Astroparticle Physics

266 13 Dark Matterearly universedevelopmentof large-scale structuresThe measurement of the COBE and WMAP satellites onthe inhomogeneities of the 2.7 Kelvin blackbody radiationhas shown that the early universe was much more homoge-neous. On the other hand, small temperature variations ofthe blackbody radiation have served as seeds for structures,which one now observes in the universe.It is generally assumed that the large-scale structureshave evolved from gravitational instabilities, which can betraced back to small primordial fluctuations in the energydensity of the early universe. Small perturbations in the energydensity are amplified due to the associated gravitationalforces. In the course of time these gravitational aggregationscollect more and more mass and thus lead to structure formation.The reason for the original microscopic small inhomogeneitiesis presumably to be found in quantum fluctua-tions. Cosmic inflation and the subsequent slow expansionhave stretched these inhomogeneities to the presently observedsize. One consequence of the idea of cosmic inflationis that the exponential growth has led to a smooth and flatuniverse which means that the density parameter Ω shouldbe very close to unity. To understand the formation of thelarge-scale structure of the universe and its dynamics in detail,a sufficient amount of mass is required because otherwisethe original fluctuations could never have been transformedinto distinct mass aggregations. It is, however, truethat the amount of visible matter only does not support thecritical density of Ω = 1 as required by inflation.To get a flat universe seems to require a second Copernicanrevolution. Copernicus had noticed that the Earth wasnot the center of the universe. Cosmologists now conjecturethat the kind of matter, of which man and Earth are made,only plays a minor rôle compared to the dark non-baryonicmatter, which is – in addition to the dark energy – neededto understand the dynamics of the universe and to reach thecritical mass density.quantum fluctuationsinflation → Ω = 1major rôleof non-baryonic dark matter13.2 Motivation for Dark Matter“If it’s not DARK, it doesn’t MATTER.”AnonymousThe idea that our universe contains dark matter is not entirelynew. Already in the thirties of the last century Zwicky