Complex evaluation of angular power spectra: Going beyond the Limber approximation

Abstract: Angular power spectra are central to the study of our Universe. In this paper, I develop a new method for the numeric evaluation and analytic estimation of the angular cross-power spectrum of two random fields using complex analysis and Picard- Lefschetz theory. The proposed continuous deformation of the integration domain resums the highly oscillatory integral into a convex integral whose integrand decays exponentially. This deformed integral can be quickly evaluated with conventional integration techniques. These methods can be used to quickly evaluate and estimate the angular power spectrum from the three-dimensional power spectrum for all angles (or multipole moments). This method is especially useful for narrow redshift bins, or samples with small redshift overlap, for which the Limber approximation has a large error.