion principle dominates, and the vacuum field density is very low. In the presence of large masses, the vacuum fields are polarized and densified, but the densification is limited by the dynamical balance between gravitational attraction and the quantum forces exerted in accordance with the Pauli exclusion principle.

    I think i understand "the vacuum fields are polarized", but could you expand
    on this?
     
  9. May 1, 2005 #8

    turbo

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    If the "gravitational infall rates" of matter and antimatter are not equivalent (a primary experiment of CERN's Athena Project, by the way) then we should expect the particle/antiparticle pairs of the quantum vacuum to be polarized by the presence of matter. Intuitively the gravitational mass equivalence will be broken due to the attraction of matter to antimatter, while the inertial masses of matter and antimatter remain equivalent.

    If this differential attraction is real, the particle/antiparticle virtual pairs of the vacuum will arise (and persist as long as allowed by the Heisenberg Uncertainty Principle) in a preferred orientation in the neighborhood of a massive object. The antiparticles will be preferentially oriented toward the large mass and the particles will be farther from the large mass. Given the self-attractive forces presented by this orientation, the ZPE pairs will pack more tightly in more massive domains. The packing will be balanced by the quantum force of the Pauli Exclusion principle as the fermions resist being pushed into the vicinity of similar objects.

    This is an elegant solution to the mechanics underlying gravity. It explains the nature of Einstein's gravitational ether and shows how gravity arises as interaction of matter with quantum fields. It also allows us to model "gravitational" lensing not as photons following geodesics in curved space-time, but as EM waves being refracted by passing through density gradients in the transmissive media (the EM fields of the vacuum). Galactic lensing and cluster lensing can therefore be studied in terms of classical optics. Just like your eyeglasses refract impinging EM waves to focus them properly on your retina, the dense EM vacuum fields in clusters refract the light passing through them. The amount and type of refraction will depend on the slope of the field's density gradient and the angle of impingement of the EM wavefront (including the overall shape of the overdense region), just like in classical optics.

    Of course, if self-attractive vacuum fields can explain why galactic clusters bind together with seemingly insufficient mass, then dark matter may be dispensed with, and if the quantum forces arising from the Pauli exclusion principle can explain how the vacuum exerts a force sufficient to dynamically balance gravitational collapse, then dark energy may be dispensed with, as well.

    (Edited for emphasis - critical concept.) The fine-tuning of the cosmological constant (in terms of vacuum energy) is a huge problem in the standard model, and I believe that the dynamical balance between gravitational attraction and fermionic repulsion of the vacuum field is the most viable explaination for the fine-tuning. If gravitational attraction and "quintessence" (or whatever else you'd like to call the repulsive force) do not arise from the very same field, then anisotropic distributions or densities in either of their fields would make fine-tuning impossible. The fine-tuning is proof that both forces (attractive and repulsive) are dynamical features of the same field.
     
    Last edited: May 2, 2005
  10. May 1, 2005 #9

    Chronos

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    You got what you asked for wolram.
     
  11. May 1, 2005 #10

    turbo

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    I cannot wait either. If the Athena project at CERN identifies a differential in the gravitational infall rates of hydrogen vs antihydrogen, vacuum polarization will be a hot field very quickly. Andrei Sakharof and others have floated the idea that gravitation and inertia arise from matter's interaction with the vacuum fields, but to my knowledge nobody has proposed a quantum effect that can explain the interaction, nor have their models been mature enought to make a falsifiable prediction regarding the mechanics of the interaction.

    That's a pretty gloomy prediction, Wolram! I hope we are inventive and inquisitive enough to avoid that, although given that there are seemingly as many string theories as there are string theorists, that Hydra may be hard to kill. I share your feeling though, and I believe that the U operates according to a few well-defined rules, and that the complexity that we observe arises out of that very simple set of rules. The more complex a theory is and the more unexplained constants it contains, the less likely it is to be correct, except as a useful approximation. The U cannot be expected to know how to follow our derived approximations for its behavior. If our approximations are incredibly tough to follow and require many years of specialized study (and the willing suspension of disbelief to embrace undetectable things like dark matter, dark energy, early inflation, etc, etc,) it should be a sign that we do not have an adequate understanding of the physics. To paraphrase Feynman, if you cannot sit down with your favorite aunt and explain to her what you do for a living, then you do not have an adequate understanding of your field.
     
    Last edited: May 1, 2005
  12. May 2, 2005 #11

    Garth

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    An 'epicycle'.

    Garth
     
  13. May 4, 2005 #12

    turbo

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    To follow up on Post #8 above, I have been trying to determine at what temperature fermionic virtual pairs of the quantum vacuum will NOT obey the Pauli Exclusion Principle and resist compaction. This led me to Deborah Jin and her work on fermion condensates. These condensates can form at 5 x 10-8K, but only if the fermions are subjected to strong pulsed magnetic fields, analagous to the tuned laser cooling used to produce Bose Einstein Condensates in the lab earlier. This temperature is orders of magnitude lower than the baseline temperature of "empty" space.

    http://jilawww.colorado.edu/~jin/

    A link to her team's preprint is on the page.
     
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