Why Dark Matter isn't what we thought it was
    By Barry Adams | March 27th 2012 05:50 AM | Print | E-mail | Track Comments
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    Barry Adams is a PhD in Theoretical Physics, he left science for many years to work as a programmer, but remained a keen amateur, reading arXiv's...

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    Although for at least 10 years, since the WMAP satellite first measured the amounts of dark matter in the universe, scientists have known the recipe for the universe (4.5% Matter,22.5% Dark Matter, 73% Dark Energy) but really didn't have a clue as to what the Dark Matter and Energy of the universe actually is. One theory of dark matter is a WIMP, a weakly interacting massive particle left over from the big bang, is the lightest super-symmetric particle, LSP. That theory fit in well with the prevailing super-string and super-symmetry theories of particle physics, and the numbers actually worked out when physicists tried predicating how much dark matter the big bang might make. But recently three separate observations in astronomy seem to have ruled this model out. At the same time particle experiments at the Tevatron and CERN have steadily ruled out super-symmetric particles at higher and higher energies, currently a the next to lightest super-particle is figured to have a mass greater than 800 GeV/c^2. But its the astronomy I want to talk about here. 

    Dark Matter was first predicted by Zwicky in the 1920s, by looking at the rotation curve of stars in a galaxy. Star orbits the centre of the galaxy, and so by measuring the how fast the star move, Zwicky could weight how much mass was inside the orbits of those stars. Isaac Newton makes this rather easy, construct a sphere with a radius of the distance of the star from the centre of the galaxy. Any matter on the inside can be thought of as acting from the centre of mass of the galaxy, any matter on the outside of our imaginary sphere can be ignored, and the orbit of the star remains the same, as if you calculated the gravity from all the stars and matter at all the positions. Its in these rotation curves that dark matter has found to be not WIMP like at all. Physicists can easy simulate what happens to groups of heavy particles interacting with gravity alone, and they a simple cusp like density profile where the density (typically written with a Greek letter varies as the radius),

                            rho =  constant (per galaxy)

    Where the power of the radius is typically between 1 and 1.5 with 1.2 being common. Such densities profile are often called NFW (Navarro-Frenk-White) profiles, and thanks to years of careful observation, we know that most spiral and elliptical galaxies  P. Salucci, and even most dwarf spheroidal galaxies Walker & Penarrubia. Instead what is seen a maximum density of 141 solar masses per cubic parsec, or 4 suns per cubic light year, after that density dark matter just doesn't seem to want to get more compressed. Astronomers instead see a constant density of around this figure at the centre of most objects.

     Making super-symmetric dark matter incompressible, isn't easy, because although it might annihilate with other dark matter particles, or even decay, the products of such decays are high energy conventional matter, and absolutely no use in propelling other dark matter particles out the way. Moreover satellite observatories, such as FERMI, HESS and PAMELA, looking for high energy gamma rays from such decays or annihilations have not found any, e.g. Illas Cholis. What astrophysicist had expected from the LSP was a strong gamma ray signal from annihilating LSPs at the centres of galaxies and globular clusters, with a rho^2 or (1/r^2a) like profile, nothing like which has been seen.

    The third and final observation is from Earth based experiment, that have finally started to detect dark matter particles, DAMA (12 sigma), COGENT (2.8 sigma) and CREST-II (4.2 or 4.7 sigma). This would be a big hooray for the super-symmetric dark matter believer, except that they've found a particles to have masses (10 – 35 GeV/c^2), that have already been ruled out by particle accelerators looking for massing energy at CERN or the Tevatron. One exception still allowed is a very light gravitino that interacts with only with the next to lightest particle was would have to be rather heavy, but such a particle would have great difficultly resisting compression as mentioned above.

    Where do these results leave the science? Dark matter researchers are happily corning ad hoc properties to particles, in particular some very odd couplings to gravity in a bid to produce stable self supporting cores. Warm Dark Matter, made of either axions or right handed (sterile) neutrinos is becoming, might support them selves by annihilations, such particles however in the mill or kilo-electron volt mass ranges. The only model I can see, that brings a maximum density and fits the the masses found by the above experiments, is Mirror Matter. This is a identical copy of particles of the standard model, but which interacts with a right handed weak force. In the Standard Model, the weak force is left handed of course. Introducing mirror matter restores the symmetry to physics, and R. Foot still seems to fit the above experiments. When the density because high enough mirror matter would start to form its own Mirror Stars and blow any remaining mirror matter out of its path, just as standard stars and supernova do. There remains the question why is there five time as much mirror matter as normally matter, are Mirror Protons five times heavier (needing a 5 times stronger, strong force: the Proton gets its mass from Gluons and Not the Higgs Bosons), or are there somehow five times as many Mirror Protons of the same mass.

    The realisation that dark matter forms a maximum density core inside galaxies, and not the predicted point cusp shape, as a shock to astrophysics that doesn't yet seem to have made it to the particle physics world yet. But it seem it may be the death nell of the super-symmetric dark matter, apart from Mirror Matter, physics hasn't yet caught up with the idea of an interacting dark particle in the 10-35 GeV range. But this is increasingly what experiments and observations seem to show.