Likelihood Analysis of CMB Temperature and Polarization Power Spectra

TL;DR

This paper introduces a new approximation method for accurately calculating the likelihood function of CMB temperature and polarization power spectra, accounting for correlations and partial sky coverage, enabling reliable parameter estimation.

Contribution

A novel general approximation for the likelihood function of correlated Gaussian fields observed on partial sky, accurate in the full-sky limit and efficient for practical use.

Findings

The approximation is accurate for ell >~ 30 in simulations.

Some Gaussian approximations yield reliable parameter constraints.

The method performs well with realistic noise and mask conditions.

Abstract

Microwave background temperature and polarization observations are a powerful way to constrain cosmological parameters if the likelihood function can be calculated accurately. The temperature and polarization fields are correlated, partial sky coverage correlates power spectrum estimators at different ell, and the likelihood function for a theory spectrum given a set of observed estimators is non-Gaussian. An accurate analysis must model all these properties. Most existing likelihood approximations are good enough for a temperature-only analysis, however they cannot reliably handle a temperature-polarization correlations. We give a new general approximation applicable for correlated Gaussian fields observed on part of the sky. The approximation models the non-Gaussian form exactly in the ideal full-sky limit and is fast to evaluate using a pre-computed covariance matrix and set of power spectrum estimators. We show with simulations that it is good enough to obtain correct results at ell >~ 30 where an exact calculation becomes impossible. We also show that some Gaussian approximations give reliable parameter constraints even though they do not capture the shape of the likelihood function at each ell accurately. Finally we test the approximations on simulations with realistically anisotropic noise and asymmetric foreground mask.