Astroparticle Physics

204 9 The Early UniverseTable 9.2Thermal history of the first 10microseconds‘scale’ T [GeV] ϱ [GeV 4 ] t [s]Planck 10 19 10 78 10 −45GUT 10 16 10 66 10 −39Electroweak 10 2 10 10 10 −11QCD 0.2 0.01 10 −5cosmoparticle physicsHiggs fieldvacuum expectation valueof the Higgs fieldspontaneous symmetrybreakingelectroweak scaleQCD scaleAt the Planck scale, i.e., with energies on the order of10 19 GeV, the limit of our ability to speculate about cosmologyand cosmoparticle physics has been reached. Noticefrom (9.39) that the time when the temperature is equal tothe Planck energy is not the Planck time but is in fact somewhatearlier, so the limit has even been overstepped somewhat.After 10 −39 seconds, when the universe had cooled totemperatures around 10 16 GeV (the ‘GUT scale’), the strong,electromagnetic, and weak interactions start to become distinct,each with a different coupling strength. One expectsthat around this temperature a phase transition related to theHiggs field of the Grand Unified Theory took place. At temperaturesabove the phase transition, the vacuum expectationvalue of the Higgs should be zero, and therefore all ele-mentary particles would be massless, including the X and Ybosons. During the transition, the GUT Higgs field acquiresa vacuum expectation value different from zero; this phenomenonis called spontaneous symmetry breaking (SSB).As a result, the X and Y bosons go from being massless tohaving very high masses on the order of the GUT scale. So,at lower temperatures, baryon-number-violating processesmediated by exchange of X and Y bosons are highly suppressed.After around 10 −11 seconds, the temperature is on theorder of 100 GeV; this is called the ‘electroweak scale’. Hereanother SSB phase transition is expected to occur wherebythe electroweak Higgs field acquires a non-zero vacuum expectationvalue. As a result, W and Z bosons as well as thequarks and leptons acquire their masses. At temperaturessignificantly lower than the electroweak scale, the massesM W ≈ 80 GeV and M Z ≈ 91 GeV are large compared tothe kinetic energies of other colliding particles, and the Wand Z propagators effectively suppress the strength of theweak interaction.At temperatures around 0.2 GeV (the ‘QCD scale’), theeffective coupling strength of the strong interaction, α s ,becomesvery large. At this point quarks and gluons become