  Simple models of single-field inflation in the very early universe can
generate the observed amplitude and scale dependence of the primordial density
perturbation, but models with multiple fields can provide an equally good fit
to current data. We show how future observations will be able to distinguish
between currently favoured models. If a curvaton field generates the primordial
perturbations after inflation, we show how the total duration of inflation can
be measured.
