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.

Saturday, 5 September 2009

Hidden Photons and New Forces - Latest Limits

Standard physics claims the world it built from just 4 forces, electromagnetism, the weak and strong nuclear forces, and gravity. This has been the known state of the act
of physics since the late forties. Is it still possible there are more unknown forces at work in the universe. Certainly cosmologist have found known matter to be just 5% of the universe, and dark matter and dark energy could easy interact with unknown forces of which we could say little. Here however I want to see whether current measurement allow for any extra force acting on the sort of matter we find on earth, i.e. acting on neutrons, protons or electrons.

Since I've been making a claim of a new force in my paper and on this blog, axitronics, it behoves to me the duty, of every so often checking physics papers to see if any new experiments rule out my force. Fortunately I'm not the only one claiming new forces. String theory generically predicts many extra U(1) (electromagnetism like) interactions which could have any range or strength. A recent paper, Naturally Light Hidden Photons in LARGE Volume String Compactifications, looks at both the theory and experimental search for such forces. I'll ignore the theory, and concentrate on the experimental limits. The diagram above is taken from that paper. The horizontal (log scale) axis represents the mass of the force carrying particle (given the force a maximum range of around hbar/mc), while the vertical axis represents how much the force mix with ordinary light, given ordinary electronics a fractional charge in whichever new force. The yellow regions are places where a new force might be useful is explaining cosmological data, and the other colour regions are excluded bye existing experiments. In fact very large parameter space exists where new forces can exists.

Which regard to my axial force nothing in the above data excludes it, firstly conversion of parity absolute forbids mixing of the axi-force carrier, the axi-photon, with ordinary photons in the vacuum, and secondly I've assumed the force to be massless in the vacuum. However to the axial force, vacuum is very hard to find, there will always be some neutrinos around, and this leads to an effective mass for axi-photon depending on the neutrino density. For the earth based experiments this gives any effective mass (based on the neutrino density in the atmosphere and presumably in vacuum in earth based labs), of about 100eV), leading to a range of about 5nm. However in deep space and based on the ordinary estimates of neutrino density, the effective mass will be about 0.3 neV (nano eletron volts), and a range of about 7mm. Thus the axial force is a natural chameleon changing with its environment, just like chameleon theories of dark energy suggest.

The most recent experiment looking for a dark energy chameleon particle, is the GammeV experiment. Which looked for light converting to other small mass particles, inside a very strong magnet. Its excluded particles up to 2meV. A much smaller mass than our prediction, so at present, it seems the axial force, dark energy theory, is still very viable.

Perphaps the best test of a fifth force, and certainly one like the axial force, is in neutron interactions. A recent measurement of the ultra cold neutrons which limits any force with range greater than about 1 micrometer, and strength stronger 10^12-10^15 times gravity or 10^24 times weaker than electromagnetism. This is a very stringent limit, but once again the chameleon type, screening of the axial force by neutrinos on the earth seems to hide the axial force very effectively. It seems to me that the axial force is still allowable to present experimental tests. This is not so true of many other forces, only the screening by the neutrino background, allows the force to hide so effectively.