HYBRID INFLATION

Usually inflation ends either by a slow rolling of the inflaton held, 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 held sigma triggered by another scalar field phi. This model looks like a hybrid of chaotic inflation with V(phi) = m(2) phi(2)/2 and the usual theory with spontaneous symmetry breaking with V(sigma) = 1/4 lambda(M(2) - lambda sigma(2))(2). The last stages of inflation in this model are supported not by the inflaton potential V(phi) but by the ''noninflationary'' potential V(sigma). Another hybrid model to be discussed here uses some budding blocks from extended inflation (Brans-Dicke theory), from new inflation (phase transition due to a nonminimal coupling of the inflaton held 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.