Cosmological models in which the dark matter consists of cold elementary particles predict that the population of dark matter haloes in the Universe should extend to masses comparable to the Earth’s, many orders of magnitude below those where galaxies can form. Using a novel multi-zoom technique, we carried out a consistent cosmological simulation of the formation of present-day haloes over the full mass range populated when dark matter is assumed to be a Weakly Interacting Massive Particle (WIMP) of mass ~100 GeV. The simulation has a dynamic range of 30 orders of magnitude in mass and resolves the internal structure of hundreds of Earth-mass haloes in as much detail as that of hundreds of rich galaxy clusters. We find that halo density profiles are universal over the entire mass range and are well described by simple two-parameter fitting formulae, such as the well-known Navarro-Frenk-White profile. Halo mass and concentration are tightly related in a way that depends on cosmology and on the nature of the dark matter. At fixed mass, concentration is independent of local environment for haloes less massive than those of typical galaxies. Haloes over the mass range (10³ – 10¹¹) M_o contribute about equally (per logarithmic interval) to the dark matter annihilation luminosity, which we find to be smaller than all previous estimates by factors ranging up to one thousand. This was a hugely challenging project that took seven years to complete, during which we uncovered and solved a number of technical problems in N-body simulations revealed only in the extreme conditions of our calculation. It was carried out on DiRAC’s Cosmology Machine (COSMA) at Durham.

References:

• Jie Wang, Sownak Bose, Carlos Frenk, Adrian Jenkins, Liang Gao, Volker Springel, and Simon White, to appear in Nature.