Astroparticle Physics

272 13 Dark MatterNACHOsultracold gas cloudsweak gravitational lensing:image distortionsseveral non-resolved star images. If, however, one of thesestars would increase in brightness due to microlensing, thiswould be noticed by the change in brightness of the wholepixel.One generally assumes that MACHOs constitute a certainfraction of dark matter. However, it is not clear, whichobjects are concerned and where these gravitational lensesreside. To understand the dynamics of galaxies there mustbe also some NACHOs (Not Astrophysical Compact HaloObjects).A certain contribution to baryonic dark matter could alsobe provided by ultracold gas clouds (temperatures < 10 K),which are very difficult to detect.Recently a promising new technique of weak gravitationallensing has been developed for the determination ofthe density of dark matter in the universe. Weak gravitationallensing is based on the fact that images of distant galaxieswill be distorted by dark matter between the observer andthe galaxy. The particular pattern of the distortions mirrorsthe mass and its spatial distribution along the line of sight tothe distant galaxy. First investigations on 145 000 galaxiesin three different directions of observation have shown thatΩ ≤ 1 and that the cosmological constant very likely playsa dominant rôle in our universe.13.2.2 Neutrinos as Dark MatterFor a long time neutrinos were considered a good candidatefor dark matter. A purely baryonic universe is in contradictionwith the primordial nucleosynthesis of the Big Bang.Furthermore, baryonic matter is insufficient to explain thelarge-scale structure of the universe. The number of neutrinosapproximately equals the number of blackbody photons.If, however, they had a small mass, they could provide a significantcontribution for dark matter.From direct mass determinations only limits for the neu-trino masses can be obtained (m νe < 3eV,m νµ < 190 keV,m ντ < 18 MeV). The deficit of atmospheric muon neutrinosbeing interpreted as ν µ –ν τ oscillations, leads to a mass ofthe ν τ neutrino of approximately 0.05 eV.Under the assumption of Ω = 1 the expected numberdensity of primordial neutrinos allows to derive an upperlimit for the total mass that could be hidden in the threeneutrino flavours. One expects that there are approximatelyupper neutrino mass limitsfrom direct measurements