Astroparticle Physics

1.4 Open Questions 171989) and at extragalactical distances (1992, Markarian421, Markarian 501). The active galactic nuclei of theMarkarian galaxies are also considered excellent candidatesources of high-energy hadronic charged cosmic rays.1.4 Open Questions“We will first understand how simplethe universe is, when we realize, howstrange it is.”AnonymousA still unsolved question of astroparticle physics is the problemof dark matter and dark energy. From the observationof orbital velocities of stars in our Milky Way and the velocitiesof galaxies in galactic clusters, it is clear that the energydensity of the visible matter in the universe is insufficient tocorrectly describe the dynamics (Fig. 1.17).Since the early nineties, the MACHO 16 and EROS 17 experimentshave searched for compact, non-luminous, Jupiterlikeobjects in the halo of our Milky Way, using the techniqueof microlensing. Some candidates have been found,but their number is nowhere near sufficient to explain themissing dark matter in the universe. One can conjecture thatexotic, currently unknown particles (supersymmetric particles,WIMPs 18 , ...), or massive neutrinos may contributeto solve the problem of the missing dark matter. A nonvanishingvacuum energy density of the universe is alsoknown to play a decisive rôle in the dynamics and evolutionof the universe.In 1998 the Super-Kamiokande experiment found evidencefor a non-zero neutrino mass by studying the relativeabundances of atmospheric electron and muon neutrinos.The observed deficit of atmospheric muon neutrinosis most readily and elegantly explained by the assumptionthat neutrinos oscillate from one lepton flavour to another(ν µ → ν τ ). This is only possible if neutrinos have mass.The presently favoured mass of 0.05 eV for ν τ ,however,isinsufficient to explain the dynamics of the universe alone.16 MACHO – search for MAssive Compact Halo Objects17 EROS – Expérience pour la Recherche d’Objets Sombres18 WIMP – Weakly Interacting Massive ParticlesFig. 1.17Orbital velocities of stars in theMilky Way in comparison withKeplerian trajectoriesMACHO, EROSsupersymmetric particlesWIMPsnon-zero neutrino masses