On general conditions for inflation

The classical evolution of a Friedmann universe, coupled to a scalar field, is examined in an attempt to find under which initial conditions chaotic inflation occurs. It is shown that if φ>m_pl/√4π this universe enters naturally into and exits out of an inflationary phase. This behavior takes place under a large variety of scalar potentials including polynomial, logarithmic and exponential potentials. Within the context of our approximation, deviation from homogeneity still leads to an inflationary phase. It is also shown that a scalar field is essential as a source for inflation. Other fields like the massive vector field are unlikely to cause an inflationary phase.