Thursday, 24 September 2009

Dark Energy from QCD

Dark Energy is the mysterious substance causing the universe to accelerate its expansions, its a fluid (or something) with more self attraction (negative pressure), than its own mass energy, which you investigate its gravity (via Einstein's General Relativity), causes a repulsive force, pushing all the galaxies in the universe apart. These blog started because I produced my own theory of what the dark energy fluid is composed of, which required adding a new force to physics. Could the all ready know forces of physics create dark energy. Up to now, everyone thought the answer was no, dark energy was from something extra not in the Standard Model. Then last week Federico Urban and Ariel Zhitnitsky published a paper "The QCD Nature of Dark Energy". It does something miraculous, it explains dark energy using one of the know existing forces, namely the strong nuclear aka Color force.

QCD or Quantum Chromodynamics is the modern theory of the force binding protons and neutrons in the nucleus, and of the binding of quarks inside a proton and neutron. It was developed in the late 1960s by Murry Gell-Mann almost others. Briefly QCD states that each quark come in three otherwise identical versions named colors. Anti-quark then have anti-colors, going by the usually rule that anti-particle have inverted properties to the ordinary particles. Different colors attract, as do colors and anti-colors, while identical colors repel. Overall matter must be color neutral, thus the allow states, are group, a red quark, a green quark, and a blue quark, making a baryon (protons and neutrons, as similar particles). Or a meson, made of colored quark and an anti-colored antiquark. The actually binding of the quarks is done by colored particles called gluons each having a color and an anti-color. There are 8 gluons as describe by the group theory of SU(3). QCD has worked very well in explaining the patterns of baryons and mesons that physicists have discovered. So if dark energy can be explained by QCD alone, physicists will have no need to invent anything more complex.

Urban and Zhitnitsky's theories of dark energy, works something like this. They begin by looking at the QCD vacuum, the state of empty space in QCD. Because quantum mechanics has uncertainty in energy for any measurement of energy, the quantum vacuum is never empty. Instead particle anti-particle pairs, and multiple gluons are constantly appearing and vanishing in the Quantum Vacuum. Calculation of the energy of the QCD vacuum would be done by averaging of all possible such configuration of the vacuum, and this might lead to a cosmological constant, a fixed energy density of the vacuum. Unfortunately the answer to this energy density is probably infinite, a problem physicist avoid by considering only the difference between state, subtracting alway two infinities in a process called re-normalisation. What Urban and Zhitnitsky do is considering what happen to the QCD vacuum as the universe expands. This expansion of the universe, forces the particle in the QCD vacuum slighly further apart, since the color force attract this particles, this gives rise to a inward pressure trying to hold the vacuum together. This in turn leads to cosmologically repulsion, because in general relativity a fluid with a self attract force, gives rise to a gravitional replusion. The calculation is hideously complicated, by the do come up with simple approximation. And there result is remarkably close to observed ammount of dark energy (2.3*10-4) eV^4 the value versus (3.6*10-4) eV^4 from there theory.

There may be many problems with this theory of Dark Energy, which has not yet been confirmed by other workers in the field. However it come from the existing standard model and thus has to be favored over other models by Occams Razor, and makes a clear prediction of the amount of dark energy at any time in the universe.

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