Astroparticle Physics

296 15 Outlookmatter dominanceprimordial antimatterannihilation radiationWhether a small CP violation is sufficient to accomplish thisremains to be seen. Since in p ¯p annihilations a substantialamount of energy is transformed into photons, the observedγ/p ratio (̂= n γ /n p ) of about 10 9 indicates that even aminute difference in the decay properties of the parent particlesof protons and antiprotons is sufficient to explain thematter dominance in the universe. Such an asymmetry couldhave its origin in Grand Unified Theories of electroweak andstrong interactions. The matter–antimatter asymmetry couldthen be explained by different decay properties of heavy Xand ¯X particles which are supposed to have existed in theearly universe. Equal numbers of X and ¯X particles decayinginto quarks, antiquarks, and leptons could lead to differentnumbers of quarks and antiquarks if baryon-number andlepton-number conservation were violated, thereby leadingto the observed matter dominance of the universe.The non-observation of primordial antimatter is still nota conclusive proof that the universe is matter dominated. Ifgalaxies made of matter and ‘antigalaxies’ made of antimatterwould exist, one would expect that they would attracteach other occasionally by gravitation. This should lead to aspectacular annihilation event with strong radiation. A clearsignal for such a catastrophe would be the emission of the511 keV line due to e + e − annihilation. It is true that such a511 keV γ -ray line has been observed – also in our galaxy– but its intensity is insufficient to be able to understand itas an annihilation of large amounts of matter and antimatter.On the other hand, it is conceivable that the annihilationradiation produced in interactions of tails of galaxies withtails of antigalaxies would establish such a radiation pressurethat the galaxies would be driven apart and the reallygiant spectacular radiation outburst would never happen.Questions about a possible dominance of matter are unlikelyto be answered in accelerator experiments alone. Onthe other hand, the early universe provides a laboratory,which might contain – at least in principle – the answers.However, the step to bridge the gap from the Planck erato present-day theories is still hidden in the mist of spaceand time.The investigations in the framework of astroparticlephysics represent an important tool for a deeper understandingof the universe, from its creation to the present and futurestate.