The standard cosmological model, the ”Lambda cold dark matter model” (LCDM) is backed up by an impressive array of data that cover a huge range of scales, from the entire observable universe, probed by measurements of temperature anisotropies in the microwave background radiation, to the scales of galaxy clusters and individual bright galaxies, sampled by large galaxy surveys. On smaller scales than this, there is no strong evidence to support the standard model. Yet, it is on such scales that the nature of the dark matter is most clearly manifest. In the standard model, the dark matter consists of cold particles, such as the lightest supersymmetric particles. There are, however, models of particle physics that predict lighter particles, such as sterile neutrinos, that would behave as warm (WDM), rather than cold (CDM), dark matter. If the dark matter is warm, free streaming in the early universe would have erased primordial fluctuations below mass scales corresponding to dwarf galaxies. The abundance and properties of dwarf galaxies could then encode information about the nature of the dark matter.
The Figure below shows two simulations from a Virgo Consortium project called “Copernicus Complexio” (COCO). These show the structure that grows in a region of diameter approximately 150 million light years in model universes in which the dark matter is cold (left) or warm (right). The WDM model is chosen to have as large a free streaming length as is allowed by observations of gas clouds seen in the early universe (the so-called “Lyman-alpha forest” in distant quasar sight-lines). There are about a hundred haloes in each volume with masses similar to that of the dark halo around the Milky Way galaxy.
Each of these is resolved with about 10 million particles making it possible for the first time to obtain a good statistical sample of well resolved subhaloes. On the scales apparent in this figure there is very little difference between the two models. However, on small scales there are large differences. In CDM tens of thousands of subhalos are visible; in WDM only a few tens are. In principle this difference should be detectable in observational studies.
The project made extensive use of the DiRAC-2 data centric facility because the simulations, with 13 billion particles each, require a machine that has both an exceptionally large memory per core and a large total memory.