Astroparticle Physics

24512 Inflation“Inflation hasn’t won the race, but so far it’sthe only horse.”Andrei LindeThe Standard Cosmological Model appears to be very successfulin describing observational data, such as the abundancesof light nuclei, the isotropic and homogeneous expansionof the universe, and the existence of the cosmicmicrowave background. The CMB’s very high degree ofisotropy and the fact that the total energy density is closeto the critical density, however, pose problems in that theyrequire a very specific and seemingly arbitrary choice of initialconditions for the universe. Furthermore, it turns outthat Grand Unified Theories (GUTs) as well as other possibleparticle physics theories predict the existence of stableparticles such as magnetic monopoles, which no one hasyet succeeded in observing. These problems can be solvedby assuming that at some very early time, the total energydensity of the universe was dominated by vacuum energy.This leads to a rapid, accelerating increase in the scale factorcalled inflation.This chapter takes a closer look at the problems mentionedabove, how inflation solves them, what else inflationpredicts, and how these predictions stand in comparison toobservations. In doing this it will be necessary to make predictionsfor the expansion of the universe from very earlytimes up to the present. For these purposes it will be sufficientto treat the universe as being matter dominated after atime of around 50 000 years, preceded by an era of radiationdomination (except for the period of inflation itself). In factit is now believed this picture is not quite true, and the currentenergy density is dominated again by a sort of vacuumenergy. This fact will not alter the arguments relevant for thepresent chapter and it will be ignored here; the topic of vacuumenergy in the present universe will be taken up again inthe next chapter.initial conditionsfor the universemonopoles?predictions of inflationeras of the universevacuum energy