Computing the Primordial Power Spectra Directly
  The tree order power spectra of primordial inflation depend upon the
norm-squared of mode functions which oscillate for early times and then freeze
in to constant values. We derive simple differential equations for the power
spectra, that avoid the need to numerically simulate the physically irrelevant
phases of the mode functions. We also derive asymptotic expansions which should
be valid until a few e-foldings before first horizon crossing, thereby avoiding
the need to evolve mode functions from the ultraviolet over long periods of
inflation.
