Characterizing Inflationary Perturbations: The Uniform Approximation
  The spectrum of primordial fluctuations from inflation can be obtained using a mathematically controlled, and systematically extendable, uniform approximation. Closed-form expressions for power spectra and spectral indices may be found without making explicit slow-roll assumptions. Here we provide details of our previous calculations, extend the results beyond leading order in the approximation, and derive general error bounds for power spectra and spectral indices. Already at next-to-leading order, the errors in calculating the power spectrum are less than a per cent. This meets the accuracy requirement for interpreting next-generation CMB observations. 
Salman Habib, Andreas Heinen, Katrin Heitmann, Gerard Jungman, and   Carmen Molina-Paris