Astroparticle Physics

12.5 Mechanisms for Inflation 255this ‘false vacuum’ to the true vacuum with V = 0. Whenthis happens, the vacuum energy will no longer dominateand the expansion will be driven by other contributions to ϱsuch as radiation.In the quantum field theories such as the Standard Modelof particle physics, different fields interact. That is, energyfrom one field can be transformed into excitations in another.This is how reactions are described in which particles arecreated and destroyed. So, when the inflaton field (see belowfor more details) moves from the false to the true vacuum,one expects its energy to be transformed into other ‘normal’particles such as photons, electrons, etc. Thus, the end of inflationsimply matches onto the hot expanding universe ofthe existing Big Bang model. It may then appear that inflationhas not made any predictions that one can verify byobservation, and as described so far no verification has beengiven. Later in this chapter, however, it will be shown howinflation provides explanations for the initial conditions ofthe Big Bang, which otherwise would have to be imposedby hand.Shortly after the original inflaton potential was proposed,it was realized by Guth and others that the modelhad important flaws. As the quantum-mechanical tunnelingis a random process, inflation should end at slightly differenttimes in different places. Therefore, some places will undergoinflation for longer than others. And because these regionscontinue to inflate, their contribution to the total volumeof the universe remains significant. Effectively, inflationwould never end. This has been called inflation’s ‘gracefulexit problem’.It was pointed out by Linde [40] and Albrecht and Steinhardt[41] that by a suitable modification of the potentialV(φ), one could achieve a graceful ending to inflation. Thepotential needed for this is shown schematically in Fig. 12.3.The field it describes is called an inflaton, as its potential isnot derived from particle physics considerations such as theHiggs mechanism, but rather its sole motivation is to providefor inflation. The local minimum in V at φ = 0isreplacedby an almost flat plateau. The field can settle downinto a metastable state near φ ≈ 0, and then effectively ‘roll’down the plateau to the true vacuum. One can show that inthis scenario, called new inflation, the exponential expansionends gracefully everywhere.inflaton fieldhow inflation endsquantum-mechanicaltunneling problemgraceful exit probleminflatonnew inflation