Extended Limber Approximation

TL;DR

This paper derives an extended version of the Limber approximation for angular power spectra, providing higher-order corrections to improve accuracy especially for narrow redshift bins and small overlaps.

Contribution

It introduces a systematic series expansion for the Limber approximation, including a second order correction to enhance precision at large multipoles.

Findings

Error in ordinary Limber approximation is O(1/ell^2)

Second order correction reduces error to O(1/ell^4)

Using ell instead of (ell+1/2) reduces accuracy

Abstract

We develop a systematic derivation for the Limber approximation to the angular cross-power spectrum of two random fields, as a series expansion in 1/(\ell+1/2). This extended Limber approximation can be used to test the accuracy of the Limber approximation and to improve the rate of convergence at large \ell's. We show that the error in ordinary Limber approximation is O(1/\ell^2). We also provide a simple expression for the second order correction to the Limber formula, which improves the accuracy to O(1/\ell^4). This correction can be especially useful for narrow redshift bins, or samples with small redshift overlap, for which the zeroth order Limber formula has a large error. We also point out that using \ell instead of (\ell+1/2), as is often done in the literature, spoils the accuracy of the approximation to O(1/\ell).