Astroparticle Physics

9.5 The Baryon Asymmetry of the Universe 20710 0Flux [photons cm -2 s -1 MeV -1 sr -1 ]10 -110 -210 -310 -410 -510 -6COMPTELSchönfelder et al. (1980)Trombka et al. (1977)White et al. (1977)1 10Photon Energy [MeV]Fig. 9.1The measured gamma-ray flux(data points) along with the levelspredicted to arise from interactionbetween domains of matter andantimatter. The upper curvecorresponds to domain sizes of20 Mpc, the lower for1000 Mpc [14, 15]International Space Station will carry out more sensitivesearches for this in the next several years [13].If there were to exist antimatter domains of the universe,then their separation from matter regions would haveto be very complete or else one would see the γ -ray fluxfrom proton–antiproton or electron–positron annihilation.The flux that one would expect depends on the size of theseparated domains. Figure 9.1 shows the measured gammarayflux (data points) along with the predicted levels (curves)that would arise from collisions of matter and antimatter regions[14, 15]. The upper curve corresponds to domain sizesof 20 Mpc and is clearly excluded by the data. The lowercurve is for domains of 1000 Mpc and it as well is incompatiblewith the measurements. So one can conclude that ifantimatter regions of the universe exist, they must be separatedby distances on the order of a gigaparsec, which isa significant fraction of the observable universe. Given thatthere is no plausible mechanism for separating matter fromantimatter over such large distances, it is far more natural toassume that the universe is made of matter, i.e., that it has anet non-zero baryon number. Also the absence of a significantflux of 511 keV γ rays from electron–positron annihilationadds to this conclusion.If one then takes as working hypothesis that the universecontains much more matter than antimatter, one needs to askhow this could have come about. One possibility is that thenon-zero baryon number existed as an initial condition, andthat this was preserved up to the present day. This is not ane + e − annihilationabsenceof annihilation radiation