How much does the curvature perturbation change after it leaves the horizon,
and when should one evaluate the power spectrum? To answer these questions we
study single field inflation models numerically, and compare the evolution of
different curvature perturbations from horizon crossing to the end of
inflation. In particular we calculate the number of efolds it takes for the
curvature perturbation at a given wavenumber to settle down to within a given
fraction of their value at the end of inflation. We find that e.g. in chaotic
inflation, the amplitude of the comoving and the curvature perturbation on
uniform density hypersurfaces differ by up to 180 % at horizon crossing
assuming the same amplitude at the end of inflation, and that it takes
approximately 3 efolds for the curvature perturbation to be within 1 % of its
value at the end of inflation.Comment: Revtex4, 11 pages, 10 figures; v2: added results section E, added
references and acknowledgements; v3: clarification added to conclusions,
version to appear in CQ
