Hybrid Inflation

Usually inflation ends either by a slow rolling of the inflaton field, which gradually becomes faster and faster, or by a first-order phase transition. We describe a model where inflation ends in a different way, due to a very rapid rolling (`waterfall') of a scalar field $\sigma$ triggered by another scalar field $\phi$. This model looks as a hybrid of chaotic inflation and the usual theory with spontaneous symmetry breaking. Another hybrid model to be discussed here uses some building blocks from extended inflation (Brans-Dicke theory), from new inflation (phase transition due to a non-minimal coupling of the inflaton field to gravity) and from chaotic inflation (the possibility of inflation beginning at large as well as at small $\sigma$). In the simplest version of this scenario inflation ends up by slow rolling, thus avoiding the big-bubble problem of extended inflation.