Astroparticle Physics

13.2 Motivation for Dark Matter 283later superclusters. Cold dark matter therefore favours a scenarioin which smaller structures would be formed first andonly later develop into larger structures (‘bottom–up’ scenario).In particular, the COBE and WMAP observations ofthe inhomogeneities of the 2.7 Kelvin radiation confirm theidea of structure formation by gravitational amplification ofsmall primordial fluctuations. These observations thereforesupport a cosmogony driven by cold dark matter, in whichsmaller structures (galaxies) are formed first and later developinto galactic clusters.The dominance of cold dark matter, however, does notexclude non-zero neutrino masses. The values favoured bythe observed neutrino anomaly in the Super-Kamiokandeand SNO experiments are compatible with a scenario ofdominating cold dark matter. In this case one would have acocktail – apart from baryonic matter – consisting predominantlyof cold dark matter with a pinch of light neutrinos.‘bottom–up’ scenarioinhomogeneitiesof blackbody radiationneutrino anomaly13.2.7 Resumé on Dark MatterIt is undisputed that large quantities of dark matter must behidden somewhere in the universe. The dynamics of galaxiesand the dynamics of galactic clusters can only be understood,if the dominating part of gravitation is caused by nonluminousmatter. In theories of structure formation in theuniverse in addition to baryonic matter (Ω b ) also other formsof matter or energy are required. These could be representedby hot dark matter (e.g., light relativistic neutrinos: Ω hot )or cold dark matter (e.g., WIMPs: Ω cold ). At the presenttime cosmologists prefer a mixture of these three components.Recent measurements of distant supernovae and preciseobservations of inhomogeneities of the blackbody radiationlead to the conclusion that the dark energy, mostly interpretedin terms of the cosmological constant Λ, alsohasa very important impact on the structure of the universe. Ingeneral, the density parameter Ω can be presented as a sumof four contributions,non-luminous matterbaryonic matterhot dark mattercold dark mattercosmological constantΩ parameterΩ = Ω b + Ω hot + Ω cold + Ω Λ . (13.25)The present state of the art in cosmology gives Ω b ≈ 0.04,Ω hot ≤ 1%, Ω cold ≈ 0.23, and Ω Λ ≈ 0.73.As demonstrated by observations of distant supernovae,the presently observed expansion is accelerated. Therefore,accelerated expansion