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Caustic Rings and Cold Dark Matter

The hierarchical cold dark matter (CDM) model for structure formation is a well defined and testable model. Direct detection is the best technique for confirming the model yet predictions for the energy and density distribution of particles on earth remain inadequate. Axially symmetric collapse of collisionless dark matter can leave observable caustic rings in phase space and this model is frequently used to make experimental predictions (Sikivie 1999). Such cold collapses inevitably suffer from radial orbit instabilities that produce unrealistic bar-like halos. Moreover, this model bears no relation to the hierarchical growth of CDM galactic halos which form via a complicated sequence of mergers and violent relaxation. This process destroys any symmetry and phase wraps existing caustics on a scale comparable to the first objects to collapse. Since axions can cluster on microscopic scales and free streaming of neutralinos only erases structure smaller than \sim 100 (GeV/m_{CDM}) A.U., the dynamical effects of caustics in the Galactic halo are expected to be negligible.