Astroparticle Physics

29315 Outlook“My goal is simple. It is complete understandingof the universe: why it is as it isand why it exists at all.”Stephen HawkingAstroparticle physics has developed from the field of cosmicrays. As far as the particle physics aspect is concerned,accelerator experiments have taken over and played a leadingrôle in this field since the sixties. However, physicistshave realized that the energies in reach of terrestrial acceleratorsrepresent only a tiny fraction of nature’s wide window,and it has become obvious that accelerator experiments willnever match the energies required for the solution of importantastrophysical and cosmological problems. Actually,recently cosmic ray physics has again taken the lead overaccelerator physics by discovering physics beyond the StandardModel, namely, by finding evidence for neutrino oscillationsin cosmic-ray experiments, which require non-zeroneutrino masses.The electromagnetic and weak interactions unify to acommon electroweak force at center-of-mass energies ofaround 100 GeV, the scale of W ± and Z masses. Alreadythe next unification of strong and electroweak interactionsat the GUT scale of 10 16 GeV is beyond any possibility toreach in accelerator experiments even in the future. This iseven more true for the unification of all interactions whichis supposed to happen at the Planck scale, where quantumeffects of gravitation become important (≈ 10 19 GeV).These energies will never be reached, not even in theform of energies of cosmic-ray particles. The highest energyobserved so far of cosmic-ray particles, which has beenmeasured in extensive-air-shower experiments (3×10 20 eV),corresponds to a center-of-mass energy of about 800 TeV inproton–proton collisions, which is 60-fold the energy thatis going to be reached at the Large Hadron Collider LHCin the year 2007. It has, however, to be considered that therate of cosmic-ray particles with these high energies is extremelylow. In the early universe (< 10 −35 s), on the otherhand, conditions have prevailed corresponding to GUT- andcosmic raysphysicsbeyond the Standard Modelelectroweak scaleGUT scalePlanck scaleLHC