Astroparticle Physics

13.2 Motivation for Dark Matter 281finite vacuum energy density (Λ ̸= 0) are not trivially coupled.Furthermore, Λ could be a dynamical constant, whichis not only of importance for the development of the earlyuniverse. Since the Λ term in the field equations appears todominate today, one would expect an accelerated expansion.The experimental determination of the acceleration parametercould provide evidence for the present effect of Λ. Todo this one would have to compare the expansion rate ofthe universe in earlier cosmological times with that of thepresent.Such measurements have been performed with the SupernovaCosmology Project at Berkeley and with the HighzSupernova Search Team at Australia’s Mount StromloSpring Observatory. The surprising results of these investigationswas that the universe is actually expanding at ahigher pace than expected (see also Sect. 8.8 and, in particular,Fig. 8.6). It is important to make sure that SN Ia uponwhich the conclusions depend explode the same way nowand at much earlier times so that these supernovae can beconsidered as dependable standard candles. This will be investigatedby looking at older and more recent supernovaeof type Ia. Involved in these projects surveying distant galaxiesare the Cerro Tololo Interamerican Observatory (CTIO)in the Chilean Andes, the Keck Telescope in Hawaii, andfor the more distant supernovae the Hubble Space Telescope(HST).cosmological constantand expanding universeSupernova CosmologyProjectHigh-z Supernova Search13.2.6 Galaxy FormationAs already mentioned in the introduction to this chapter, thequestion of galaxy formation is closely related to the problemof dark matter. Already in the 18th century philosopherslike Immanuel Kant and Thomas Wright have speculatedabout the nature and the origin of galaxies. Today it seems tobe clear that galaxies originated from quantum fluctuations,which have been formed right after the Big Bang.With the Hubble telescope one can observe galaxies upto redshifts of z = 3.5 (λ observed = (1 + z)λ emitted ); thiscorresponds to 85% of the total universe. The idea of cosmicinflation predicts that the universe is flat and expandsforever, i.e., the Ω parameter is equal to unity.The dynamics of stars in galaxies and of galaxies ingalactic clusters suggests that less than ≈ 5% of matter isin form of baryons. Apart from the vacuum energy the mainpart of matter leading to Ω = 1 has to be non-baryonic,Kant, Wrightquantum fluctuationsHubble telescopeΩ parameter