Wednesday, October 19, 2011

Cold dark matter in difficulties

Cold dark matter scenario assumes that dark matter consists of exotic particles having extremely weak interactions with ordinary matter and which clump together gravitationally. These concentrations of dark matter would grow and attract ordinary matter forming eventually the galaxies.

Cold dark matter scenario has several problems.

  1. Computer simulations support the view that dark matter should be densely packed in galactic nuclei. This prediction is problematic since the constant velocity spectrum of distant stars rotting around galactic nucleus requires that the mass of dark matter within sphere of radius R is proportional to R so that the density of dark matter would decrease as 1/r2. This if one assumes that the distribution of dark matter is spherically symmetric.
  2. Observations show that in the inner parts of galactic disk velocity spectrum depend linearly on the radial distance (see this). Dark matter density should be constant in good approximation (assuming spherical symmetry) whereas the cold dark matter model represent is strong peaking of the mass density in the galactic center. This is known as core/cusp problem.
  3. Cold dark matter scenario predicts also large number of dwarf galaxies with mass which would be about one thousandth of that for the Milky Way. They are not observed. This is known as missing satellites problem.
  4. Cold dark matter scenario predicts signficant amounts of low angular momentum material which is not observed.

Cold dark matter scenario is however in difficulties as one learns from Science Daily article Dark Matter Mystery Deepens. Observational data about the structure of dar matter in dwarf galaxies is however in conflict with this picture. New measurements about two dwarf galaxies tell that dark matter distribution is smooth. Dwarf galaxies are believed to contain 99 per cent of dark matter and are therefore ideal for the attempts to understand dark matter. Dwarf galaxies differ from ordinary ones in that stars inside them move like bees in beehive instead of moving along nice circular orbits. The distribution of the dark matter was found to be uniform over a region with diameter of several hundred light years which corresponds to the size scale of the galactic nucleus. For comparison purposes note that Milky Way has at its center a bar like structure with size between 3300-16000 ly. Notice also that also in ordinary galaxies constant density core is highly suggestive (core/cusp problem) so that dwarf galaxies and ordinary galaxies need not be so different after all.

In TGD framework the simplest model for the galactic dark matter assumes that galaxies are like pearls in a necklace. Necklace would be long magnetic flux tube carrying dark energy identified as magnetic energy and galaxies would be bubbles inside the flux tube which would have thicknened locally. Similar model would apply to start. The basic prediction is that the motion of stars along flux tube is free apart from the gravitational force caused by the visible matter. Constant velocity spectrum for distant stars follows from the logarithmic gravitational potential of the magnetic flux tube and cylindrical symmetry would be absolutely essential and distinguish the model from the cold dark matter scenario.

What can one say about the dwarf galaxies in TGD framework? The thickness of the flux tube is a good guess for the size scale in which dark matter distribution is approximately constant: this for any galaxy (recall that dark and ordinary matter would have formed as dark energy transforms to matter). The scale of hundred light years is roughly by a factor of 1/10 smaller than the size of the center of the Milky Way nucleus. The natural question is whether the dark matter distribution could spherically symmetric and constant in this scale also for ordinary galaxies. If so, the cusp/core problem would disappear and orinary galaxies and dwarf galaxies would not differ in an essential manner as far as dark matter is considered. The problem would be essentially that of cold dark matter scenario.

For details and background see the chapter Cosmic strings of "Physics in Many-Sheeted Space-time".

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