Astroparticle Physics

282 13 Dark Matterpropertiesof non-baryonic matter‘cold’ and ‘hot’ particlesmodels of galaxy formationquantum fluctuationsand gravitational instabilities‘protogalaxies’‘top–down’ scenariowhich means it must consist of particles that do not occurin the Standard Model of elementary particle physics.The behaviour of this matter is completely different fromnormal matter. This dark matter interacts with other matterpredominantly via gravitation. Therefore, collisions of darkmatterparticles with known matter particles must be veryrare. As a consequence of this, dark-matter particles loseonly a small fraction of their energy when moving throughthe universe. This is an important fact when one discussesmodels of galaxy formation.Candidates for dark matter are subdivided into ‘cold’and ‘hot’ particles. The prototype of hot dark matter is theneutrino (m ν ̸= 0), which comes in at least in three differentflavour states. Low-mass neutrinos are certainly insufficientto close the universe. Under cold dark matter one normallysubsumes heavy weakly interacting massive particles(WIMPs) or axions.The models of galaxy formation depend very sensitivelyon whether the universe is dominated by hot or cold darkmatter. Since in all models one assumes that galaxies haveoriginated from quantum fluctuations which have developedto larger gravitational instabilities, two different cases canbe distinguished.If the universe would have been dominated by the lowmassneutrinos, fluctuations below a certain critical masswould not have grown to galaxies because fast relativisticneutrinos could easily escape from these mass aggregations,thereby dispersing the ‘protogalaxies’. For a neutrino massof 20 eV a critical mass of about 10 16 solar masses is required,so that structure formation can really set in. Withsuch large masses one lies in the range of the size of superclusters.Neutrinos as candidates for dark matter thereforewould favour a scenario, in which first the large structures(superclusters), later clusters, and eventually galaxies wouldhave been formed (‘top–down’ scenario). This would implythat galaxies have formed only for z ≤ 1. However, fromHubble observations one already knows that even for z ≥ 3large populations of galaxies existed. This is also an argumentto exclude a neutrino-dominated universe.Massive, weakly interacting and mostly non-relativistic(i.e., slow) particles, however, will be bound gravitationallyalready to mass fluctuations of smaller size. If cold dark matterwould dominate, initially small mass aggregations wouldcollapse and grow by further mass attractions to form galaxies.These galaxies would then develop galactic clusters and