We consider the classical evolution of the inflaton field $ϕ(t)$ and the Hubble parameter $H(t)$ in homogeneous and isotropic single-field inflation models. Under an extremely broad assumption, we show that the universe generically emerges from an initial singularity in a non-inflating state where the kinetic energy of the inflaton dominates its potential energy, $\dotϕ^2 \gg V(ϕ)$. In this kinetically-dominated regime, the dynamical equations admit simple analytic solutions for $ϕ(t)$ and $H(t)$, which are independent of the form of $V(ϕ)$. In such models, these analytic solutions thus provide a simple way of setting the initial conditions from which to start the (usually numerical) integration of the coupled equations of motion for $ϕ(t)$ and $H(t)$. We illustrate this procedure by applying it to spatially-flat models with polynomial and exponential potentials, and determine the background evolution in each case; generically $H(t)$ and $|ϕ(t)|$ as well as their time derivatives decrease during kinetic dominance until $\dotϕ^2\sim V(ϕ)$, marking the onset of a brief period of fast-roll inflation prior to a slow roll phase. We also calculate the approximate spectrum of scalar perturbations produced in each model and show that it exhibits a generic damping of power on large scales. This may be relevant to the apparent low-$\ell$ falloff in the CMB power spectrum.

Accepted by Physical Review D, 20 pages, 14 figures. v3 contains a significant correction to the proof, submitted as an errata to PRD