Particle physics provides some natural Dark Matter (DM) candidates like WIMPs
(Weakly Interacting Massive Particles). Although these particles are very weakly
interacting with ordinary matter, there are direct and indirect detection experiments
possible to catch them. Direct detection is based on scattering processes of the
DM particles with other particles in a laboratory detector. Indirect detection
exploits the annihilation of DM particles. This annihilation leads to products
like gamma-rays that could be observed, for example with the recently launched GLAST satellite.
Both methods are currently used, but up to now there is no clear evidence for any detection.
Such a discovery marks a crucial test for the DM theory.
Since the DM particles are interacting only very weakly with other particles
the experiments are rather challenging and good theoretical predictions
on the expected DM distribution are needed to fine-tune them.
The scales that are probed by detection experiments are extremely small
compared to what cosmologists are used to. Both direct and indirect detection
schemes are therefore sensitive to very small-scale features in DM.
DM is supposed to be mainly cold: CDM (cold dark matter). Cold refers to the very
small primordial velocity dispersion. Therefore the dynamical evolution of CDM
produces distinct small-scale features in the form of streams and caustics (see Figure 1).
A low stream number at a given location produces a quite clumpy velocity distribution.
Caustics on the other hand lead to very high CDM densities. These features influence
the expected detector signals. For example, caustics might boost the
annihilation flux due to their high density. A low stream number on the
other hand would produce characteristic features in detectors searching for DM.
The main tool of modern cosmology to learn about cosmic structure formation and
the DM distribution are cosmological supercomputer simulations. Such simulations are
limited by computational power in terms of the number of particles they use to represent
the DM distribution. Therefore it was up to now not possible to directly resolve the required
Mark Vogelsberger, Simon White, Volker Springel (all MPA) and Amina Helmi
(University of Groningen) therefore invented a new simulation technique to resolve
these structures for the first time in current state-of-the-art N-body simulations.
The MPA scientists implemented this new technique into the current version of MPA's GADGET code,
one of the leading codes for cosmological simulations.
Figure 2 demonstrates the identification of caustics in the DM distribution
that becomes possible with this new method. It shows a calculation done for a
spherically symmetric DM halo. Plotted is the number of caustics DM particles have passed
while orbiting in the halo potential. The green line shows the simulation result revealed with the
new method whereas the red line represents the analytic result. It is quite striking how well
the two agree in terms of the passed caustic number. This demonstrates the very accurate caustic
identification of the newly invented method that can therefore be used to estimate the caustic
boost factors of the annihilation radiation.
Another application is the estimate of the DM stream number near the solar position.
From a simple halo model for the Milky Way that takes into account its radial dependent triaxial shape,
the method can be used to show that the number of streams near the solar neighbourhood
should be of the order of 100.000. The reason for this high number is the fast decrease
in stream density while the DM particles are orbiting in the halo. It turns out that
stream densities in general decrease like 1/(t/torbital)3 (see Figure 3)
in the halo, and that chaotic orbits are also expected where the stream density decreases
even faster. These low stream densities lead to the high stream number and to the
conclusion that direct detection experiments should encounter a quite smooth velocity
distribution. Therefore detectors on earth can quite safely assume a smooth DM velocity
Mark Vogelsberger, Simon White, Amina Helmi, Volker Springel
Mark Vogelsberger, Simon White, Amina Helmi and Volker Springel
Monthly Notices of the Royal Astronomical Society, Volume 385, Issue 1, pp. 236-254.