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QCD vs. Spatial Expansion

  1. Apr 18, 2015 #1
    This thread involves a little general relativity and a little particle physics.

    Observations indicate that space is expanding at an accelerated rate.

    Given this would continue, even atomic nuclei should be torn apart leaving single particles, isolated from one another by causal horizons.

    What would ensue once the size of the causal horizon approached the length of asymptotic freedom of the constituent quarks of a meson or baryon?
  2. jcsd
  3. Apr 18, 2015 #2


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    To make this statement complete, you should add "at cosmological scales" right after expanding. The stars for example don't expand (those things are generally referred to as gravitationally bound objects).

    So your idea of torn apart matter is because of a misconception. One way to see the distinction is that in cosmology you are describing an homogeneous and isotropic dust cloud (all the constituent contributions are averaged in this cosmological fluid), whereas the spacetime around a star is described to a good extend by the Schwarzchild metric (probably a more precise answer would be by a more complicated metric, since the space around the star is not vacuum).

    I don't understand this question, if you could re-ask it in another way?
    When the "size" of the universe became large enough (when the energy dropped below the QCD scale), the free quark-gluon plasma underwent a phase transition and hadronized, resulting to pions (which later annihilated) and neutrons/protons.
  4. Apr 19, 2015 #3
    How would I request this thread be moved the Special and General Relativity forum?
  5. Apr 21, 2015 #4
    It depends on the nature of dark energy, but if the dark energy pressure remains constant (as in the simplest scenario, where dark energy is a vacuum energy of some kind, i.e. the cosmological constant) then what you describe will never happen. The dark energy density would remain constant per unit spacetime volume, but because objects are moving apart from each other, there gets to be more and more spacetime between them, so the expansion gets faster and faster, i.e. accelerates. But for things which aren't moving apart, i.e. things that are bound together (by electromagnetic/nuclear forces as for atoms, and by gravity for solar system/galaxy/galaxy cluster scales), then the force pulling them apart will not increase with time. So they will never get "ripped" apart.

    But, if dark energy IS increasing in density with time, then what you describe could eventually happen. Personally I don't think it is likely :). Note also that the evidence currently is compatible with the constant scenario. But as for what exactly would happen at the scale you describe, I guess you mean something around the confinement scale, I have no idea :).
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