Astroparticle Physics

294 15 OutlookTheory of Everythingorigin of cosmic rayscosmic acceleratorsneutrino astronomybeyond the Standard ModelPlanck-scale energies. The search for stable remnants ofthe GUT or Planck time probably allows to get informationabout models of the all-embracing theory (TOE – Theory ofEverything). These relics could manifest themselves in exoticobjects like heavy supersymmetric particles, magneticmonopoles, axions, or primordial black holes.Apart from the unification of all interactions the problemof the origin of cosmic rays has still not been solved. Thereis a number of known cosmic accelerators (supernova explosions,pulsars, active galactic nuclei, M87(?), ...), but itis completely unclear how the highest energies (> 10 20 eV)are produced. Even the question of the identity of these par-ticles (protons?, heavy nuclei?, photons?, neutrinos?, newparticles?) has not been answered. It is conjectured that activegalactic nuclei, in particular those of blazars, are ableto accelerate particles to such high energies. If, however,protons or gamma rays are produced in these sources, ourfield of view into the cosmos is rather limited due to theshort mean free path of these particles (λ γp ≈ 10 Mpc,λ γγ ≈ 10 kpc). Therefore, the community of astroparticlephysicists is optimistic to be able to explore the universewith neutrino astronomy where cosmic neutrinos are pro-duced in a similar way to γ rays in cosmic beam-dump experimentsvia pion decays. Neutrinos directly point back tothe sources, they are not subject to deflections by magneticfields and they are not attenuated or even absorbed by interactions.The enigmatic neutrinos could also give a small contributionto the dark matter, which obviously dominates theuniverse. The investigations on the flavour composition ofatmospheric neutrinos have shown that a deficit of muonneutrinos exists, which obviously can only be explained byoscillations. Such neutrino oscillations require a non-zeroneutrino mass, which carries elementary particle physics alreadybeyond the well-established Standard Model. It wasclear already for a long time that the Standard Model of elementaryparticles with its 26 free parameters cannot be thefinal answer, however, first hints for a possible extension ofthe Standard Model do not originate from accelerator experimentsbut rather from investigations of cosmic rays.Neutrinos alone are by far unable to solve the problemof dark matter. To which extent exotic particles (WIMPs,SUSY particles, axions, quark nuggets, ...) contribute tothe invisible mass remains to be seen. In addition, there is